P. N. Romani

TITAN'S SURFACE BRIGHTNESS TEMPERATURES

Radiance from the surface of Titan can be detected from space through a spectral window of low opacity in the thermal infrared at 19 μm (530 cm–1). By combining Composite Infrared Spectrometer observations from Cassinis first four years, we have mapped the latitude distribution of zonally averaged surface brightness temperatures. The measurements are corrected for atmospheric opacity as derived from the dependence of radiance on the emission angle. At equatorial latitudes near the Huygens landing site, the surface brightness temperature is found to be 93.7 ± 0.6 K, in excellent agreement with the in situ measurement. Temperature decreases toward the poles, reaching 90.5 ± 0.8 K at 87°N and 91.7 ± 0.7 K at 88°S. The meridional distribution of temperature has a maximum near 10°S, consistent with Titans late northern winter.

Cassini infrared Fourier spectroscopic investigation

The composite infrared spectrometer (CIRS) is a remote sensing instrument to be flown on the Cassini orbiter. CIRS will retrieve vertical profiles of temperature and gas composition for the atmospheres of Titan and Saturn, from deep in their tropospheres to high in their stratospheres. CIRS will also retrieve information on the thermal properties and composition of Saturns rings and Saturnian satellites. CIRS consists of a pair of Fourier Transform Spectrometers (FTSs) which together cover the spectral range from 10-1400 cm-1 with a spectral resolution up to 0.5 cm-1. The two interferometers share a 50 cm beryllium Cassegrain telescope. The far-infrared FTS is a polarizing interferometer covering the 10-600 cm-1 range with a pair of thermopile detectors, and a 3.9 mrad field of view. The mid-infrared FTS is a conventional Michelson interferometer covering 200-1400 cm-1 in two spectral bandpasses: 600-1100 cm- 1100-1400 cm(superscript -1 with a 1 by 10 photovoltaic HgCdTe array. Each pixel of the arrays has an approximate 0.3 mrad field of view. The HgCdTe arrays are cooled to approximately 80K with a passive radiative cooler.

Detection of the Methyl Radical on Neptune

We report the first detection of the methyl radical (CH3) in the upper atmosphere of Neptune. Observations with the Short-Wavelength Spectrometer of the Infrared Space Observatory (ISO) satellite at a resolving power of 2200 revealed several emission features from the ν2 Q-branch of CH3 around 16.50 μm. The column density of methyl radicals above the 0.2 mbar level is 1.6+1.2−0.9 × 1013 molecules cm-2. Results are compared with predictions of photochemical models. The CH3 abundance is mostly sensitive to the eddy mixing profile and to the poorly known methyl recombination rate at low pressure. Using Slagle et al.s expression for this rate, the present observations imply an eddy mixing coefficient at the methane homopause in the range of 2-9 × 106 cm2 s-1. Recombination rates higher than used in current photochemical models would lead to larger eddy mixing coefficients.

Millimeter-wave observations of Saturn, Uranus, and Neptune - CO and HCN on Neptune

Saturn, Uranus, and Neptune were observed at millimeter wavelengths with the IRAM 30 m telescope. The major result is the detection of CO and HCN in Neptunes stratosphere, with respective mixing ratios of (6.5 +/- 3.5) x 10 exp -7 and (3 +/- 1.5) x 10 exp -10. CO seems to be present in Neptunes troposphere as well and to slowly decrease with altitude (scale height about 200 km). HCN is probably formed from reactions between CH3 and N, which can be supplied in sufficient amounts by escape from Tritons atmosphere. The origin of CO, however, is more problematic, because: (1) thermochemical models fail to reproduce the observed abundance by a factor of about 1000; and (2) an external source would require a very large flux of oxygen. CO appears to be at least 15 times less abundant on Uranus than on Neptune. Finally, an upper limit of 10 exp -7 for CO in Saturns stratosphere suggests an internal origin for Saturnian CO.

