Tetsuya Tokano

Seasonal variation of Titans atmospheric structuresimulated by a general circulation model

The seasonal variation of Titans atmospheric structure with emphasis on thestratosphere is simulated by a three-dimensional general circulation model. The model includesthe transport of haze particles by the circulation. The likely pattern of meridional circulation isreconstructed by a comparison of simulated and observed haze and temperature distribution. TheGCM produces a weak zonal circulation with a small latitudinal temperature gradient, in conflictwith observation. The direct reason is found to be the excessive meridional circulation. Underuniformly distributed opacity sources, the model predicts a pair of symmetric Hadley cells nearthe equinox and a single global cell with the rising branch in the summer hemisphere belowabout z = 230 km and a thermally indirect cell above the direct cell near the solstice. Theinterhemispheric circulation transports haze particles from the summer to the winter hemisphere,causing a maximum haze opacity contrast near the solstice and a smaller contrast near theequinox, contrary to observation. On the other hand, if the GCM is run under modified coolingrate in order to account for the enhancement in nitriles and some hydrocarbons in the northernhemisphere near the vernal equinox, the meridional cell at the equinox becomes a single cell withrising motions in the autumn hemisphere. A more realistic haze opacity distribution can bereproduced at the equinox. However, a pure transport effect (without particle growth bymicrophysics, etc.) would not be able to cause the observed discontinuity of the global hazeopacity distribution at any location. The stratospheric temperature asymmetry can be explainedby a combination of asymmetric radiative heating rates and adiabatic heating due to verticalmotion within the thermally indirect cell. A seasonal variation of haze particle number density isunlikely to be responsible for this asymmetry. It is likely that a thermally indirect cell covers theupper portion of the main haze layer. An artificial damping of the meridional circulation enablesthe formation of high-latitude jets in the upper stratosphere and weaker equatorial superrotation.The latitudinal temperature distribution in the stratosphere is better reproduced.

Modelling of thunderclouds and lightning generation on Titan

Abstract The likelihood of lightning generation in Titans troposphere is investigated by theoretical consideration and a numerical one-dimensional time-dependent thundercloud model. The main cloud electrification hypotheses proposed for terrestrial thunderstorms are examined taking into account recent knowledge concerning Titans atmospheric state, especially condensation, and the lower ionosphere. Titans thunderclouds may be quite rare because of the difficult methane nucleation and low convection energy, but once moist convection is triggered, a massive cloud containing slowly falling solid and liquid methane particles may be expected. In this case the cloud rapidly attaches a large amount of free electrons, which are abundant in Titans troposphere due to the low abundance of electrophilic species. The entirely negative space charge in the cloud causes within a few minutes a temporary maximum electric field of 2×10 6 V m −1 which is sufficient to initiate 20 km long negative cloud-to-ground lightning strokes in Titans lower troposphere. The collisional charging, on the other hand, appears to be less efficient since the charge transfer itself may be limited at Titans cold temperatures and no substantial charge redistribution takes place in the cloud due to the weak updraft and gravitation.

Ionospheric layer induced by meteoric ionization in Titan's atmosphere

Abstract The ablation of meteoroids and the ionization of metallic ions in the atmosphere of Titan has been investigated. The ionization rates of the most abundant metals in cometary meteoroids, Si + ,Mg + and Fe + , are calculated from the meteoroid mass loss rate and the ionization probability of each metal. We have modeled the ion-neutral chemistry of metallic ions and calculated the concentration of the most abundant metal ions and electrons. We found that long-lived metallic ions considerably change the predictions of the electron density by the models which only consider solar radiation and electrons trapped in the magnetosphere of Saturn. The inclusion of metallic ions in the upper ionospheric models leads to an increase in the electron concentration below 800 km. We conclude that an ionospheric layer should be present at around 700 km with an electron density peak similar in magnitude to the one produced by solar radiation at 1000 km or by cosmic rays at 90 km.

