Seasonal stratospheric photochemistry on Uranus and Neptune
Julianne I. Moses, Leigh N. Fletcher, Thomas K. Greathouse, Glenn S. Orton, Vincent Hue
SSeasonal Stratospheric Photochemistry on Uranus andNeptune
Julianne I. Moses a , Leigh N. Fletcher b , Thomas K. Greathouse c , GlennS. Orton d , Vincent Hue c a Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA b Department of Physics and Astronomy, University of Leicester, University Road,Leicester, LE1 7RH, UK c Southwest Research Institute, San Antonio, TX 78228, USA d Jet Propulsion Laboratory, MS 183-501, Pasadena, CA 91109, USA
Abstract
A time-variable 1D photochemical model is used to study the distributionof stratospheric hydrocarbons as a function of altitude, latitude, and sea-son on Uranus and Neptune. The results for Neptune indicate that in theabsence of stratospheric circulation or other meridional transport processes,the hydrocarbon abundances exhibit strong seasonal and meridional varia-tions in the upper stratosphere, but that these variations become increasinglydamped with depth due to increasing dynamical and chemical time scales.At high altitudes, hydrocarbon mixing ratios are typically largest where thesolar insolation is the greatest, leading to strong hemispheric dichotomiesbetween the summer-to-fall hemisphere and winter-to-spring hemisphere. Atmbar pressures and deeper, slower chemistry and diffusion lead to latitudevariations that become more symmetric about the equator. On Uranus, thestagnant, poorly mixed stratosphere confines methane and its photochemicalproducts to higher pressures, where chemistry and diffusion time scales re-main large. Seasonal variations in hydrocarbons are therefore predicted to be
Preprint submitted to Icarus March 29, 2018 a r X i v : . [ a s t r o - ph . E P ] M a r ore muted on Uranus, despite the planet’s very large obliquity. Radiative-transfer simulations demonstrate that latitude variations in hydrocarbons onboth planets are potentially observable with future JWST mid-infrared spec-tral imaging. Our seasonal model predictions for Neptune compare well withretrieved C H and C H abundances from spatially resolved ground-basedobservations (no such observations currently exist for Uranus), suggestingthat stratospheric circulation — which was not included in these models —may have little influence on the large-scale meridional hydrocarbon distribu-tions on Neptune, unlike the situation on Jupiter and Saturn. Keywords:
Atmospheres, chemistry; Photochemistry; Uranus; Neptune; Atmospheres,composition
1. Introduction
Infrared and ultraviolet observations reveal that the stratospheric compo-sition of Uranus and Neptune is being altered by solar-driven photochemistry,despite the great distance of these planets from the Sun (see the reviews ofAtreya et al., 1991; Bishop et al., 1995; Yung and DeMore, 1999). Methanephotolysis by solar ultraviolet radiation triggers the production of acety-lene (C H ), ethylene (C H ), ethane (C H ), methylacetylene (CH C H),diacetylene (C H ), and other complex hydrocarbons, many of which havebeen observed on Uranus and Neptune (see Burgdorf et al., 2006; Orton et al.,2014c, and references therein). These photochemically produced species areradiatively active at mid-infrared wavelengths and can affect many aspectsof the planetary atmosphere, such as its thermal structure, aerosol struc-2ure, energy balance, dynamical motions, and ionospheric structure. A fullunderstanding of the three-dimensional (3D) time-variable behavior of pho-tochemically produced species is therefore important for understanding manyaspects of atmospheric physics and chemistry on Uranus and Neptune.The non-zero obliquity (axial tilt) of Uranus and Neptune results in aseasonal dependence of solar insolation (see Fig. 1) that affects the produc-tion and loss rates of photochemically active constituents. Uranus, withits extreme ∼ ◦ obliquity and rotational pole nearly in line with its or-bital plane, experiences very unusual seasons compared to other Solar-Systemplanets. Averaged over a year, high latitudes on Uranus receive greater solarinsolation than low latitudes (see Fig. 2). Much of the planet alternates be-tween being almost fully illuminated and being in almost complete darknessfor half a year at a time (with one year on Uranus being equal to 84 Earthyears), creating an opportunity for dramatic changes in hydrocarbon produc-tion as a function of season. Neptune’s more moderate 28.3 ◦ obliquity resultsin seasonal forcing similar to that on the Earth, Mars, and Saturn, with lowlatitudes receiving a greater annual average solar insolation than high lati-tudes. Thus, averaged over a year, hydrocarbon production rates on Neptunewill be greater at low latitudes than high latitudes. Given that a Neptuneyear is 165 Earth years, the winter high-latitude regions on Neptune endurelong periods of time without sunlight, and the reduction in photochemicalproduction of stratospheric hydrocarbons during the long polar winter couldpotentially affect global hydrocarbon abundances and/or result in differentmeridional distributions of hydrocarbons than shorter-period planets withsimilar obliquities, such as Saturn. 3 ranus: Daily mean solar insolation (W m -2 ) P l ane t o c en t r i c La t i t ude ( deg r ee s ) L s (degrees) Neptune: Daily mean solar insolation (W m -2 ) P l ane t o c en t r i c La t i t ude ( deg r ee s ) L s (degrees) Uranus: Daily mean actinic flux (W m -2 ) P l ane t o c en t r i c La t i t ude ( deg r ee s ) L s (degrees) Neptune: Daily mean actinic flux (W m -2 ) P l ane t o c en t r i c La t i t ude ( deg r ee s ) L s (degrees) Figure 1: (Top) Mean daily solar insolation (W m − per planetary day) incident onto aunit horizontal surface at the top of the atmosphere of Uranus (Left) and Neptune (Right)as a function of planetocentric latitude and season, where season is represented by solarlongitude L s . (Bottom) Mean daily actinic flux (W m − per planetary day) at the topof the atmosphere of Uranus (Left) and Neptune (Right) as a function of planetocentriclatitude and season. Note that a molecule being photodissociated does not care whatdirection the photon is coming from, just what the local photon flux is; therefore, theactinic flux, which is the solar flux without accounting for the cosine dependence of thesolar zenith angle, is more relevant to the photochemistry discussion than the insolationat a “surface.” ea r l y A v e r age I n s o l a t i on ( W m - ) Planetocentric Latitude (degrees)UranusNeptune . . . -50 0 50 Figure 2: Annual average solar insolation at Uranus and Neptune as a function of latitude.Unlike the situation on other planets in the solar system, the polar regions of Uranus receivea higher annual average insolation than the equatorial region.
Although several one-dimensional (1D) photochemical models for Uranusand Neptune have been developed in the past (e.g., Atreya and Ponthieu,1983; Romani and Atreya, 1988, 1989; Romani et al., 1993; Summers andStrobel, 1989; Bishop et al., 1990, 1992, 1998; Moses et al., 1992, 1995, 2005;Lellouch et al., 1994; Dobrijevic and Parisot, 1998; Dobrijevic et al., 2010;B´ezard et al., 1999; Schulz et al., 1999; Orton et al., 2014c; Moses and Poppe,2017), all previous models were designed for either global-average conditionsor specific latitudes and times. Here, we present results from a 1D time-variable model that tracks the seasonal variation of photochemically pro-duced hydrocarbons as a function of altitude for different latitudes. Themodels are similar to those of Moses and Greathouse (2005) in that the 1D5odels for the different latitudes are not connected to each other via atmo-spheric circulation or any type of meridional transport, and the temperaturestructure is kept constant with latitude and time. Hue et al. (2015) showthat for Saturn, the expected seasonal variations in stratospheric tempera-tures have only a minor influence on the abundances of the observable hy-drocarbons, except in high-latitude regions during winter, where downwarddiffusion of hydrocarbons is faster as a result of atmospheric compression dueto the lower temperatures. On the other hand, atmospheric dynamics canalter the vertical and meridional distribution of stratospheric constituentsin potentially more significant ways. Comparisons of seasonal 1D photo-chemical models with the observed vertical and meridional distribution ofhydrocarbon abundances can therefore provide useful information on the na-ture and strength of atmospheric dynamics, as well as atmospheric chemistry(see the Jupiter and Saturn studies of Moses and Greathouse, 2005; Lianget al., 2005; Nixon et al., 2007, 2010; Guerlet et al., 2009, 2010; Friedson andMoses, 2012; Sinclair et al., 2013, 2014; Zhang et al., 2013; Sylvestre et al.,2015; Fletcher et al., 2015, 2016; Hue et al., 2015, 2016; Moses et al., 2015).We describe our photochemical models in section 2, present and discussthe results for the seasonal and meridional variations in hydrocarbons onUranus and Neptune in section 3, compare the theoretical predictions withavailable observations in section 4, and discuss implications for future obser-vations and modeling in section 5. 6 . Photochemical model
The coupled set of continuity equations describing the vertical distribu-tion of chemical constituents in the upper atmosphere of Uranus and Neptuneis solved simultaneously by finite-difference methods, using the Caltech/JPL1D KINETICS model (e.g., Allen et al., 1981; Yung et al., 1984). The Uranusmodel contains 181 vertical grid points, ranging from 5.6 bar to 1 × − mbar, with at least three grid points per scale height to ensure accuracy;the background temperature structure is taken from Orton et al. (2014b).The Neptune model contains 198 vertical grid points ranging from 5.0 bar to1 × − mbar, with the background temperature structure taken from Moseset al. (2005), which was originally based on Orton et al. (1992), Roques et al.(1994), and Wang and Yelle (1992). There is no physically meaningful reasonfor the different number of grid points for each planet — the numbers werechosen simply to conform to previous models for these planets (Orton et al.,2014c; Moses and Poppe, 2017). Figure 3 shows the model temperature gridfor both planets. Although the temperature-pressure profile is assumed tobe constant with latitude, the altitude grid changes with latitude due to thevariation of gravity with latitude and radius on a rapidly spinning, oblateplanet (e.g., Lindal et al., 1985). Thirty different latitudes are considered,ranging from − ◦ to 87 ◦ planetocentric latitude, every 6 degrees. The chemical-kinetics inputs to the model are described in Moses et al.(2005, 2015) and references therein, with the complete updated reaction listpresented in Moses and Poppe (2017). A total of ∼
70 hydrocarbon and oxy-7 ranusNeptune
Temperature (K) P r e ss u r e ( m ba r) Figure 3: Temperature profiles for Uranus and Neptune (as labeled) adopted in the model. gen species are considered, interacting through ∼
500 chemical reactions, with113 of these being photolysis reactions. The model includes condensation ofC H , C H , C H , C H , C H , C H , H O, and CO , all of which willcondense on both Uranus and Neptune (for the condensation procedure, seeMoses et al., 2000b). Multiple Rayleigh scattering of the dominant gas-phaseconstituents is included, but aerosol scattering and absorption are not — thestratospheric aerosols are optically thin (Pollack et al., 1987; Moses et al.,1995; Karkoschka and Tomasko, 2009, 2011). The chemical production andloss rates and atmospheric transmission profiles are diurnally averaged, whichis appropriate given that photolysis time scales and chemical time constantsfor the hydrocarbons are longer than a planetary day ( ∼ ∼ vapor makes it past the tropopause cold trap and into thestratosphere, just as water vapor does on the Earth. To streamline the cur-rent calculations, we neglect tropospheric methane condensation and insteadfix the CH volume mixing ratio at the lower boundary of the model to berepresentative of the value observed in the lower stratosphere — 1.6 × − for Uranus (based on Orton et al., 2014c) and 1.2 × − for Neptune (basedon Fletcher et al., 2010; Lellouch et al., 2010, 2015). This assumption leadsto an inaccurate methane profile in the troposphere and therefore inaccu-rate tropospheric hydrocarbon chemistry (which is relatively unimportant atthese deeper altitudes, in any case), so the tropospheric results are ignoredthroughout the paper; only the stratospheric results are discussed. We alsofix the mixing ratios of helium and CO at the lower boundary of the model.On Uranus, we assume the lower-boundary volume mixing ratio to be 15%for helium (Lindal et al., 1987) and 1.0 × − for CO (i.e., well below theupper limit of Teanby and Irwin, 2013). On Neptune, we assume the lower-boundary volume mixing ratio to be 19% for helium (Conrath et al., 1991b)and 8 × − (Luszcz-Cook and de Pater, 2013) for CO. The rest of the hy-drocarbon and oxygen species are assumed to have concentration gradientsof zero at the lower boundary, allowing the species to flow freely from theirsource regions at higher altitudes through the lower boundary at a maximumpossible rate.For the models presented here, we assume that the CH mixing ratioat the base of the stratosphere is constant with latitude. Karkoschka andTomasko (2009, 2011), Sromovsky et al. (2011, 2014), Irwin et al. (2012),9ice et al. (2013), de Kleer et al. (2015), and Luszcz-Cook et al. (2016) pro-vide observational evidence that the methane abundance in the tropospherevaries with latitude on both Uranus and Neptune, with the polar regionsbeing depleted in methane with respect to equatorial regions. The high-latitude depletion in CH is particularly strong for Uranus. As discussedin the above references, these variations most