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The impact of methane convection on the circulation of Titan is investigated in the Titan Atmospheric Model (TAM), using a simplified Betts–Miller (SBM) moist convection parameterization scheme. We vary the reference relative humidity ( R H S B M ) and relaxation timescale of convection ( τ ) parameters of the SBM scheme. Titan’s atmosphere is mostly insensitive to changes in τ , but convective instability and precipitation are highly impacted by changes in R H S B M . Convection behavior changes from infrequent ( R H S B M to near-continuous precipitation at the poles during summer at high R H S B M (85%). The intermediate regime ( R H S B M =70%–80%) consists of frequent events ( ∼ 10 per Titan year) of moderate intensity that are limited in meridional extent to their respective hemisphere. Using results from the Titan Regional Atmospheric Modeling System (TRAMS) and observations, we tune the parameters of the SBM parameterization with optimum values of RH=80% and τ =28800 s. We present a simulated decadal climatology that qualitatively matches observations of Titan’s humidity and cloud activity and generally resembles previous results with TAM. Comparing this simulation to one without moist convection demonstrates that convection strengthens the meridional circulation, warms the mid-levels and cools the surface at the poles, and magnifies zonal-mean global moisture anomalies.

Using an in-house developed one dimensional photo-chemical model (1D-PCM), which considers production and loss of 11 ions namely, CO 2 + , CO + , C + , N 2 + , N + , He + , O + ( 2 D ) , O + ( 2 P ) , O + ( 4 S ) , O 2 + and NO + , characteristics of the V2 layer in the Venus ionosphere has been studied. It is noted that existing ionospheric model for the Venus ionosphere, such as the IonA (Ionization in Atmospheres) model, not only over/under estimate the peak electron density of V2 layer, it also has significant departures from the observations on the solar zenith angle and solar activity control. The IonA model uses VenusGRAM model (Venus Global Reference Atmosphere Model) as input for the neutral density and temperature and considers Venus atmosphere consisting of CO2, O, and N2 molecules only. Further, it oversimplifies the ion chemistry by assuming Venus ionosphere to have O 2 + as the only dominant ion species. Using VTS3 model, an empirical model based on measurements from Orbiter Neutral Mass Spectrometer on Pioneer Venus Orbiter (PVO) which considers profiles of six neutrals (CO2, O, CO, He, N, and N2), we modified IonA model, named as IonA-VTS3, to find that it reproduced the altitude of V2 peak electron density (hmV2) quite well. However, the model still lacked in reproducing observed peak V2 electron density (NmV2). The in-house developed one dimensional photo-chemical model (1D-PCM) not only estimated NmV2 accurately, the hmV2 was also reproduced quite well. Comparison of Venera and PVO radio occultation measurements with 1D-PCM and IonA-VTS3 calculations reveals the role of complex chemical reactions in determining the features of peak altitude and density of V2 layer during different solar activity periods. We surmised that differences in the observed and IonA modeled peak V2 layer altitudes were due to the limitations associated with VenusGRAM neutral density model. It shows that variations in the neutral density controls the V2 layer peak density height. 1D-PCM calculations also showed that the complex chemistry including production and loss reactions of 11 ions could reproduce the variations in the peak density of Venusian ionosphere during different solar activity conditions. It suggests that the ion-chemistry has wider control over the peak plasma density in the Venus ionosphere.

The deep convective cloud–environment feedback loop is likely important to Titan's global methane, energy, and momentum cycles, just as it is for Earth's global water, energy, and momentum budgets. General circulation models of Titan's atmosphere are unable to explicitly simulate deep convection and must instead parameterize the impact of this important subgrid-scale phenomenon on the model-resolved atmospheric state. The goal of this study is to better quantify through cloud resolving modeling the effects of deep convective methane storms on their environment and to feed that information forward to improve parameterizations in global models. Dozens of atmospheric profiles unstable with respect to deep moist convection are extracted from the global Titan Atmospheric Model (TAM) and used to initialize the cloud-resolving Titan Regional Atmospheric Modeling System (TRAMS). Mean profiles of heating/cooling and moistening/drying of the large-scale environment in TRAMS indicate that Titan's deep convection forces the environment in a manner analogous to Earth: Large-scale subsidence of the environmental air warms and dries the environment, but clouds can also moisten the environment through the detrainment and evaporation of condensate near cloud top. Relative humidity profiles and characteristic convective time scales are derived to guide the tuning of the deep convective parameterization implemented in TAM, as described in a companion paper. The triggering of convection, the dry convective mixing of the planetary boundary layer, and the entrainment of environmental air into rising air parcels are found to be critical to determining whether a deep convective cloud will form. Only profiles with relatively large convective available potential energy (CAPE) and well mixed planetary boundary layers with high relative humidity were found to produce storms. Environments with low-level thermal inversions and planetary boundary layers with low relative humidity or rapidly decreasing moisture with height failed to generate deep convection in TRAMS despite positive CAPE.