Isotopic Ratios in Titan's Methane: Measurements and Modeling

The existence of methane in Titan’s atmosphere (∼ 6% level at the surface) presents a unique enigma, as photochemical models predict that the current inventory will be entirely depleted by photochemistry in a timescale of ∼20 Myr. In this paper, we examine the clues available from isotopic ratios ( 12 C/ 13 C and D/H) in Titan’s methane as to the past atmosphere history of this species. We first analyze recent infrared spectra of CH4 collected by the Cassini Composite Infrared Spectrometer, measuring simultaneously for the first time the abundances of all three detected minor isotopologues: 13 CH4, 12 CH3D, and 13 CH3D. From these we compute estimates of 12 C/ 13 C = 86.5 ± 8.2 and D/H = (1.59 ± 0.33) × 10 −4 , in agreement with recent results from the Huygens GCMS and Cassini INMS instruments. We also use the transition state theory to estimate the fractionation that occurs in carbon and hydrogen during a critical reaction that plays a key role in the chemical depletion of Titan’s methane: CH4 +C 2H → CH3 +C 2H2. Using these new measurements and predictions we proceed to model the time evolution of 12 C/ 13 C and D/H in Titan’s methane under several prototypical replenishment scenarios. In our Model 1 (no resupply of CH4), we find that the present-day 12 C/ 13 C implies that the CH4 entered the atmosphere 60–1600 Myr ago if methane is depleted by chemistry and photolysis alone, but much more recently—most likely less than 10 Myr ago—if hydrodynamic escape is also occurring. On the other hand, if methane has been continuously supplied at the replenishment rate then the isotopic ratios provide no constraints, and likewise for the case where atmospheric methane is increasing. We conclude by discussing how these findings may be combined with other evidence to constrain the overall history of the atmospheric methane.

Hydrocarbons in Neptune's stratosphere from Voyager infrared observations

Emission from the acetylene and ethane bands at 729 and 822 cm-1detected in the Voyager infrared spectra of Neptune has been analyzed. A large selection of low-spatial resolution spectra was used to derive the disk-averaged abundances of C2H2 and C2H6. Under the assumption of uniform vertical distributions above the saturation region, a C2H2 mixing ratio of 6−4+14 x 10 −8 and a C2H6 mixing ratio of 1.5−0.5+2.5 x 10−6 were inferred. The accuracy of the retrievals is limited by the large uncertainty in the stratospheric temperature structure. The maximum contribution to the observed C2H2 and C2H6emission comes from the 0.2- and 0.7-mbar regions, respectively. Mixing ratio profiles derived from photochemical modeling, which are not constant with height above the saturation region, indicate that the hydrocarbon emission is most sensitive to the assumed eddy diffusion coefficient in the millibar region. Either the C2H2 or the C2H6 emission can be reproduced by the photochemical model to within the accuracy of the retrievals, but not both simultaneously. Best fits to both emission features simultaneously occur with C2H2 mixing ratios a factor of 2 too high and C2H6 mixing ratios a factor of 2 too low. We consider this agreement satisfactory considering the unknowns in the chemical and photolytic processes. A set of Voyager spectra at higher spatial resolution was used to study the latitudinal variation of the C2H2 emission between 30°N and 80°S. Zonal mean radiances at the C2H2 peak show a minimum near 50°–60°S and maxima near the south pole and equator. This behavior is similar to that observed at 350 and 250 cm−1, where the lower stratosphere and troposphere are sounded. The mid-latitude minimum can be explained by a fivefold depletion of acetylene or a temperature decrease of about 15 K (or any combination of the two effects) in the 0.03- to 2-mbar region. The latitude variation in the C2H2emission could result from a circulation pattern forced from deep levels, with upwelling at mid-latitudes and subsidence at low and high latitudes.

Methane photochemistry and haze production on Neptune

A numerical model was used to study methane photochemistry in the stratosphere of Neptune. The observed mixing ratio of methane, 2%, forces photolysis to occur near the CH4 homopause. For an assumed nominal value of the eddy mixing coefficient of 106 cm 2 sec -1 at the CH4 homopause, the predicted average mixing ratios of CzH6 and C2Hz, 1.5 x 10 -6 and 6 x 10 -7, respectively, agree well with observations in the infrared. The acetylene and ethane abundances are weakly dependent upon the strength of the eddy mixing and directly proportional to it. Haze production from methane photochemistry results from the formation of hydrocarbon ices and polyacetylenes. The calculated mixing ratios of C2H6, C2H2, and C4H2 are large enough to cause condensation to their respective ices near the tropopause. These hazes are capable of providing the necessary aerosol optical depth at the appropriate pressure levels required by observations of Neptune in the visible and near IR. Polyacetylene formation from C2H2 photolysis is limited by the low quantum yield of dissociation for acetylene, efficient recycling of its photolysis products by the other hydrocarbons, and the greatly reduced solar flux at Neptune. Comparisons of model predictions to Uranus show both a lower ratio of polyacetylene production to hydrocarbon ice and a lower likelihood of UV postprocessing of the acetylene ice to polymers on Neptune compared to Uranus. This is in agreement with the observed difference in the single scattering aibedo of the stratospheric aerosols in the visible between Uranus and Neptune, with the aerosols on Neptune being brighter.