likely arise from large-scalecirculation patterns affecting the methane abundance in its condensationregion in the upper troposphere, with CH enhancements occurring in up-welling regions and depletions in downwelling regions, but the full detailsof this process have yet to be worked out. Moreover, it is currently unclearwhether the latitude dependence extends into the stratosphere or not, or evenhow the methane is injected into the stratosphere in the first place. Doesmethane “leak” out of the troposphere into the stratosphere preferentiallyin regions where the tropopause is warmer and the local saturation vaporpressure higher (e.g., Orton et al., 2007), or is the methane dynamically in-jected into the stratosphere preferentially in regions with strong convectiveuplift (e.g., Karkoschka and Tomasko, 2011; Sromovsky et al., 2014; de Pa-ter et al., 2014)? Given the lack of current information on the stratosphericbehavior of methane as a function of latitude, our constant-with-latitude as-sumption provides a first-order look at the problem that can be altered whenmore data become available, such as with future observations from the JamesWebb Space Telescope (JWST) (Norwood et al., 2016). We do note, however,that when Greathouse et al. (2011) assumed a larger stratospheric methaneabundance at the equator relative to the poles at Neptune, the temperaturepredictions from their radiative seasonal models were in closer agreement to10he observed temperatures, suggesting a latitude dependence of CH in thestratosphere of Neptune, at least, is possible.The oxygen-bearing species in our model are assumed to derive from ex-ternal sources such as comets (Lellouch et al., 2005; Hesman et al., 2007;Luszcz-Cook and de Pater, 2013) or interplanetary dust (Moses 1992, Poppe2016; see also Cavali´e et al. 2014 & Moses and Poppe 2017), with CO havinga possible significant internal source from the deeper troposphere (Fegley andPrinn, 1986; Lodders and Fegley, 1994; Fletcher et al., 2010; Luszcz-Cook andde Pater, 2013; Cavali´e et al., 2017). Other major oxygen species from theinterior, such as H O and CO , will condense at depth in the troposphere,with negligible amounts being transported into the stratosphere. To accountfor the external source of oxygen, we assume that H O, CO, and CO areflowing in at the top of the atmosphere on both planets. The relative fluxesfor these species are taken from Moses and Poppe (2017), who investigatedthe coupled oxygen-hydrocarbon photochemistry on Uranus and Neptune,and constrained the influx rates through comparisons of models with strato-spheric observations of H O (Feuchtgruber et al., 1997, 1999; Lellouch et al.,2010), CO (Lellouch et al., 2005; Hesman et al., 2007; Fletcher et al., 2010;Luszcz-Cook and de Pater, 2013; Teanby and Irwin, 2013; Cavali´e et al.,2014), and CO (Feuchtgruber et al., 1999; Meadows et al., 2008; Ortonet al., 2014c). For Uranus, these influx rates are 1.2 × H O moleculescm − s − , 2.7 × CO molecules cm − s − , and 2.8 × CO moleculescm − s − at the top boundary. For Neptune, the assumed influx rates are2.0 × H O molecules cm − s − , 2.0 × CO molecules cm − s − , and2.3 × CO molecules cm − s − at the top boundary.11ote that our assumption of a constant influx at the top boundary pro-vides a reasonable approximation to the situation in which the oxygen is sup-plied by interplanetary dust particles, but the vertical profiles of the oxygenspecies would be more complicated if the bulk of the oxygen were supplied bya relatively recent large cometary impact, as has been suggested for Neptunein particular (see Lellouch et al., 2005; Hesman et al., 2007; Luszcz-Cook andde Pater, 2013; Moses and Poppe, 2017). We also assume a downward flux ofatomic H at the upper boundary of both planets to account for an additionalthermospheric source. We assume an influx rate of 5 × and 1 × Hatoms cm − s − , respectively, at the top boundary of our Uranus and Nep-tune models based on Moses et al. (2005). All other species are assumed tohave zero-flux boundary conditions at the top of the atmosphere. As is typical in 1D photochemical models, vertical transport is assumed tooperate through eddy and molecular diffusion. The eddy diffusion coefficientprofile is one of the major free parameters for such models. Figure 4 showsthe diffusion coefficients adopted in our models. For Uranus, we assumethe vertical eddy diffusion coefficient K zz = 2430 cm s − , independent ofaltitude, based on Orton et al. (2014c). That assumption places the methanehomopause at the ∼ × − mbar pressure level. For Neptune, we assume K zz = 10 . P . , for P < . . P . , for 28 mbar < P < . , for P >
28 mbar , (1)for K zz in cm s − and P in mbar, which places the methane homopause levelat ∼ × − mbar. These eddy diffusion coefficient profiles were chosen incombination with our adopted lower-stratospheric CH mixing ratio and ourupdated giant-planet chemical reaction list to provide a reasonable fit tothe global-average stratospheric hydrocarbon abundances on both Uranusand Neptune (see Orton et al., 2014c; Moses and Poppe, 2017). For thenominal models presented here, the K zz profile is assumed to be independentof latitude. The assumptions regarding the molecular diffusion coefficientsare described in Moses et al. (2000a). P r e ss u r e ( m ba r) Diffusion coefficient (cm s -1 ) Uranus Neptune
Figure 4: Profiles of the eddy diffusion coefficients (solid lines) and CH molecular diffu-sion coefficients (dot-dashed lines) for Uranus (green) and Neptune (blue) adopted in themodels. For the solar ultraviolet flux, we take the average of the solar-cycle min-13mum and maximum fluxes presented in Woods and Rottman (2002). Al-though the hydrocarbon abundances at high altitudes are sensitive to theinstantaneous solar flux, such that the solar-cycle variations should show upreadily in the mixing ratio profiles in the upper stratosphere (e.g., Mosesand Greathouse, 2005), chemical and transport time scales are much longerin the middle and lower stratosphere where the infrared observations areprobing. Therefore, average solar flux values are sufficient for our purposes.We also include an isotropic source of stellar background UV radiation (takenfrom Mathis et al., 1983) and solar Lyman α photons that are being scat-tered from atomic hydrogen in the local interplanetary medium (LIPM).This LIPM source of photolyzing radiation is increasingly important to thephotochemistry the farther the planet is located from the Sun, as was firstdemonstrated by Strobel et al. (1990) and Bishop et al. (1992). The LIPMsource is needed particularly for the seasonal models presented here, as thescattered Lyman alpha provides a mechanism for CH to be photolyzed evenduring the long polar winter on these planets, when high latitudes do notreceive direct sunlight.The magnitude of this LIPM source depends on both the solar cycleand heliocentric distance. Before the Voyager encounter with Neptune, theUltraviolet Spectrometer (UVS) instrument recorded a background Lymanalpha LIPM intensity of roughly 340 Rayleighs (Broadfoot et al., 1989). Dueto apparent calibration issues with the Voyager measurement (Gangopadhyayet al., 2005), we revise this value downward by a factor of ∼
2, and based onmodeling and observations (Gangopadhyay et al., 2005; Qu´emerais et al.,2009), we assume the intensity at Uranus is a factor of ∼ π (cid:82) I ( τ, µ ) dµ , where I is the intensityincident on a small surface area with cosine of the incidence angle µ atvertical optical depth τ within the atmosphere. From Beer’s law, I ( τ, µ ) = I exp( − τ /µ ), and the actinic flux becomes F ( τ ) = 2 πI (cid:0) e − τ − τ [ − Ei( − τ )] (cid:1) , (2)where Ei is the exponential integral. In general, Lyman alpha photons fromthis LIPM source are absorbed at higher altitudes than the direct solar Ly-man alpha source.Unlike the case on Saturn, the small, faint ring systems of Uranus andNeptune do not cast sufficient shadows on the planets to notably affect thesolar flux received and so are ignored in the calculations. We first keep the season fixed at northern vernal equinox conditions ( L s = 0 ◦ ) and run the 1D models for the different latitudes until steady-statesolutions are achieved for that constant season. Then, we run the time-variable seasonal models with the above vernal equinox results as our initialconditions. The heliocentric distance and relevant solar zenith angle andflux calculations are updated every planetary day during the seasonal modelruns. The planetary orbital positions as a function of time (and L s ) for twofull planetary years are obtained from the JPL Horizons ephemeris calculator(Giorgini et al., 1996). The values for Uranus begin at L s = 0 ◦ in November,1839 and end two planetary years later at L s = 0 ◦ in December, 2007; the15alues for Neptune begin at L s = 0 ◦ in September, 1716 and end two plan-etary years later at L s = 0 ◦ in March, 2046. The full two-year time periodwas chosen originally to avoid any potential discontinuities due to the solarcycle being different at the beginning and end of the most recent planetaryyear when repeating the yearly calculations, but with the choice of solar-cycleaverages, the full two-year sequence is unnecessary. By the same token, theresults for a specific L s for Neptune from this time period are relevant to thesame L s in the more recent era beyond 2007. Because the chemical speciesthat are produced in the upper stratosphere can take thousands of years todiffuse down to the base of the model atmosphere, this two-planetary-yearsequence is run again and again, with the results from the end of the secondyear being fed as initial conditions to the beginning of new two-year calcu-lation, until a repeatable annual result is obtained. The Uranus model wasiterated for a total of 80 Uranus years ( ∼ ∼
3. Results and Discussion
The photochemical product abundances vary with altitude, latitude, andtime on Uranus and Neptune due to seasonal forcing (see full model output inthe Supplementary Material). We begin by discussing the seasonal variationson Neptune, which are reasonably straightforward and qualitatively similarto those derived for Saturn (Moses and Greathouse, 2005; Hue et al., 2015).We then discuss seasonal variations on Uranus, which differ from those on16eptune and Saturn — not only because of the planet’s extreme axial tilt,but because of the very weak vertical mixing on Uranus. The results for bothplanets as a function of latitude are then presented, followed by the columndensities as a function of both latitude and season.17 igure 5: Mixing ratio profiles for key hydrocarbons (as labeled) on Neptune as a functionof season for 3 ◦ N planetocentric latitude (Top), 45 ◦ planetocentric latitude (Middle), and81 ◦ planetocentric latitude (Bottom). Seasons are shown every 30 ◦ in L s . Data pointsare from various published observations (global average or low latitude, acquired during L s = 230–300 ◦ ) (Caldwell et al., 1988; B´ezard et al., 1991; Bishop et al., 1992; Ortonet al., 1992; Kostiuk et al., 1992; Yelle et al., 1993; B´ezard et al., 1998; Schulz et al., 1999;Meadows et al., 2008; Fletcher et al., 2010; Greathouse et al., 2011; Lellouch et al., 2015),and unpublished Infrared Space Observatory hydrocarbon observations (Bruno B´ezard,personal communication, 2001). (continued) . 19ig. 5. (continued) . 20 .1. Seasonal variations in mixing ratios on Neptune The vertical mixing-ratio profiles of several observed (or potentially ob-servable) hydrocarbons on Neptune are shown in Fig. 5 as a function of seasonfor representative low, middle, and high latitudes (top, middle, and bottompanels, respectively). Seasonal variations in the mixing ratios are appar-ent at altitudes above the few-millibar region for most of the photochemicalproducts. Greater seasonal variation is exhibited at high latitudes than lowlatitudes, simply because the seasonal variation in solar insolation is greaterat high latitudes than low latitudes (recall Fig. 1). Note that methane itselfdoes not vary much with either season or latitude because its vertical profileis dominated by eddy and molecular diffusion rather than chemistry; the cu-mulative photochemical products do not rival the abundance of the parentCH molecules, so photodestruction leads to only a very minor change in themixing-ratio profile of methane.For stable species with relatively long chemical lifetimes, such as C H and C H , the seasonal variations are confined to high altitudes and becomeprogressively smaller in magnitude at deeper pressures (see also Fig. 6). Thisbehavior stems from the fact that both the diffusion time scales and chemicaltime constants for these species generally increase with increasing pressure(see Fig. 7, which displays the time constants at − ◦ latitude for the vernalequinox situation at the start of the model run). Chemical time constantsdo vary considerably with both latitude and season, but the general trendof shorter time constants at higher altitudes holds true for all situations.Both the diffusion time scales and chemical lifetimes are less than a Neptuneseason in the region from ∼ µ bar, where the C H and C H pro-21 eliocentric Longitude Ls (degrees) Heliocentric Longitude Ls (degrees) M o l e F r a c t i on M o l e F r a c t i on M o l e F r a c t i on M o l e F r a c t i on M o l e F r a c t i on M o l e F r a c t i on M o l e F r a c t i on M o l e F r a c t i on C H C H C H C H C H C H C H C H Figure 6: Mixing ratios of acetylene (left) and ethane (right) on Neptune as a functionof season (represented by solar longitude L s ) at 1.1 × − mbar (top), 0.01 mbar (sec-ond from top), 0.1 mbar (third from top), and 1 mbar (bottom). Results are shown forfive different latitudes: − ◦ (red), − ◦ (orange), − ◦ (green), − ◦ (blue), and − ◦ (magenta). Seasonal variations are are prominent at lower pressures (higher altitudes) butbecome muted at higher pressures (lower altitudes), and are much greater at high latitudesthan low latitudes. The 81 ◦ S latitude region receives no direct sunlight for an extendedperiod during the winter, leading to a strong minimum during that period. Note alsothat the position of the minimum mixing ratio shifts to later in the year as the pressureincreases in the atmosphere. (For interpretation of the references to color in this figurelegend, the reader is referred to the online version of this article.) duction rates peak in the upper stratosphere. Therefore, at ∼ H and C H abundances respond quickly to changes in insolation. Theincreased insolation during the summer season leads to increased C H andC H production rates and an increased abundance of hydrocarbon photo-chemical products in general (see Figs. 5 & 6). The short vertical diffusion22ime scales ensure that the photochemically produced species are transportedrapidly to lower altitudes. Conversely, lower photochemical production ratesin the winter lead to lower abundances of hydrocarbons at high altitudes, asthe molecules produced in previous seasons have already been transportedaway. Photolysis by direct solar ultraviolet photons shuts off during the longpolar winter, causing a strong dip in the high-altitude C H and C H abun-dances at high latitudes in the winter season (see Figs. 5 & 6), which quicklyrecovers as soon as the region receives direct sunlight again. However, solarLyman α radiation scattered by H atoms in the local interplanetary mediumcontinues to contribute to CH photolysis even in the polar night, preventinga much stronger depletion in hydrocarbon abundances.At 0.01 mbar, the chemical lifetime of C H approaches a Neptune seasonand that of C H exceeds a Neptune year (Fig. 7). The C H and C H mix-ing ratios in this pressure region are strongly influenced by molecules beingtransported into the region from higher altitudes, albeit with a time delay,which introduces a phase lag into the seasonal behavior. Seasonal variationsare still apparent, but become more muted, and the minimum and maximumin the mixing ratios are shifted away from the solstices toward later timesin the winter and summer seasons. Those phase lags continue to grow withdepth, and the seasonal variations become reduced in magnitude, until al-most no seasonal variations remain apparent in the few mbar region. At thatpoint, the yearly averaged solar insolation controls the overall abundances,rather than the seasonally variable insolation. Other relatively chemicallystable, long-lived species such as C H and C H exhibit similar behavior asC H and C H , with seasonal variations confined to higher altitudes, and23 igure 7: Time constants for photochemical loss (Left) and net photochemical lifetime(Right) above the condensation region on Neptune for C H (red solid line) and C H (blue dashed line), along with the diffusion time constant for C H (green dot-dashed line,nearly identical to that for C H ), compared to a Neptune year (vertical black triple-dot-dashed line) and a Neptune season (orange dotted line) for − ◦ latitude, vernal equinox.The photochemical loss time scale is defined as the species concentration divided by thechemical loss rate; the net photochemical time scale is defined as the species concentrationdivided by the absolute magnitude of the chemical production rate minus the loss rate forthe species. The latter gives a better measure of the species stability in the presence ofefficient recycling processes; the sharp peaks occur where production and loss rates arenearly equal. The diffusion time scale is defined as the square of the generalized scaleheight divided by the generalized diffusion coefficient (see Moses and Greathouse, 2005).Note that chemical and diffusion time scales are shorter than a Neptune season in thepeak production region from 1 to a few µ bar but the net chemical lifetimes become longerthan a Neptune season in the middle and lower stratosphere. (For interpretation of thereferences to color in this figure legend, the reader is referred to the online version of thisarticle.) phase lags introduced in the ∼ H , CH C H, C H , and CH that haveshorter chemical lifetimes at depth continue to experience seasonal varia-24ions at all altitudes above their condensation regions (i.e., for the first threeaforementioned species, which condense). The seasonal behavior of thesespecies can be complicated, as both diffusion and in situ photochemistry af-fect the behavior, photodestruction of other seasonally variable species suchas C H and C H contribute to their production, and photochemically pro-duced radicals such as atomic H affect both production and destruction (seeMoses et al., 2000a, 2005, 2015, for details). This more complicated chem-istry (as well as uncertainties in the thermal structure for the observationalabundance determinations) contributes to the apparent disagreements be-tween the CH C H and C H abundances predicted by the models and in-ferred from observations. The model-data mismatch for these species is notof great concern here, because each chemical rate coefficient in the model issubject to uncertainties, which when combined, lead to factors of a few un-certainties in the predicted abundances (e.g., Dobrijevic et al., 2003, 2010).Accounting for these uncertainties in our seasonal models is beyond the scopeof this paper.Full results for the abundances of all species in the model as a functionof latitude and season can be found in the Supplementary Material for thisarticle. The mixing-ratio profiles for several observed (or potentially observable)hydrocarbons on Uranus are shown in Fig. 8 as a function of season forrepresentative low, middle, and high latitudes. The first thing to note fromFig. 8 is that seasonal variations in hydrocarbon abundances are minor onUranus compared to Neptune. Small changes in the C H and C H mixing25atio with season are predicted in the ∼ ∼ × − mbar on Uranus — a pressure roughly threeorders of magnitude greater than the homopause level on Neptune — andso the CH is not carried as high up in the stratosphere on Uranus as it ison Neptune. Both C H and C H have their peak production region near ∼ H production rate exceeds its loss rate, andbecause more C H is produced in the summer season and less in the winter,the C H mixing ratio at 0.1 mbar is greatest in the summer-to-fall timeframe and least in the winter-to-spring, with the phase lag due to the longdiffusion and chemistry time constants being apparent in Fig. 9. In contrast,the loss rate of C H from photolysis exceeds the production rate at altitudesabove the 0.2-mbar level, so more of the C H that is being carried upwardfrom its peak production region is destroyed in the summer than the winter,and the mixing ratio of C H is greatest in the winter/spring and smallest inthe summer/fall. At pressures of 1 mbar and greater, diffusion and chemicaltime constants are so long that no seasonal changes are expected for C H and C H at these pressures. 26 igure 8: Mixing ratio profiles for key hydrocarbons (as labeled) on Uranus as a functionof season for 3 ◦ N planetocentric latitude (Top), 45 ◦ planetocentric latitude (Middle), and81 ◦ planetocentric latitude (Bottom). Seasons are shown every 30 ◦ in L s . Data pointsare from various published observations (Encrenaz et al., 1986, 1998; Orton et al., 1987;Herbert et al., 1987; Caldwell et al., 1988; Yelle et al., 1989; Bishop et al., 1990; B´ezardet al., 1999; Orton et al., 2014c). (continued) .Even the species with shorter chemical lifetimes, such as C H and CH C Hexperience relatively little seasonal variation in comparison with Neptune, be-cause of the long diffusion time scales and lack of presence of these moleculesat high altitudes where time scales are typically shorter. We would thereforenot expect to observe much in the way of seasonal variations in hydrocarbonabundances on Uranus unless vertical transport — or stratospheric circula-tion in general — changes with season. One exception is C H , whose mixingratio varies by as much as an order of magnitude in the ∼ eliocentric Longitude Ls (degrees)C H C H M o l e F r a c t i on M o l e F r a c t i on M o l e F r a c t i on C H C H . m ba r . m ba r o S27 o S45 o S63 o S81 o S C H C H . m ba r . m ba r o S27 o S45 o S63 o S81 o S Heliocentric Longitude Ls (degrees) M o l e F r a c t i on M o l e F r a c t i on M o l e F r a c t i on Figure 9: Mixing ratios of acetylene (left) and ethane (right) on Uranus as a function ofseason (represented by solar longitude L s ) at 0.1 mbar (top), 0.2 mbar (middle), and 1mbar (bottom). Results are shown for five different latitudes: − ◦ (red), − ◦ (orange), − ◦ (green), − ◦ (blue), and − ◦ (magenta). Seasonal variations are muted in generalon Uranus but are more pronounced at lower pressures (higher altitudes). (For interpre-tation of the references to color in this figure legend, the reader is referred to the onlineversion of this article.) Note, however, that if stratospheric temperatures vary with season, thecolumn abundance of all the condensable hydrocarbons will be greater at lat-itudes and seasons where the lower-stratospheric temperatures are warmer,because the available region over which the molecules can remain in thevapor phase expands to deeper pressures (i.e., the condensation region be-comes more narrowly confined to the coldest pressure levels surrounding thetropopause). On Uranus, where the stratosphere is colder and the columndensity of hydrocarbons is so low anyway due to sluggish atmospheric mix-ing, changes in the pressure level of condensation due to seasonal temperaturechanges (e.g., Conrath et al., 1990) may have more of an effect on observed29ydrocarbon abundances than changes due to seasonal photochemistry itself.This effect applies to all hydrocarbons that can condense in the lower strato-sphere, including C H and C H , and would be observable if remote-sensingobservations probe deep enough to sense the condensation region near the ∼ Figure 10: Time constants for photochemical loss (Left) and net photochemical lifetime(Right) above the condensation region on Uranus for C H (red solid line) and C H (bluedashed line), along with the diffusion time constant for C H (green dot-dashed line, nearlyidentical to that for C H ), compared to a Uranus year (vertical black triple-dot-dashedline) and a Uranus season (orange dotted line) for − ◦ latitude, vernal equinox. (Forinterpretation of the references to color in this figure legend, the reader is referred to theonline version of this article.) As with seasonal variations, latitude variations in mixing ratios are morepronounced at higher altitudes. Fig. 11 illustrates how the C H , C H ,CH C H, and C H mixing ratios on Neptune vary as a function of latitudeat four specific seasons: L s = 0 ◦ (dotted orange curves), 90 ◦ (solid bluecurves), 180 ◦ (dashed green curves), and 270 ◦ (solid red curves). At high30ltitudes (e.g., pressures less than ∼ H and C H become moresymmetric about the equator, with the annual average actinic flux dominat-ing the latitude variations (e.g., recall Fig. 2). On Neptune, the variations inannual average solar irradiation with latitude lead to a greater 1-mbar abun-dance of C H , C H , and CH C H at low latitudes in comparison with highlatitudes, while species with short chemical lifetimes, such as C H , continueto exhibit hemispheric asymmetries at pressures of 1 mbar (see Fig. 11).Note also for the shorter-lived species that the increasing phase lag withdepth can lead to hemispheric asymmetries at 1 mbar that are completelyout of phase with those at 0.1 mbar. This effect is important to keep inmind when considering observations that are more sensitive to deeper levels— the abundances are out of phase at these depths in comparison to theinstantaneous seasonal forcing, and the vertical profiles change significantlywith season and latitude, which is important to keep in mind when choosingpriors for retrievals.The sluggish vertical mixing and large axial tilt on Uranus result in no-table differences in latitude variations on Uranus (see Fig. 12), in compar-ison to Neptune. The longer vertical diffusion time scales and shorter yearon Uranus result in seasonal variations being damped at 1 mbar on Uranus,and even species with relatively short photochemical lifetimes, like C H and31 igure 11: Mixing ratios of ethane and acetylene (Left) and methylacetylene and diacety-lene (Right) at 0.1 mbar (Top) and 1 mbar (bottom) on Neptune for four different seasons: L s = 0 ◦ (dotted orange), 90 ◦ (solid blue), 180 ◦ (dashed green), and 270 ◦ (solid red). (Forinterpretation of the references to color in this figure legend, the reader is referred to theonline version of this article.) C H , are controlled by the annual average actinic flux at these pressures.Because the annual average daily insolation is greater at the poles than theequator on the highly tilted Uranus (Fig. 2), most photochemically producedspecies at 1 mbar have a larger mixing ratio at high latitudes than low lat-itudes. Ethane is an exception. The low-altitude homopause on Uranusresults in different dominant hydrocarbon reactions in the region in whichmethane is photolyzed than on the other giant planets. Photolysis reactionscontrol both the production and loss of ethane in the homopause region,32hich act to counteract each other, and there are no effective in situ chem-ical loss processes for C H at 1 mbar. The net column production rate forC H therefore does not vary much as a function of latitude. The C H thatis produced near the homopause region diffuses downward, and because wehave assumed that the eddy diffusion coefficient profile does not vary withlatitude, the ethane abundance at depth on Uranus is relatively constantwith latitude. Figure 12: Same as Fig. 11, except for Uranus.