Limnological Structure of Titan's Hydrocarbon Lakes and Its Astrobiological Implication

Cassini radar recently detected several putative liquid hydrocarbon lakes in the polar region of Saturns moon Titan. Such lakes may contain organic sediments deposited from the atmosphere that would promote prebiotic-type chemistry driven by cosmic rays, the result of which could be the production of more complex molecules such as nitrogen-bearing organic polymer or azides. The physical properties of the lake and their temporal evolution under Titans present climatic setting were investigated by means of a one-dimensional lake thermal stratification model. Lakes can undergo various evolutions, depending on the initial composition and depth of the lake and hydrocarbon abundance in the near-surface atmosphere. Pure methane ponds, which may occasionally form when heavy methane hailstones reach the surface, would be transitory in that they would evaporate, freeze up, and eventually dry up. On the other hand, lakes filled with a mixture of methane, ethane, and nitrogen would be more stable; and freezing or drying would not necessarily occur in most cases. Such lakes undergo a seasonal cycle of thermal stratification in spring and early summer and convective overturning in other seasons. The summer thermal stratification near the lake surface could be destabilized by bottom heating as a result of an enhanced geothermal heat flux, e.g., in the vicinity of cryovolcanoes. Most likely the composition of the lake and atmosphere would come to equilibrium by way of a small amount of evaporation, but the lake-atmosphere system could be repeatedly brought out of equilibrium by irregular precipitation. The viability of prebiotic-like chemistry in the lake may depend on many lake parameters, such as temperature, liquid or frozen state, and convective mixing. Moreover, convective mixing may drive suspension of solid acetylene and other sediments on the lake bottom and redistribution of dissolved gases, which might be relevant for putative life-forms that consume hydrogen and solid acetylene.

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

Lightning activity on Titan: can Cassini detect it?

Although no lightning discharges were observed during the Voyager 1 9yby of Titan, this lack of evidence does not rule out the existence of lightning phenomena which could be detected by the Radio and Plasma Wave Science (RPWS) instrument on board of the Cassini spacecraft. The existence of lightning or other electromagnetic discharges has been suggested to explain the formation of hydrocarbons and nitriles in the context of Titan’s complex organic chemistry. Although thunderclouds may be a rare phenomenon in Titan’s lower atmosphere, recent investigations show that such clouds may cause temporary maximum electrical =elds in the order of 2× 10 6 Vm −1 su>cient to initiate 20 km long Earth-like cloud-to-ground type 2 lightning strokes. Since such clouds are likely to be correlated with strongconvection near the subsolar point, we expect possible lig htning9ashes to occur only on the dayside. Recent telescopic infrared observations have detected localized, high, short-lived clouds on Titan. We have calculated the 9ash characteristics, frequency spectrum, maximum spectral energy and the electromagnetic energy radiated into the troposphere by using a wave guide model of lightning currents for Titan’s lightning strokes. Our study indicates that cloud-to-ground lightning strokes on Titan would be comparable with so-called type 2 lightning strokes on Earth. Their total radiated energy to the far =eld could be about 130 kJ and their maximum energy at a frequency of about 4 kHz. In order to estimate the capability of the Cassini=RPWS instrument to detect lightning discharges during several close Titan 9ybys, we distinguish the atmospheric regions, where the propagation of electromagnetic waves is unperturbed or where it is impossible. We found that the Cassini=RPWS instrument should be able to detect electromagnetic signals generated from a representative cloud-to-ground lightning stroke in Titan’s lower atmosphere in a frequency range above 500 kHz or 1 MHz up to 200 Titan radii away. We suggest that for the search of lightning signals the RPWS high-frequency receiver HF2 with its H2-1E or H2-1E=F receiver modes and low integration times D t of 10 or 20 ms should be chosen. Since the lightning 9ash rate might be low (i1 9ash per hour) it is important to have longobservation times. An analysis of all Cassini trajectories of Titan close 9ybys shows that the spacecraft would have the opportunity to observe Titan’s dayside within 100 Titan radii duringnearly all 9ybys. This time should be longenoug h, even if the lightning 9ash rate is low. c

Growth mechanisms and dune orientation on Titan

Dune fields on Titan cover more than 17% of the moons surface, constituting the largest known surface reservoir of organics. Their confinement to the equatorial belt, shape, and eastward direction of propagation offer crucial information regarding both the wind regime and sediment supply. Herein, we present a comprehensive analysis of Titans dune orientations using automated detection techniques on nonlocal denoised radar images. By coupling a new dune growth mechanism with wind fields generated by climate modeling, we find that Titans dunes grow by sediment transport on a nonmobile substratum. To be fully consistent with both the local crestline orientations and the eastward propagation of Titans dunes, the sediment should be predominantly transported by strong eastward winds, most likely generated by equinoctial storms or occasional fast westerly gusts. Additionally, convergence of the meridional transport predicted in models can explain why Titans dunes are confined within ±30° latitudes, where sediment fluxes converge.