THE NITROGEN ISOTOPIC RATIO IN JUPITER'S ATMOSPHERE FROM OBSERVATIONS BY THE COMPOSITE INFRARED SPECTROMETER ON THE CASSINI SPACECRAFT

The Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft made infrared observations of Jupiters atmosphere during the flyby of 2000 December to 2001 January. The unique database in the 600-1400 cm-1 region with 0.53 and 2.8 cm-1 spectral resolutions obtained from the observations permits retrieval of global maps of the thermal structure and composition of Jupiters atmosphere, including the distributions of 14NH3 and 15NH3. Analysis of Jupiters ammonia distributions from three isolated 15NH3 spectral lines in eight latitudes is presented for evaluation of the nitrogen isotopic ratio. The nitrogen isotopic ratio 14N/15N (or 15N/14N) in Jupiters atmosphere in this analysis is calculated to be 448 ± 62 [or (2.23 ± 0.31) × 10-3]. This value of the ratio determined from CIRS data is found to be in very close agreement with the value previously obtained from the measurements by the Galileo Probe Mass Spectrometer. Some possible mechanisms to account for the variation of Jupiters observed isotopic ratio relative to those of various astrophysical environments are discussed.

Seasonal Changes in Titan's Surface Temperatures

Seasonal changes in Titan’s surface brightness temperatures have been observed by Cassini in the thermal infrared. The Composite Infrared Spectrometer measured surface radiances at 19 μm in two time periods: one in late northern winter (LNW; Ls = 335 ◦ ) and another centered on northern spring equinox (NSE; Ls = 0 ◦ ). In both periods we constructed pole-to-pole maps of zonally averaged brightness temperatures corrected for effects of the atmosphere. Between LNW and NSE a shift occurred in the temperature distribution, characterized by a warming of ∼0.5 K in the north and a cooling by about the same amount in the south. At equinox the polar surface temperatures were both near 91 K and the equator was at 93.4 K. We measured a seasonal lag of ΔLS ∼ 9 ◦ in the meridional surface temperature distribution, consistent with the post-equinox results of Voyager 1 as well as with predictions from general circulation modeling. A slightly elevated temperature is observed at 65 ◦ S in the relatively cloud-free zone

Reanalysis of Voyager 2 UVS Occultations at Uranus: Hydrocarbon Mixing Ratios in the Equatorial Stratosphere

Abstract Application of noise filtering and inversion techniques to single-channel UVS lightcurves obtained during the Voyager 2 solar occultation at Uranus has yielded tighter constraints on the structure and composition of the upper equatorial stratosphere at the time of the encounter. Specifically, atmospheric pressure and temperature profiles in the altitude region bracketted by total number densities 2 × 10 15 cm −3 and 5 × 10 16 cm −3 have been derived, based on the observed H 2 Rayleigh scattering opacity profiles ( wavelengths > 153 nm ) with an assumed helium mixing ration of 0.15: at the density level 3.3 (±0.2) × 10 16 cm −3 , the pressure is 0.60 (±0.01) mbar with a temperature 133 (±8) K. Lightcurves obtained at shorter wavelengths reveal the mixing ratio of accetylene to be increasing (0.7 × 10 −8 –1.5 × 10 −8 ) with increasing pressure in the pressure interval 0.10−0.30 mbar. Ethane is also present at these pressures with a mixing ratio of roughly 10 −8 ; if methane is present at mixing ratios in excess of roughly 3 × 10 −7 , then the ethane estimate may need to be halved. Comparison with photochemical models indicates values of 10 3 -10 -4 cm 2 sec −1 for the eddy mixing coefficient at the methane homopause, depending on the manner in which eddy mixing is assumed to vary with atmospheric number density. It has not been possible to obtain meaningful results from the stellar occultations, which are characterized by poor signal-to-noise ratios.