At higher altitudes on Uranus (e.g., 0.1 mbar in Fig. 12), the photochem-ically produced species exhibit hemispheric dichotomies, with most speciesbeing more abundant in the summer-to-fall hemisphere than the winter-to-spring hemisphere, as on Neptune. However, C H is again an exception.33t the CH homopause on Uranus, CH is less abundant than on the othergiant planets and is less effective at shielding the C H from photolysis.Seasonal variations in solar irradiation therefore lead to a reduction in thehigh-altitude C H abundance in the summer/fall hemisphere in comparisonwith the winter/spring hemisphere.Keep in mind that the models presented in this paper do not considermeridional circulation or differences in vertical transport or methane abun-dance with latitude. Such processes could end up controlling latitude varia-tions in photochemically produced species abundances on the giant planets,particularly on Uranus, where chemical and diffusion time constants are longand latitudinal/seasonal variations are predicted to be weak. Because the hydrocarbon mixing ratios change significantly with altitudeand the because the observed emission is proportional to the column abun-dance of the molecules when the emission lines are optically thin, it is conve-nient to consider the column abundance above various pressure regions whenpredicting observable changes as a function of latitude and time. Figs. 13–17show how the column abundances of various hydrocarbons above differentpressure levels vary as a function of latitude and season on Neptune.34 H (at 0.01 mbar)C H (at 0.1 mbar)C H (at 1 mbar) La t i t ude ( deg r ee s ) - La t i t ude ( deg r ee s ) - La t i t ude ( deg r ee s ) - L s (degrees) column density (10 cm -2 )0 50 100 150 200 250 300 350 L s (degrees) column density (10 cm -2 )0 50 100 150 200 250 300 350 L s (degrees) column density (10 cm -2 ) Figure 13: The column abundance of ethane on Neptune above 0.01 mbar (Top), 0.1mbar (Middle), and 1 mbar (Bottom), as a function of planetocentric latitude and season( L s ). The dashed vertical line represents the time of the Voyager encounter, and the solidvertical line represents 7 September 2018, the date of the next Neptune opposition. s (degrees) L s (degrees)L s (degrees)C H (at 0.01 mbar)C H (at 0.1 mbar)C H (at 1 mbar) La t i t ude ( deg r ee s ) - La t i t ude ( deg r ee s ) - La t i t ude ( deg r ee s ) - column density (10 cm -2 ) column density (10 cm -2 ) column density (10 cm -2 ) Figure 14: Same as Fig. 13, except for acetylene. H (at 0.01 mbar)C H (at 0.1 mbar)C H (at 1 mbar) La t i t ude ( deg r ee s ) - La t i t ude ( deg r ee s ) - La t i t ude ( deg r ee s ) - column density (10 cm -2 )column density (10 cm -2 )0 50 100 150 200 250 300 350 L s (degrees) column density (10 cm -2 )0 50 100 150 200 250 300 350 L s (degrees) L s (degrees) Figure 15: Same as Fig. 13, except for ethylene. H (at 0.01 mbar)C H (at 0.1 mbar)C H (at 1 mbar) La t i t ude ( deg r ee s ) - La t i t ude ( deg r ee s ) - La t i t ude ( deg r ee s ) - column density (10 cm -2 )column density (10 cm -2 )0 50 100 150 200 250 300 350 L s (degrees) column density (10 cm -2 )0 50 100 150 200 250 300 350 L s (degrees) L s (degrees) Figure 16: Same as Fig. 13, except for methylacetylene. H (at 0.01 mbar)C H (at 0.1 mbar)C H (at 1 mbar) La t i t ude ( deg r ee s ) - La t i t ude ( deg r ee s ) - La t i t ude ( deg r ee s ) - column density (10 cm -2 )column density (10 cm -2 )0 50 100 150 200 250 300 350 L s (degrees) column density (10 cm -2 )0 50 100 150 200 250 300 350 L s (degrees) L s (degrees) Figure 17: Same as Fig. 13, except for diacetylene.
39s was discussed in section 3.1, the abundances of most of the hydro-carbon photochemical products closely track the seasonal variation of solarinsolation at high altitudes, causing the hydrocarbon column densities above0.01 mbar to be much greater in the summer hemisphere in comparison tothe winter hemisphere. However, chemical and diffusion time scales increasewith depth in the atmosphere, leading to increasing phase lags in the abun-dance maxima and minima with depth at any particular latitude, along withmeridional profiles of column abundance that become more symmetric aboutthe equator at greater depths. These changes are readily apparent for C H (Fig. 13), C H (Fig. 14), and CH C H (Fig. 16).Similar trends are seen for C H (Fig. 15) and C H (Fig. 17), althoughthese two species also exhibit some additional interesting behavior. Note fromFig. 15 that the C H column abundance above 1 mbar begins to increasestrongly during the high-latitude winter months. This behavior results fromthe fact that photolysis is normally an effective loss process for C H inthe 0.1-10 mbar region, but direct solar UV photolysis is absent during thewinter polar night. Although the stellar background UV source and scatteredLIPM Lyman alpha photons are still available at that time, the overall C H photolysis rate is greatly diminished, and the local production rate of C H exceeds its loss rate. The dominant production of C H at this time occursthrough C H + H + M → C H + M, followed by C H + H → C H + H, causing a net conversion of C H into C H during the long winterpolar nights at ∼ H from higher altitudes intothis region also contributes to the increase. For C H , Fig. 17 illustratesthat high-altitude production of C H occurs most readily during the long40igh-latitude summers, when the sun never sets, with much less productionat other latitudes/times. The seasonal and meridional behavior at 1 mbaris more complicated and reflects the diffusion source from higher altitudes,with its associated phase lag, as well as in situ production and loss. Oneinteresting feature that shows up in the C H column density plot at 1 mbaris a brief local maximum as the sunlight returns to the high-latitude regionsin the late winter or spring. Figure 18 illustrates how the column abundances of several hydrocarbonphotochemical products vary with latitude and season on Uranus. Becausethe hydrocarbons are vertically confined to deeper stratospheric levels dueto the weak atmospheric mixing on Uranus, we do not show the results formultiple pressures, but instead plot the column abundance above 0.25 mbar,which is near the peak of the contribution function for most of the observedhydrocarbon bands in the
Spitzer /IRS infrared observations (Orton et al.,2014b,c).The column densities of C H , CH C H, and C H exhibit maxima in thesummer-to-fall hemispheres and minima in the winter-to-spring hemispheres,with strong hemispheric dichotomies apparent during most seasons. As withNeptune, this result is caused by the greater photochemical production ratesduring periods and locations with higher mean daily solar insolation, com-bined with a phase lag due to the diffusion of hydrocarbons from higheraltitudes. Unlike Neptune, however, these species do not have a maximumcolumn abundance near the equator when averaged over a year, as can beseen from the generally low column abundances in the equatorial region.41 H (at 0.25 mbar) column density (10 cm -2 ) - La t i t ude ( deg r ee s ) L s (degrees) C H (at 0.25 mbar) - La t i t ude ( deg r ee s ) L s (degrees) cm -2 ) CH C H (at 0.25 mbar) - La t i t ude ( deg r ee s ) L s (degrees) cm -2 ) C H (at 0.25 mbar) - La t i t ude ( deg r ee s ) L s (degrees) cm -2 ) Figure 18: The column abundance of acetylene (Top left), ethane (Top right), methy-lacetylene (Bottom left), and diacetylene (Bottom right) on Uranus above 0.25 mbar, asa function of planetocentric latitude and season ( L s ). The dashed vertical line representsthe time of the Voyager encounter, and the solid vertical line represents the next Uranusopposition on 24 October 2018.
The abundance of diacetylene is particularly sensitive to solar irradiation.Because the C H chemical lifetime is short, the diacetylene column densitydrops precipitously in the winter polar night once the sun drops below thehorizon, as is obvious in Fig. 18.As discussed in Section 3.3, C H is an exception to the aforementioned42rends in the hydrocarbon distributions with latitude and season. At 0.25mbar and lower pressures, photolysis effectively destroys C H , so it survivesat these pressures more readily at low latitudes and in the winter-to-springhemispheres, where the mean daily solar insolation is the lowest.