Spatial inhomogeneity of the martian subsurface water distribution: implication from a global water cycle model

Abstract In an effort to test and to understand the global hydrogen distribution in the shallow subsurface of Mars retrieved by the Mars Odyssey gamma-ray spectrometer, the present state and movement of water are investigated by a coupled global subsurface–atmosphere water cycle model. It was found that the observed global subsurface hydrogen distribution is largely consistent with the modeled global water cycle, so a large fraction of hydrogen is likely to exist as water, at low and mid latitudes in the form of adsorbed water. Under the present climate the water content in the shallow subsurface becomes higher in the northern hemisphere than in the southern hemisphere as a result of global water cycle, regardless of the initial water distribution in the soil or adsorptive capacity. The higher annual maximum soil temperature in the south, stronger net northward transport of atmospheric water vapor, and the emission of vapor from the northern residual polar cap in northern summer contribute to this hemispheric asymmetry. The generally higher adsorptive capacity of clay minerals in the northern plains may further increase this bias. The longitudinal inhomogeneity is caused by several factors, such as thermal inertia, adsorptive capacity, and atmospheric surface pressure. The water abundance is locally high in low thermal inertia regions (e.g., Arabia Terra) and at deep places where the surface pressure is high (e.g., Hellas); it is low in soil with a low adsorptive capacity (e.g., Tharsis) and high thermal inertia regions (e.g., Solis Planum). Most of the soil humidity near the surface at low and mid latitudes may originate from the atmosphere. The model implies that the upper soil layer should be largely ice-free because otherwise an excessive sublimation and vapor emission into the atmosphere in warm seasons would violate the observational constraints. Moreover, the more uniform latitudinal variation of the observed hydrogen abundance near the surface compared to that of deeper layers is indicative of the presence of adsorbed water instead of ground ice because the adsorbed water content does not as steeply depend on latitude as the ground ice stability. Concerning the regolith mineralogy, montmorillonite can much better account for the observed water cycle than palagonite. While the presence of permanent ground ice appears likely in the polar region below a thin layer, large seasonal cycle of phase change between pore ice and adsorbed water may be possible. Regolith adsorption/desorption is neither negligible nor crucial for the seasonal atmospheric water cycle, but the surface–atmosphere coupling is a major prerequisite for the long-term evolution of subsurface water distribution.

Surface Temperatures on Titan During Northern Winter and Spring

Meridional brightness temperatures were measured on the surface of Titan during the 2004-2014 portion of the Cassini mission by the Composite Infrared Spectrometer. Temperatures mapped from pole to pole during five two year periods show a marked seasonal dependence. The surface temperature near the south pole over this time decreased by 2 K from 91.7 plus or minus 0.3 to 89.7 plus or minus 0.5 K while at the north pole the temperature increased by 1 K from 90.7 plus or minus 0.5 to 91.5 plus or minus 0.2 K. The latitude of maximum temperature moved from 19 S to 16 N, tracking the subsolar latitude. As the latitude changed, the maximum temperature remained constant at 93.65 plus or minus 0.15 K. In 2010 our temperatures repeated the north-south symmetry seen by Voyager one Titan year earlier in 1980. Early in the mission, temperatures at all latitudes had agreed with GCM predictions, but by 2014 temperatures in the north were lower than modeled by 1 K. The temperature rise in the north may be delayed by cooling of sea surfaces and moist ground brought on by seasonal methane precipitation and evaporation.

The dynamics of Titan's troposphere

While the Voyager mission could essentially not reveal the dynamics of Titans troposphere, useful information was obtained by the Cassini spacecraft and, particularly, by the Huygens probe that landed on Titans surface; this information can be interpreted by means of numerical models of atmospheric circulation. The meridional circulation is likely to consist of a large Hadley circulation asymmetric about the equator, but is susceptible to disruption by turbulence in clouds. The zonal wind in the troposphere is comparable to or even weaker than that in the terrestrial troposphere and contains zones of easterlies, much in contrast to the super-rotating stratosphere. Unique to Titan is the transition from a geostrophic to cyclostrophic wind balance in the upper troposphere. While Earth-like storm systems associated with baroclinic instability are absent, Saturns gravitational tide introduces a planetary wave of wavenumber 2 and a periodical variation in the wind direction in the troposphere. Unlike on Earth, the wind over the equatorial surface is westerly. The seasonal reversal in the Hadley circulation sense and zonal wind direction is predicted to have a substantial influence on the formation of dunes as well as variation of Titans rotation rate and length of day.