4. Comparisons with observations
Low signal-to-noise ratios prevented mapping of the hydrocarbon infraredemission features during the
Voyager flybys of Uranus and Neptune, so nomeasurements of photochemical product abundances as a function of lati-tude were ever reported from the only spacecraft to ever encounter theseplanets. The large heliocentric distances and cold atmospheric temperaturesof Uranus and Neptune also pose challenges for obtaining spatially resolvedobservations of hydrocarbon photochemical products from Earth-based tele-scopes. Hammel et al. (2007) and Orton et al. (2007) were the first to showspatially resolved images of C H emission on Neptune, and Greathouse et al.(2011) were the first to present meridional distributions of C H and C H .Greathouse et al. (2011) used the TEXES spectrograph at the Gemini North8-m telescope in October 2007 (Neptune L s = 275.4 ◦ ) to obtain high-spectral-resolution, spatially resolved observations of emission from H , CH , C H ,and C H at thermal-infrared wavelengths. Emission from the S(1) rota-tional line of H and the ν band of CH were used to constrain stratospherictemperature fields, which then allowed Greathouse et al. (2011) to retrieveethane and acetylene mixing ratios as a function of pressure and latitude.Their results at the peak of the contribution functions from the strong mid-IR C H and C H lines are shown in Fig. 19, in comparison with our results43t L s = 280 ◦ for the same pressure levels. NeptunePlanetocentric latitude (degrees) M o l e F r a c t i on Figure 19: Model predictions for the meridional variation of the ethane mixing ratio at1 mbar (red curve) and the acetylene mixing ratio at 0.45 mbar (blue curve) at L s =280 ◦ , in comparison with the retrievals of Greathouse et al. (2011) from Gemini/TEXESobservations at L s = 275.4 ◦ . Our seasonal photochemical model provides an excellent fit to the Greathouseet al. (2011) retrieved meridional distribution of C H at 1 mbar, with bothmodel and data exhibiting a peak mixing ratio at the equator that then dropsoff gradually toward higher latitudes in both hemispheres. For acetylene, theobservations indicate a slight maximum in the C H mixing ratio at − ◦ lat-itude, with mixing ratios dropping gradually toward both lower and higherlatitudes. This C H meridional behavior is not predicted by the models,44hich instead exhibit an equatorial maximum mixing ratio with more-or-less symmetric behavior across the equator at this season (Fig. 19), and acolumn abundance above 1 mbar that peaks at − ◦ latitude (see Fig. 14).However, the C H meridional distribution from the model, as well as anad hoc constant-with-latitude distribution, are both consistent with the re-trievals to within observational uncertainties. This model-data consistencyfor Neptune hydrocarbon distributions is in sharp contrast to similar com-parisons for Jupiter (Liang et al., 2005; Nixon et al., 2007, 2010; Zhang et al.,2013; Fletcher et al., 2016) and Saturn (Moses and Greathouse, 2005; Guer-let et al., 2009, 2010; Friedson and Moses, 2012; Sinclair et al., 2013, 2014;Sylvestre et al., 2015; Hue et al., 2015), for which stratospheric circulationand/or other meridional/vertical transport processes have been suggested asinfluencing the large-scale meridional hydrocarbon distributions. The goodmodel-data comparisons here for Neptune suggest that either stratosphericwinds are weaker and have less of an effect on hydrocarbon distributions onNeptune, or that degeneracies in retrievals and/or uncertainties in modelsand their related assumptions are resulting in a fortuitous coincidence.Spatially resolved observations of C H on Neptune from two differentmid-infrared observational data sets (Keck/LWS spectra from 2003 and Gemini-S/TReCS spectra from August 2007) were also presented by Fletcher et al.(2014). A factor of ∼ H mixing ratio between the two data sets highlights the difficulties inherent incalibrating mid-infrared ground-based spectra and suggests that degeneraciesbetween temperatures and abundances are a perennial issue for the retrievals,particularly when dealing with low-resolution spectra. Fletcher et al. (2014)45lso point to poor weather conditions and the lack of a Cohen standard starfor calibration of the 2007 Gemini-S spectra. The Fletcher et al. retrievals ofthe Keck data from 2003 exhibit a maximum in the 1-mbar C H abundanceat the equator, declining gradually toward higher latitudes — consistentwith both our photochemical models and the retrievals of Greathouse et al.(2011). The Fletcher et al. retrievals from the Gemini-S data from 2007have a higher overall C H mixing ratio and fine-scale meridional structure,including a local minimum at the equator and the south pole, that are notconsistent the photochemical models or with the retrievals of Greathouseet al. (2011). Fletcher et al. (2014) suggest that differences between the 2003and 2007 data sets are caused by either poor weather for the 2007 observa-tions, calibration issues, or by changes in lower-stratospheric temperaturesthat are not sampled reliably in retrievals of low-resolution spectra.From the time of the Voyager encounter with Neptune in 1989 to the2007 observations of Greathouse et al. (2011) and Fletcher et al. (2014),the southern hemisphere of Neptune had swung more prominently into viewfrom the Earth, and our models suggest that the overall column abundanceof C H above 1 mbar would have been increasing over that time period inthe southern hemisphere. This trend is superficially consistent with the re-port of Hammel et al. (2006) of an increase in global-average ethane emissionover the time period from 1985 to 2003. On the other hand, Hammel et al.(2006) observe a decrease in emission from 2003 to 2004 that is not consis-tent with the photochemical models, and Fletcher et al. (2014) argue for apossible slight decrease in the global-average C H mixing ratio from 2003 tomid-2007, with a sharper increase in late 2007, none of which are predicted46y the models. The hydrocarbon emission depends on both atmospherictemperatures and abundances, and it remains to be seen whether the pho-tochemical model predictions are consistent with the observed time-variabletrends and/or whether other factors such as adiabatic heating/cooling andvariations in abundances due to transport effects are in play.No retrievals of hydrocarbon abundances as a function of latitude haveever been published for Uranus. The stratosphere of Uranus is colder thanthat of Neptune, and detecting any hydrocarbon emission other than fromCH and C H from Earth-based telescopes has typically been difficult. The Voyager encounter with Uranus in 1986 occurred near southern summer sol-stice. Analyses of the ultraviolet solar occultation during the
Voyager en-counter yielded C H and C H vertical profiles near the equatorial region(Herbert et al., 1987; Bishop et al., 1990). The peak C H mixing ratio de-rived from this UV occultation is less than that derived from global-average Infrared Space Observatory (ISO) observations obtained in 1996 (Encrenazet al., 1998). Because the southern hemisphere of Uranus was still domi-nating the whole-disk observations in 1996, our photochemical model predic-tions for enhanced southern hemispheric C H during this time period arequalitatively consistent with the greater C H abundance seen by Encrenazet al. (1998), in comparison with the equatorial region during the Voyager encounter (see Fig. 18).Later
Spitzer /IRS observations of Uranus acquired near the 2007 equinoxsuggest a very slight decrease in the C H abundance in comparison with theEncrenaz et al. (1998) ISO observations. This result, too, appears qualita-tively consistent with the models, as the emission from the predicted greater47bundance of C H in the southern hemisphere would still be dominatingthe whole-disk observations at the equinox (Fig. 18), and the hemispheric-averaged C H column abundance decreased between 1998 and 2007 in themodel. The models predict a continued decrease in the C H abundance inthe southern hemisphere over the next few years, accompanied by an increasein the northern hemisphere abundance.
5. Implications for future observations and modeling
Large ground-based telescopes are currently capable of spatially resolvingUranus and Neptune at mid-infrared wavelengths (e.g., Greathouse et al.,2011; Fletcher et al., 2014; Orton et al., 2014a), and our models providehypotheses for the distribution of hydrocarbons that can be tested with suchfuture observations. In addition, the
James Webb Space Telescope (JWST),which is due to be operational shortly (currently delayed launch date March-to-June, 2019), will be able to provide spatially-resolved spectroscopic mapsof Uranus and Neptune. Of particular interest onboard JWST is the MediumResolution Spectrometer (MRS) of the MIRI instrument (Rieke et al., 2015),an Integral Field Unit (IFU) with the capability to provide spatially-resolved5-28 µ m spectroscopy across the planetary disks (Norwood et al., 2016).The resulting spectral maps will provide key observational tests of the two-dimensional hydrocarbon distributions predicted by our models.We use the time-, latitude- and altitude-dependent hydrocarbon profilesfrom our models to simulate the expected spectral radiance and brightnesstemperatures that could be observed by the MIRI instrument. Our assumedspatially-uniform stratospheric temperatures were combined with the two-48imensional (latitude, altitude) tropospheric temperatures as measured byVoyager-2/IRIS (Orton et al., 2015; Fletcher et al., 2014) to provide a realisticestimate of the temperature distribution, although we note that stratospherictemperature contrasts, if present, would significantly alter our results. Weemploy the NEMESIS optimal estimation retrieval algorithm (Irwin et al.,2008) in forward-modelling mode, simulating the top-of-atmosphere spectralradiance for all pixels on the observable disk, accounting for the latitude,longitude, and emission angle of the individual pixel element. The spatialorientation and size of each disk was calculated for their 2018 oppositions(3.7” for Uranus on October 24th; 2.4” for Neptune on September 7th). Thelaunch delay to 2019 will not have a significant effect on these simulations,given the long seasonal timescales of both worlds. However, JWST is onlycapable of observing during a limited range of solar elongations, such thatthe planets will be observed closer to quadrature with a slightly degradedspatial resolution (angular diameter 3.6” for Uranus, 2.3” for Neptune).The MIRI forward-model simulations include both collision-induced con-tinuum emission from H -H , H -He and H -CH , as well as emission andabsorption from the gaseous species in our photochemical model. Sources ofspectral line data are described in Fletcher et al. (2014) and were used togenerate k -distributions (absorption coefficients ranked in order of strengthon a wavelength, temperature, and pressure grid) for use in NEMESIS. These k -distributions were generated for CH and its isotopologues, C H , C H ,C H , C H , C H , C H , CO and C H . The tables covered each of the 12sub-bands (i.e., four IFUs with three subbands each) of MIRI between 5-28 µ m, using the correct variation of the spectral resolution ( R ∼ − µ m to1.10” at 28.45 µ m), respectively, and as the MIRI consortium is implement-ing dithering techniques to optimize sampling of the target field, we electedto base our simulation on the 0.11”/pixel plate scale of the MIRI imager,combined with the diffraction-limited performance of the 6.5-m mirror. Thisplate scale significantly oversamples the FWHM of JWST’s primary mirrorand is smaller than the best plate scale of the MIRI IFU, but it is repre-sentative of the expected noise-free quality with an optimal use of telescopedithering.Disk images at every wavelength were convolved with a Gaussian, repre-senting the diffraction-limited spatial resolution of a 6.5-m primary mirror,and then averaged spectrally to highlight interesting hydrocarbon features.Finally, a brightness-temperature cross section was extracted along the cen-tral meridian (to show meridional variations) and along a latitude circle atthe sub-Earth latitude (to show the variation of brightness temperature withemission angle). 50
20 40 60 80 Y P i x e l X PixelDisk Image: 18.6 µ m Disk Image: 12.2 µ m Disk Image: 7.7 µ m X Pixel Meridional: 18.6 µ m B r i gh t ne ss T e m pe r a t u r e ( K ) -90 -60 -30 0 30 60 90 Latitude
Meridional: 7.7 µ m -90 -60 -30 0 30 60 90 Latitude B r i gh t ne ss T e m pe r a t u r e ( K ) Meridional: 12.2 µ m -90 -60 -30 0 30 60 90 Latitude B r i gh t ne ss T e m pe r a t u r e ( K ) Meridional: 13.6 µ m -90 -60 -30 0 30 60 90 Latitude B r i gh t ne ss T e m pe r a t u r e ( K ) B r i gh t ne ss T e m pe r a t u r e ( K ) Emission Angle: 18.6 µ m Emission Angle
Emission Angle: 7.7 µ m Emission Angle B r i gh t ne ss T e m pe r a t u r e ( K ) Emission Angle
Emission Angle: 12.2 µ m B r i gh t ne ss T e m pe r a t u r e ( K ) Emission Angle
Emission Angle: 13.6 µ m B r i gh t ne ss T e m pe r a t u r e ( K ) X Pixel -050100150200250
Disk Image: 13.6 µ m X Pixel
Figure 20: Synthetic images of Neptune (Top row) relevant to the next opposition on 7September 2018, simulating what the JWST MIRI instrument would be able to see if itwere operating at that time: (Left) in the tropospheric continuum at 18.6 µ m, (Secondfrom left) in the methane emission band at 7.7 µ m, (Second from right) in ethane emissionat 12.2 µ m, and (Right) in acetylene emission at 13.6 µ m. The dashed line marks thecentral meridian, and the black dot is at the position of the south pole. Brightness-temperature cross sections in the same four wavelength regions were also extracted (Middlerow) along the central meridian to highlight latitude variations and (Bottom row) along alatitude circle at the sub-Earth latitude to highlight variations in brightness temperaturewith emission angle.
20 40 60 80 Y P i x e l X PixelDisk Image: 18.6 µ m Disk Image: 12.2 µ m Disk Image: 7.7 µ m X PixelMeridional: 18.6 µ m B r i gh t ne ss T e m pe r a t u r e ( K ) -90 -60 -30 0 30 60 90 Latitude
Meridional: 7.7 µ m -90 -60 -30 0 30 60 90 Latitude B r i gh t ne ss T e m pe r a t u r e ( K ) Meridional: 12.2 µ m -90 -60 -30 0 30 60 90 Latitude B r i gh t ne ss T e m pe r a t u r e ( K ) Meridional: 13.6 µ m -90 -60 -30 0 30 60 90 Latitude B r i gh t ne ss T e m pe r a t u r e ( K ) B r i gh t ne ss T e m pe r a t u r e ( K ) Emission Angle: 18.6 µ m Emission Angle
Emission Angle: 7.7 µ m Emission Angle B r i gh t ne ss T e m pe r a t u r e ( K ) Emission Angle
Emission Angle: 12.2 µ m B r i gh t ne ss T e m pe r a t u r e ( K ) Emission Angle
Emission Angle: 13.6 µ m B r i gh t ne ss T e m pe r a t u r e ( K ) X Pixel
Disk Image: 13.6 µ m X Pixel
54 86 69 7553 88 69 74
53 8887 6867 747352 8786 6867 73
Figure 21: Same as Figure 20, except for Uranus at the next opposition on 24 October2018, and the black dot is at the north pole. µ m. The assumed Voyager-era thermal structure (Conrath et al., 1998; Fletcher et al., 2014), with broadmid-latitude temperature minima sandwiched between warmer equatorialand high-latitude regions — which was assumed for these synthetic imagesbut was not considered in the photochemical models — shows up readily inthe 18.6 µ m images. For a lack of information to the contrary, we have as-sumed a uniform stratospheric thermal structure and spatially uniform eddydiffusion coefficients in both the photochemical model and synthetic images,which result in roughly uniform methane vertical profiles across the planet.The images in methane emission at 7.7 µ m are therefore relatively bland,with limb brightening dominating the observed emission. If any latitudi-nally variable stratospheric temperatures are actually present on Neptune,these should show up readily in all the hydrocarbon emission bands. Limbbrightening is also apparent in the ethane emission at 12.2 µ m and acety-lene emission at 13.6 µ m, but the meridional emission cross sections alsoshow evidence for the predicted compositional gradients. For example, the1-mbar low-latitude column-density maximum in C H during this season(see Fig. 13) enhances the low-latitude emission at 12.2 µ m, while the strongenhancement in the stratospheric column abundance on C H at high south-ern latitudes is readily apparent in the 13.6 µ m images.Figure 21 shows the simulated MIRI images for Uranus. As with Nep-tune, the Voyager-derived tropospheric thermal structure shows a temper-ature minimum at mid-latitudes (Flasar et al., 1987; Conrath et al., 1998;Orton et al., 2015), which shows up readily in the 18.6 µ m images. The as-53umed uniform stratospheric temperature fields lead to rather bland imagesin the stratospheric emission features, with limb brightening dominating theemission (and note that the brightest emission is not actually right at thelimb itself, due to an artifact of the point-spread function of the observatory,convolving dark space with the bright emission from the limb). The compo-sitional gradients are less pronounced on Uranus than on Neptune, so thereis in general less structure apparent in the Uranus images at 12.2 and 13.6 µ m. Our models predict a greater column abundance of C H at low lati-tudes than high latitudes during this season (see Fig. 18), whereas the C H column abundance is more uniform with latitude, and hints of this behaviorappear in the 12.2 and 13.6 µ m cross sections, respectively, although anythermal contrasts in the stratosphere as a function of latitude would likelyobscure these differences in the real images.It should be kept in mind that our seasonal photochemical models do notaccount for changes in stratospheric temperatures with latitude or season,for possible variations in the stratospheric methane profile with latitude orseason, or for variations due to the advection of species. Such effects couldpotentially have a major influence on the hydrocarbon emission profiles andvertical/meridional distributions, with meridional temperature gradients inparticular having a likely large effect on the stratospheric emission featuresseen with MIRI. Future models should add complexity in stages, such as cou-pling the photochemical model with a radiative model that can accuratelypredict seasonally variable temperatures, incorporating possible meridionalchanges in the methane profile, and testing 2D behavior with different zonallyaverage circulation scenarios. Eventually, incorporating simple C H x chem-54stry and realistic hydrocarbon-based radiative transfer into general circula-tion models would be the ultimate end goal for predictions of the distributionof hydrocarbons as a function of latitude and season on the giant planets.
6. Conclusions
Time-variable photochemical models are used to investigate how hydro-carbon photochemical products vary as a function of altitude, latitude, andseason on Uranus and Neptune. The results indicate that meridional andseasonal gradients in hydrocarbon abundances can persist on these planetsin the absence of stratospheric circulation.Based on our theoretical calculations and comparisons with observations,we draw the following conclusions for Neptune: • Seasonal variations in hydrocarbon abundances on Neptune are pre-dicted to be greater at high latitudes than low latitudes because of thelarger seasonal variations in solar insolation at high latitudes. • The larger mean daily solar insolation in the summer hemisphere leadsto greater photochemical production and higher abundances of hydro-carbon photochemical products in the summer hemisphere than thewinter hemisphere. • Seasonal variations in hydrocarbon abundances on Neptune are mostpronounced at high altitudes and become progressively weaker at depthbecause diffusion time constants and chemical lifetimes increase withincreasing pressure. Hydrocarbons respond quickly to changes in thesolar flux at high altitudes, but the greater time constants at depth55ntroduce phase lags in the seasonal response to the changing insolation,such that the abundance maxima for long-lived hydrocarbons such asC H and C H shift to later in the summer season with increasingdepth. • In the absence of advection or other stratospheric transport processes,meridional variations in hydrocarbon abundances will exist on Nep-tune and will again be greater at high altitudes than low altitudes. Atpressures greater than a few mbar, chemical and transport time scalesare longer than a Neptune season, and the yearly averaged solar fluxthen controls the hydrocarbon abundance variation with latitude. Be-cause the yearly average insolation is larger at the equator than thepoles at Neptune, most hydrocarbons at pressures greater than a mbarare expected to exhibit abundance maxima at low latitudes, decreasinggradually toward higher latitudes. Hydrocarbons that have short pho-tochemical lifetimes at mbar pressures, such as C H and C H , exhibitadditional complicated seasonal behavior. • Model predictions for the meridional variation of C H and C H onNeptune compare well with retrievals from the spatially resolved Gem-ini infrared spectral observations of Greathouse et al. (2011), and theKeck observations of Fletcher et al. (2014) (to within uncertainties ofthe observational analyses), suggesting that stratospheric circulationis less important in controlling the large-scale hydrocarbon meridionaldistributions on Neptune than it is on Jupiter and Saturn (cf. Lianget al., 2005; Moses and Greathouse, 2005; Nixon et al., 2007; Guerlet56t al., 2009; Sinclair et al., 2013; Sylvestre et al., 2015; Hue et al., 2015;Fletcher et al., 2016).Neptune’s axial tilt is only a couple degrees greater than that of Saturn,and the predicted seasonal variations on Neptune share many similaritieswith those on Saturn (cf. Moses and Greathouse, 2005; Hue et al., 2015).Seasonal variations on Uranus, on the other hand, differ notably from bothSaturn and Neptune. Although the extreme axial tilt of Uranus is partiallyresponsible for these differences, the very weak atmospheric mixing on Uranusplays a larger role. Our main conclusions with respect to Uranus are thefollowing: • The weak vertical transport on Uranus confines hydrocarbons to rela-tively low altitudes. The vertical diffusion time constants and chemicallifetimes tend to be large in the pressure region where the complexhydrocarbons are being produced, which results in more muted sea-sonal variations on Uranus than on Neptune. Some seasonal variationis expected in the 0.1-1 mbar region of Uranus (leading to notablehemispheric dichotomies in hydrocarbon abundances, with generallygreater abundances in the summer-to-fall hemisphere than the winter-to-spring hemisphere), but virtually no seasonal changes are predictedat pressures greater than 1 mbar. • Most hydrocarbons on Uranus are predicted to have a greater abun-dance at the poles than the equator, due to the latitude variation of theannual average solar insolation on the highly tilted Uranus. Ethane isan exception to this trend because of effective photolysis loss that can57ompete with production; C H abundances are on average greater atlow latitudes than high latitudes.Solar Lyman alpha photons scattered from atomic hydrogen in the in-terplanetary medium provide an important source of photolyzing radiationfor both Uranus and Neptune, particularly in the high-latitude polar winter,where sunlight is absent for long periods of time each year. Seasonal varia-tions at high altitudes would be even more significant without this extra UVsource.Based on our photochemical model results, we simulated the emissionfrom Uranus and Neptune that would be observed using the MIRI instrumentonboard JWST. We find that hydrocarbon variations with latitude shouldshow up readily in MIRI images of Neptune but would be more muted forUranus. Keep in mind, however, that the models presented here do notconsider possible seasonal variations in stratospheric temperatures, possiblelatitude variations in the stratospheric methane abundance or its verticalprofile, or possible perturbations due to stratospheric circulation, waves, orother transport processes. These factors could potentially play a major rolein controlling large-scale vertical and meridional hydrocarbon distributions,particularly on Uranus, where chemical and diffusion time constants are long.Because many of the hydrocarbon photochemical products condense in thecold lower stratospheres of these planets, any (likely) changes in temperaturewith season could strongly affect the total column density of the condens-able products, due to the high sensitivity of the species’ vapor pressures totemperatures, although the gas-phase chemistry variations themselves arenot very sensitive to temperature (e.g., Moses and Greathouse, 2005; Moses58t al., 2015). Differences in the methane abundance at high versus low lati-tudes have already been identified in the tropospheres of Uranus and Neptune(Karkoschka and Tomasko, 2009, 2011; Sromovsky et al., 2011, 2014; Irwinet al., 2012; Tice et al., 2013; de Kleer et al., 2015; Luszcz-Cook et al., 2016),and if such differences extend into the stratosphere, would have a major ef-fect on photochemical production rates. Seasonal forcing likely drives strato-spheric circulation on Uranus and Neptune (e.g., Flasar et al., 1987; Conrathet al., 1990, 1991a), and could have important consequences for hydrocar-bon distributions. The models presented here represent a first-order solutionthat when compared to observations can provide valuable insights into thephysical and chemical processes at play in the stratospheres of Uranus andNeptune. Future 1D, 2D, and 3D models could add complexity, as needed,to explain observations. We present our full model results in the journalsupplementary material in the hopes that they will be of use in analyzingfuture observations, including potential spatially resolved infrared spectralobservations from JWST (e.g., Norwood et al., 2016) and potential futurespacecraft missions to the ice giants (e.g., Hofstadter et al., 2017; Mousiset al., 2017).Hydrocarbon photochemical products (both vapor and aerosols) on Uranusand Neptune help control stratospheric heating, cooling, and energy balance,which in turn influence atmospheric dynamics. The gas-phase hydrocar-bons reveal the complex chemistry at play in these atmospheres and can actas tracers to illuminate dynamical motions. Furthering our understandingof the three-dimensional time-variable behavior of hydrocarbons on Uranusand Neptune is therefore important for furthering our understanding of the59omplex chemical, radiative, and dynamical couplings and feedbacks thatcharacterize our solar-system ice giants.
7. Acknowledgments
This material is based on research supported by the National Aeronauticsand Space Administration (NASA) Science Mission Directorate under grantNNX13AH81G from the Planetary Atmospheres Research Program. Theoxygen chemistry portion was supported by NASA grant NNX13AG55G.Fletcher was supported by a Royal Society Research Fellowship and EuropeanResearch Council Consolidator Grant (under the European Union’s Horizon2020 research and innovation programme, grant agreement No. 723890) atthe University of Leicester. Orton acknowledges support from NASA to theJet Propulsion Laboratory, California Institute of Technology.
References
Allen, M., Yung, Y. L., Waters, J. W., 1981. Vertical transport and pho-tochemistry in the terrestrial mesosphere and lower thermosphere (50-120km). J. Geophys. Res. 86, 3617–3627.Atreya, S. K., Ponthieu, J. J., 1983. Photolysis of methane and the ionosphereof Uranus. Planet. Space Sci. 31, 939–944.Atreya, S. K., Sandel, B. R., Romani, P. N., 1991. Photochemistry andvertical mixing. In: Bergstralh, J. T., Miner, E. D., Matthews, M. S.(Eds.), Uranus. Univ. Arizona Press, Tucson, pp. 110–146.60´ezard, B., Feuchtgruber, H., Moses, J. I., Encrenaz, T., 1998. Detection ofmethyl radicals (CH ) on Saturn. Astron. Astrophys. 334, L41–L44.B´ezard, B., Romani, P. N., Conrath, B. J., Maguire, W. C., 1991. Hydro-carbons in Neptune’s stratosphere from Voyager infrared observations. J.Geophys. Res. 96, 18,961–18,975.B´ezard, B., Romani, P. N., Feuchtgruber, H., Encrenaz, T., 1999. Detectionof the methyl radical on Neptune. Astrophys. J. Letters 515, 868–872.Bishop, J., Atreya, S. K., Herbert, F., Romani, P., 1990. Reanalysis of Voy-ager 2 UVS occultations at Uranus: Hydrocarbon mixing ratios in theequatorial stratosphere. Icarus 88, 448–464.Bishop, J., Atreya, S. K., Romani, P. N., Orton, G. S., Sandel, B. R., Yelle,R. V., 1995. The middle and upper atmosphere of Neptune. In: Cruik-shank, D. P., Matthews, M. S., Schumann, A. M. (Eds.), Neptune andTriton. Univ. Arizona Press, Tucson, pp. 427–487.Bishop, J., Atreya, S. K., Romani, P. N., Sandel, B. R., Herbert, F., 1992.Voyager 2 ultraviolet spectrometer solar occultations at Neptune: Con-straints on the abundance of methane in the stratosphere. J. Geophys.Res. 97, 11,681–11,694.Bishop, J., Romani, P. N., Atreya, S. K., 1998. Voyager 2 ultraviolet spec-trometer solar occultations at Neptune: Photochemical modeling of the125–165 nm lightcurves. Planet. Space Sci. 46, 1–20.Broadfoot, A. L., Atreya, S. K., Bertaux, J. L., Blamont, J. E., Dessler, A. J.,Donahue, T. M., Forrester, W. T., Hall, D. T., Herbert, F., Holberg, J. B.,61unten, D. M., Krasnopolsky, V. A., Linick, S., Lunine, J. I., Mcconnell,J. C., Moos, H. W., Sandel, B. R., Schneider, N. M., Shemansky, D. E.,Smith, G. R., Strobel, D. F., Yelle, R. V., 1989. Ultraviolet spectrometerobservations of Neptune and Triton. Science 246, 1459–1466.Burgdorf, M., Orton, G., van Cleve, J., Meadows, V., Houck, J., 2006. Detec-tion of new hydrocarbons in Uranus’ atmosphere by infrared spectroscopy.Icarus 184, 634–637.Caldwell, J., Wagener, R., Fricke, K.-H., 1988. Observations of Neptune andUranus below 2000 A with the IUE. Icarus 74, 133–140.Cavali´e, T., Moreno, R., Lellouch, E., Hartogh, P., Venot, O., Orton, G. S.,Jarchow, C., Encrenaz, T., Selsis, F., Hersant, F., Fletcher, L. N., 2014.The first submillimeter observation of CO in the stratosphere of Uranus.Astron. Astrophys. 562, A33.Cavali´e, T., Venot, O., Selsis, F., Hersant, F., Hartogh, P., Leconte, J.,2017. Thermochemistry and vertical mixing in the tropospheres of Uranusand Neptune: How convection inhibition can affect the derivation of deepoxygen abundances. Icarus 291, 1–16.Conrath, B. J., Flasar, F. M., Gierasch, P. J., 1991a. Thermal structureand dynamics of Neptune’s atmosphere from Voyager measurements. J.Geophys. Res. 96, 18,931–18,939.Conrath, B. J., Gautier, D., Lindal, G. F., Samuelson, R. E., Shaffer, W. A.,1991b. The helium abundance of Neptune from Voyager measurements. J.Geophys. Res. 96, 18,907–18,919. 62onrath, B. J., Gierasch, P. J., Leroy, S. S., 1990. Temperature and circula-tion in the stratosphere of the outer planets. Icarus 83, 255–281.Conrath, B. J., Gierasch, P. J., Ustinov, E. A., 1998. Thermal structure andpara hydrogen fraction on the outer planets from Voyager IRIS measure-ments. Icarus 135, 501–517.de Kleer, K., Luszcz-Cook, S., de Pater, I., ´Ad´amkovics, M., Hammel, H. B.,2015. Clouds and aerosols on Uranus: Radiative transfer modeling ofspatially-resolved near-infrared Keck spectra. Icarus 256, 120–137.de Pater, I., Fletcher, L. N., Luszcz-Cook, S., DeBoer, D., Butler, B., Ham-mel, H. B., Sitko, M. L., Orton, G., Marcus, P. S., 2014. Neptune’s globalcirculation deduced from multi-wavelength observations. Icarus 237, 211–238.Dobrijevic, M., Cavali´e, T., H´ebrard, E., Billebaud, F., Hersant, F., Selsis, F.,2010. Key reactions in the photochemistry of hydrocarbons in Neptune’sstratosphere. Planet. Space Sci. 58, 1555–1566.Dobrijevic, M., Ollivier, J. L., Billebaud, F., Brillet, J., Parisot, J. P., 2003.Effect of chemical kinetic uncertainties on photochemical modeling results:Application to Saturn’s atmosphere. Astron. Astrophys. 398, 335–344.Dobrijevic, M., Parisot, J. P., 1998. Effect of chemical kinetics uncertaintieson hydrocarbon production in the stratosphere of Neptune. Planet. SpaceSci. 46, 491–505. 63ncrenaz, T., Combes, M., Atreya, S. K., Romani, P. N., Fricke, K., 1986.A study of the upper atmosphere of Uranus using the IUE. Astron. Astro-phys. 162, 317–322.Encrenaz, T., Feuchtgruber, H., Atreya, S. K., Bezard, B., Lellouch, E.,Bishop, J., Edgington, S., Degraauw, T., Griffin, M., Kessler, M. F., 1998.ISO observations of Uranus: The stratospheric distribution of C H andthe eddy diffusion coefficient. Astron. Astrophys. 333, L43–L46.Fegley, Jr., B., Prinn, R. G., 1986. Chemical models of the deep atmosphereof Uranus. Astrophys. J. 307, 852–865.Feuchtgruber, H., Lellouch, E., de Graauw, T., B´ezard, B., Encrenaz, T.,Griffin, M., 1997. External supply of oxygen to the atmospheres of thegiant planets. Nature 389, 159–162.Feuchtgruber, H., Lellouch, E., Encrenaz, T., Bezard, B., Coustenis, A.,Drossart, P., Salama, A., de Graauw, T., Davis, G. R., 1999. Oxygen inthe stratospheres of the giant planets and Titan. In: Cox, P., Kessler,M. (Eds.), The Universe as Seen by ISO. Vol. SP-427 of ESA SpecialPublication. pp. 133–136.Flasar, F. M., Conrath, B. J., Pirraglia, J. A., Gierasch, P. J., 1987. Voy-ager infrared observations of Uranus’ atmosphere - Thermal structure anddynamics. J. Geophys. Res. 92, 15011–15018.Fletcher, L. N., de Pater, I., Orton, G. S., Hammel, H. B., Sitko, M. L., Irwin,P. G. J., 2014. Neptune at summer solstice: Zonal mean temperatures fromground-based observations, 2003-2007. Icarus 231, 146–167.64letcher, L. N., Drossart, P., Burgdorf, M., Orton, G. S., Encrenaz, T., 2010.Neptune’s atmospheric composition from AKARI infrared spectroscopy.Astron. Astrophys. 514, A17.Fletcher, L. N., Greathouse, T. K., Orton, G. S., Sinclair, J. A., Giles, R. S.,Irwin, P. G. J., Encrenaz, T., 2016. Mid-infrared mapping of Jupiter’s tem-peratures, aerosol opacity and chemical distributions with IRTF/TEXES.Icarus 278, 128–161.Fletcher, L. N., Irwin, P. G. J., Sinclair, J. A., Orton, G. S., Giles, R. S.,Hurley, J., Gorius, N., Achterberg, R. K., Hesman, B. E., Bjoraker, G. L.,2015. Seasonal evolution of Saturn’s polar temperatures and composition.Icarus 250, 131–153.Friedson, A. J., Moses, J. I., 2012. General circulation and transport in Sat-urn’s upper troposphere and stratosphere. Icarus 218, 861–875.Gangopadhyay, P., Izmodenov, V. V., Shemansky, D. E., Gruntman, M.,Judge, D. L., 2005. Reappraisal of the Pioneer 10 and Voyager 2 Ly α intensity measurements. Astrophys. J. 628, 514–519.Giorgini, J. D., Yeomans, D. K., Chamberlin, A. B., Chodas, P. W., Ja-cobson, R. A., Keesey, M. S., Lieske, J. H., Ostro, S. J., Standish,E. M., Wimberly, R. N., 1996. JPL’s on-line solar system data service.In: AAS/Division for Planetary Sciences Meeting Abstracts C H andC H in Saturn’s stratosphere from CIRS/Cassini limb and nadir observa-tions. Icarus 209, 682–695.Guerlet, S., Fouchet, T., B´ezard, B., Simon-Miller, A. A., Flasar, F. M., 2009.Vertical and meridional distribution of ethane, acetylene and propane inSaturn’s stratosphere from CIRS/Cassini limb observations. Icarus 203,214–232.Hammel, H. B., Lynch, D. K., Russell, R. W., Sitko, M. L., Bernstein, L. S.,Hewagama, T., Jun. 2006. Mid-Infrared Ethane Emission on Neptune andUranus. Astrophys. J. Letters 644, 1326–1333.Hammel, H. B., Sitko, M. L., Lynch, D. K., Orton, G. S., Russell, R. W.,Geballe, T. R., de Pater, I., 2007. Distribution of ethane and methaneemission on Neptune. Astron. J. 134, 637–641.Herbert, F., Sandel, B. R., Yelle, R. V., Holberg, J. B., Broadfoot, A. L.,Shemansky, D. E., Atreya, S. K., Romani, P. N., 1987. The upper atmo-sphere of Uranus - EUV occultations observed by Voyager 2. J. Geophys.Res. 92, 15093–15109.Hesman, B. E., Davis, G. R., Matthews, H. E., Orton, G. S., 2007. Theabundance profile of CO in Neptune’s atmosphere. Icarus 186, 342–353.66ofstadter, M., Simon, A., Atreya, S., Banfield, D., Fortney, J., Hayes, A.,Hedman, M., Hospodarsky, G., Mandt, K., Masters, A., Showalter, M.,Soderlund, K., Turrini, D., Turtle, E. P., Elliott, J., Reh, K., 2017. A visionfor ice giant exploration. In: Planetary Science Vision 2050 Workshop. Vol.1989 of LPI Contributions. p. 8115.Hue, V., Cavali´e, T., Dobrijevic , M., Hersant, F., Greathouse, T. K., 2015. 2Dphotochemical modeling of Saturn’s stratosphere. Part I: Seasonal varia-tion of atmospheric composition without meridional transport. Icarus 257,163–184.Hue, V., Greathouse, T. K., Cavali´e, T., Dobrijevic, M., Hersant, F., 2016.2D photochemical modeling of Saturn’s stratosphere. Part II: Feedbackbetween composition and temperature. Icarus 267, 334–343.Irwin, P. G. J., Teanby, N. A., Davis, G. R., Fletcher, L. N., Orton, G. S.,Calcutt, S. B., Tice, D. S., Hurley, J., 2012. Further seasonal changesin Uranus’ cloud structure observed by Gemini-North and UKIRT. Icarus218, 47–55.Irwin, P. G. J., Teanby, N. A., de Kok, R., Fletcher, L. N., Howett, C. J. A.,Tsang, C. C. C., Wilson, C. F., Calcutt, S. B., Nixon, C. A., Parrish,P. D., 2008. The NEMESIS planetary atmosphere radiative transfer andretrieval tool. Journal of Quantitative Spectroscopy and Radiative Transfer109, 1136–1150.Karkoschka, E., Tomasko, M. G., 2009. The haze and methane distributionson Uranus from HST-STIS spectroscopy. Icarus 202, 287–309.67arkoschka, E., Tomasko, M. G., 2011. The haze and methane distributionson Neptune from HST-STIS spectroscopy. Icarus 211, 780–797.Kostiuk, T., Romani, P., Espenak, F., Bezard, B., 1992. Stratospheric ethaneon Neptune - Comparison of groundbased and Voyager IRIS retrievals.Icarus 99, 353–362.Lellouch, E., Hartogh, P., Feuchtgruber, H., Vandenbussche, B., de Graauw,T., Moreno, R., Jarchow, C., Cavali´e, T., Orton, G., Banaszkiewicz, M.,Blecka, M. I., Bockel´ee-Morvan, D., Crovisier, J., Encrenaz, T., Fulton,T., K¨uppers, M., Lara, L. M., Lis, D. C., Medvedev, A. S., Rengel, M.,Sagawa, H., Swinyard, B., Szutowicz, S., Bensch, F., Bergin, E., Bille-baud, F., Biver, N., Blake, G. A., Blommaert, J. A. D. L., Cernicharo, J.,Courtin, R., Davis, G. R., Decin, L., Encrenaz, P., Gonzalez, A., Jehin, E.,Kidger, M., Naylor, D., Portyankina, G., Schieder, R., Sidher, S., Thomas,N., de Val-Borro, M., Verdugo, E., Waelkens, C., Walker, H., Aarts, H.,Comito, C., Kawamura, J. H., Maestrini, A., Peacocke, T., Teipen, R.,Tils, T., Wildeman, K., 2010. First results of Herschel-PACS observationsof Neptune. Astron. Astrophys. 518, L152.Lellouch, E., Moreno, R., Orton, G. S., Feuchtgruber, H., Cavali´e, T., Moses,J. I., Hartogh, P., Jarchow, C., Sagawa, H., 2015. New constraints on theCH vertical profile in Uranus and Neptune from Herschel observations.Astron. Astrophys. 579, A121.Lellouch, E., Moreno, R., Paubert, G., 2005. A dual origin for Neptune’scarbon monoxide? Astron. Astrophys. 430, L37–L40.68ellouch, E., Romani, P. N., Rosenqvist, J., 1994. The vertical Distributionand Origin of HCN in Neptune’s Atmosphere. Icarus 108, 112–136.Liang, M.-C., Shia, R.-L., Lee, A. Y.-T., Allen, M., Friedson, A. J., Yung,Y. L., 2005. Meridional transport in the stratosphere of Jupiter. Astrophys.J. Lett. 635, L177–L180.Lindal, G. F., Lyons, J. R., Sweetnam, D. N., Eshleman, V. R., Hinson, D. P.,1987. The atmosphere of Uranus - Results of radio occultation measure-ments with Voyager 2. Journal of Geophysical Research 92, 14987–15001.Lindal, G. F., Sweetnam, D. N., Eshleman, V. R., 1985. The atmosphereof Saturn: An analysis of the Voyager radio occultation measurements.Astron. J. 90, 1136–1146.Lodders, K., Fegley, Jr., B., 1994. The origin of carbon monoxide in Nep-tunes’s atmosphere. Icarus 112, 368–375.Luszcz-Cook, S. H., de Kleer, K., de Pater, I., Adamkovics, M., Hammel,H. B., 2016. Retrieving Neptune’s aerosol properties from Keck OSIRISobservations. I. Dark regions. Icarus 276, 52–87.Luszcz-Cook, S. H., de Pater, I., 2013. Constraining the origins of Neptune’scarbon monoxide abundance with CARMA millimeter-wave observations.Icarus 222, 379–400.Mathis, J. S., Mezger, P. G., Panagia, N., 1983. Interstellar radiation field anddust temperatures in the diffuse interstellar matter and in giant molecularclouds. Astron. Astrophys. 128, 212–229.69eadows, V. S., Orton, G., Line, M., Liang, M.-C., Yung, Y. L., van Cleve,J., Burgdorf, M. J., 2008. First Spitzer observations of Neptune: Detectionof new hydrocarbons. Icarus 197, 585–589.Moses, J. I., 1992. Meteoroid ablation in Neptune’s atmosphere. Icarus 99,368–383.Moses, J. I., Allen, M., Yung, Y. L., 1992. Hydrocarbon nucleation andaerosol formation in Neptune’s atmosphere. Icarus 99, 318–346.Moses, J. I., Armstrong, E. S., Fletcher, L. N., Friedson, A. J., Irwin, P. G. J.,Sinclair, J. A., Hesman, B. E., 2015. Evolution of stratospheric chemistryin the Saturn storm beacon region. Icarus 261, 149–168.Moses, J. I., B´ezard, B., Lellouch, E., Gladstone, G. R., Feuchtgruber, H.,Allen, M., 2000a. Photochemistry of Saturn’s atmosphere. I. Hydrocarbonchemistry and comparisons with ISO observations. Icarus 143, 244–298.Moses, J. I., Fouchet, T., B´ezard, B., Gladstone, G. R., Lellouch, E., Feucht-gruber, H., 2005. Photochemistry and diffusion in Jupiter’s stratosphere:Constraints from ISO observations and comparisons with other giant plan-ets. J. Geophys. Res. 110, E08001.Moses, J. I., Greathouse, T. K., 2005. Latitudinal and seasonal models ofstratospheric photochemistry on Saturn: Comparison with infrared datafrom IRTF/TEXES. J. Geophys. Res. 110, E09007.Moses, J. I., Lellouch, E., B´ezard, B., Gladstone, G. R., Feuchtgruber, H.,Allen, M., 2000b. Photochemistry of Saturn’s atmosphere. II. Effects of aninflux of external oxygen. Icarus 145, 166–202.70oses, J. I., Poppe, A. R., 2017. Dust ablation on the giant planets: Con-squences for stratospheric photochemistry. Icarus 297, 33–58.Moses, J. I., Rages, K., Pollack, J. B., 1995. An analysis of Neptune’s strato-spheric haze using high-phase-angle Voyager images. Icarus 113, 232–266.Mousis, O., Atkinson, D. H., Cavali´e, T., Fletcher, L. N., Amato, M. J.,Aslam, S., Ferri, F., Renard, J.-B., Spilker, T., Venkatapathy, E., Wurz,P., Aplin, K., Coustenis, A., Deleuil, M., Dobrijevic, M., Fouchet, T., Guil-lot, T., Hartogh, P., Hewagama, T., Hofstadter, M. D., Hue, V., Hueso,R., Lebreton, J.-P., Lellouch, E., Moses, J., Orton, G. S., Pearl, J. C.,Sanchez-Lavega, A., Simon, A., Venot, O., Waite, J. H., Achterberg, R. K.,Atreya, S., Billebaud, F., Blanc, M., Borget, F., Brugger, B., Charnoz, S.,Chiavassa, T., Cottini, V., d’Hendecourt, L., Danger, G., Encrenaz, T.,Gorius, N. J. P., Jorda, L., Marty, B., Moreno, R., Morse, A., Nixon, C.,Reh, K., Ronnet, T., Schmider, F.-X., Sheridan, S., Sotin, C., Vernazza,P., Villanueva, G. L., 2017. Scientific rationale for Uranus and Neptune insitu explorations. ArXiv e-prints.Nixon, C. A., Achterberg, R. K., Conrath, B. J., Irwin, P. G. J., Teanby,N. A., Fouchet, T., Parrish, P. D., Romani, P. N., Abbas, M., Leclair, A.,Strobel, D., Simon-Miller, A. A., Jennings, D. J., Flasar, F. M., Kunde,V. G., May 2007. Meridional variations of C H and C H in Jupiter’satmosphere from Cassini CIRS infrared spectra. Icarus 188, 47–71.Nixon, C. A., Achterberg, R. K., Romani, P. N., Allen, M., Zhang, X.,Teanby, N. A., Irwin, P. G. J., Flasar, F. M., 2010. Abundances of Jupiter’s71race hydrocarbons from Voyager and Cassini. Planet. Space Sci. 58, 1667–1680.Norwood, J., Moses, J., Fletcher, L. N., Orton, G., Irwin, P. G. J., Atreya,S., Rages, K., Cavali´e, T., S´anchez-Lavega, A., Hueso, R., Chanover, N.,2016. Giant planet observations with the James Webb Space Telescope.Publications of the Astronomical Society of Pacific 128, 018005.Orton, G., Encrenaz, T., Leyrat, C., Puetter, R., Friedson, A., 2007. Evi-dence for methane escape and strong seasonal and dynamical perturbationsof Neptune’s atmospheric temperatures. Astron. Astrophys. 473, L5–L8.Orton, G. S., Aitken, D. K., Smith, C., Roche, P. F., Caldwell, J., Snyder,R., 1987. The spectra of Uranus and Neptune at 8-14 and 17-23 microns.Icarus 70, 1–12.Orton, G. S., Fletcher, L. N., Encrenaz, T., Leyrat, C., Roe, H. G., Fujiyoshi,T., Pantin, E., 2015. Thermal imaging of Uranus: Upper-tropospherictemperatures one season after Voyager. Icarus 260, 94–102.Orton, G. S., Fletcher, L. N., Moses, J. I., Lellouch, E., Moreno, R., Swin-yard, B. M., Hofstadter, M. D., Greathouse, T. K., 2014a. Thermal emis-sion constraints on the atmospheres of Uranus and Neptune. In: Workshopon the Study of the Ice Giant Planets. Vol. 1798 of LPI Contributions. p.2002.Orton, G. S., Fletcher, L. N., Moses, J. I., Mainzer, A. K., Hines, D., Hammel,H. B., Martin-Torres, F. J., Burgdorf, M., Merlet, C., Line, M. R., 2014b.72id-infrared spectroscopy of Uranus from the Spitzer Infrared Spectrom-eter: 1. Determination of the mean temperature structure of the uppertroposphere and stratosphere. Icarus 243, 494–513.Orton, G. S., Lacy, J. H., Achtermann, J. M., Parmar, P., Blass, W. E.,1992. Thermal spectroscopy of Neptune: The stratospheric temperature,hydrocarbon abundances, and isotopic ratios. Icarus 100, 541–555.Orton, G. S., Moses, J. I., Fletcher, L. N., Mainzer, A. K., Hines, D., Hammel,H. B., Martin-Torres, J., Burgdorf, M., Merlet, C., Line, M. R., 2014c. Mid-infrared spectroscopy of Uranus from the Spitzer infrared spectrometer:2. Determination of the mean composition of the upper troposphere andstratosphere. Icarus 243, 471–493.Pollack, J. B., Rages, K., Pope, S. K., Tomasko, M. G., Romani, P. N.,1987. Nature of the stratospheric haze on Uranus: Evidence for condensedhydrocarbons. J. Geophys. Res. 92, 15037–15065.Poppe, A. R., 2016. An improved model for interplanetary dust fluxes in theouter Solar System. Icarus 264, 369–386.Qu´emerais, E., Lallement, R., Sandel, B. R., Clarke, J. T., 2009. Interplan-etary Lyman α observations: Intensities from Voyagers and line profilesfrom HST/STIS. Space Science Reviews 143, 151–162.Rieke, G. H., Wright, G. S., B¨oker, T., Bouwman, J., Colina, L., Glasse, A.,Gordon, K. D., Greene, T. P., G¨udel, M., Henning, T., Justtanont, K., La-gage, P.-O., Meixner, M. E., Nørgaard-Nielsen, H.-U., Ray, T. P., Ressler,73. E., van Dishoeck, E. F., Waelkens, C., 2015. The Mid-Infrared Instru-ment for the James Webb Space Telescope, I: Introduction. Publicationsof the Astronomical Society of Pacific 127, 584.Romani, P. N., Atreya, S. K., 1988. Methane photochemistry and methaneproduction on Neptune. Icarus 74, 424–445.Romani, P. N., Atreya, S. K., 1989. Stratospheric aerosols from CH photo-chemistry on Neptune. Geophys. Res. Lett. 16, 941–944.Romani, P. N., Bishop, J., Bezard, B., Atreya, S., 1993. Methane photochem-istry on Neptune: Ethane and acetylene mixing ratios and haze production.Icarus 106, 442–463.Roques, F., Sicardy, B., French, R. G., Hubbard, W. B., Barucci, A.,Bouchet, P., Brahic, A., Gehrels, J.-A., Gehrels, T., Grenier, I., Le Bertre,T., Lecacheux, J., Maillard, J. P., McLaren, R. A., Perrier, C., Vilas,F., Waterworth, M. D., 1994. Neptune’s upper stratosphere, 1983-1990:ground-based stellar occultation observations III. Temperature profiles.Astron. Astrophys. 288, 985–1011.Schulz, B., Encrenaz, T., B´ezard, B., Romani, P. N., Lellouch, E., Atreya,S. K., 1999. Detection of C H in Neptune from ISO/PHT-S observations.Astron. Astrophys 350, L13–L17.Sinclair, J. A., Irwin, P. G. J., Fletcher, L. N., Greathouse, T., Guerlet, S.,Hurley, J., Merlet, C., 2014. From Voyager-IRIS to Cassini-CIRS: Interan-nual variability in Saturn’s stratosphere? Icarus 233, 281–292.74inclair, J. A., Irwin, P. G. J., Fletcher, L. N., Moses, J. I., Greathouse,T. K., Friedson, A. J., Hesman, B., Hurley, J., Merlet, C., 2013. Seasonalvariations of temperature, acetylene and ethane in Saturn’s atmospherefrom 2005 to 2010, as observed by Cassini-CIRS. Icarus 225, 257–271.Sromovsky, L. A., Fry, P. M., Kim, J. H., 2011. Methane on Uranus: The casefor a compact CH cloud layer at low latitudes and a severe CH4