Thermal cooling rates QC and solar heating rates QH in the atmosphere of Venus at altitudes between 0 and 100 km are investigated using the radiative transfer and radiative balance simulation techniques described by Haus et al. (2015b, 2016). QC strongly responds to temperature profile and cloud parameter changes, while QH is less sensitive to these parameters. The latter mainly depends on solar insolation conditions and the unknown UV absorber distribution. A parameterization approach is developed that permits a fast and reliable calculation of temperature change rates Q for different atmospheric model parameters and that can be applied in General Circulation Models to investigate atmospheric dynamics. A separation of temperature, cloud parameter, and unknown UV absorber influences is performed. The temperature response parameterization relies on a specific altitude and latitude-dependent cloud model. It is based on an algorithm that characterizes Q responses to a broad range of temperature perturbations at each level of the atmosphere using the Venus International Reference Atmosphere (VIRA) as basis temperature model. The cloud response parameterization considers different temperature conditions and a range of individual cloud mode factors that additionally change cloud optical depths as determined by the initial latitude-dependent model. A QH response parameterization for abundance changes of the unknown UV absorber is also included. Deviations between accurate calculation and parameterization results are in the order of a few tenths of K/day at altitudes below 90 km. The parameterization approach is used to investigate atmospheric radiative equilibrium (RE) conditions. Polar mesospheric RE temperatures above the cloud top are up to 70 K lower and equatorial temperatures up to 10 K higher than observed values. This radiative forcing field is balanced by dynamical processes that maintain the observed thermal structure.

During accretion, the young rocky planets are so hot that they become endowed with a magma ocean. From that moment, the mantle convective thermal flux control the cooling of the planet and an atmosphere is created by outgassing. This atmosphere will then play a key role during this cooling phase. Studying this cooling phase in details is a necessary step to explain the great diversity of the observed telluric planets and especially to assess the presence of surface liquid water. We used here a radiative-convective 1D atmospheric model (H2O, CO2) to study the impact of the Bond albedo on the evolution of magma ocean planets. We derived from this model the thermal emission spectrum and the spectral reflectance of these planets, from which we calculated their Bond albedos. Compared to Marcq et al. (2017), the model now includes a new module to compute the Rayleigh scattering, and state of the art CO2 and H2O gaseous opacities data in the visible and infrared spectral ranges. We show that the Bond albedo of these planets depends on their surface temperature and results from a competition between Rayleigh scattering from the gases and Mie scattering from the clouds. The colder the surface temperature is, the thicker the clouds are, and the higher the Bond albedo is. We also evidence that the relative abundances of CO2 and H2O in the atmosphere strongly impact the Bond albedo. The Bond albedo is higher for atmospheres dominated by the CO2, better Rayleigh scatterer than H2O. Finally, we provide the community with an empirical formula for the Bond albedo that could be useful for future studies of magma ocean planets., 16 pages, 10 figures, 5 tables. Accepted for publication in the Journal Icarus the 27 August 2018

The original Strobel et al. (Strobel, D.F., Zhu, X., Summers, M.E., Stevens, M.E. [1996]. Icarus 120, 266–289) model for Pluto’s stratospheric density and thermal structure is augmented to include a radial momentum equation with radial velocity associated with atmospheric escape of N2 and in the energy equation to also include the solar far ultraviolet and extreme ultraviolet (FUV–EUV) heating in the upper atmosphere and adiabatic cooling due to hydrodynamic expansion. The inclusion of radial velocity introduces important negative feedback processes such as increased solar heating leading to enhanced escape rate and higher radial velocity with stronger adiabatic cooling in the upper atmosphere accompanied by reduced temperature. The coupled set of equations for mass, momentum, and energy are solved subject to two types of upper boundary conditions that represent two different descriptions of atmospheric escape: Jeans escape and hydrodynamic escape. For the former which is physically correct, an enhanced Jeans escape rate is prescribed at the exobase and parameterized according to the direct simulation Monte Carlo kinetic model results. For the latter, the atmosphere is assumed to remain a fluid to infinity with the escape rate determined by the temperature and density at the transonic point subject to vanishing temperature and pressure at infinity. For Pluto, the two escape descriptions approach the same limit when the exobase coincides with the transonic level and merge to a common escape rate ∼1028 N2 s−1 under elevated energy input. For Pluto’s current atmosphere, the hydrodynamic approach underestimates the escape rate by about 13%. In all cases, the escape rate is limited by the solar FUV–EUV power input. Specific results for the New Horizons Pluto flyby July 2015 are escape rate ∼3.5 × 1027 N2 s−1, exobase at 8r0 ∼ 9600 km, with Jeans λ ∼ 5 for a reference Pluto atmosphere model. With Pluto’s highly elliptic orbit and variable solar activity affecting its atmosphere, Pluto’s escape rates’ range is (1–10) × 1027 N2 s−1, exobase radius is bounded by ∼(5–13)r0, and at the exobase Pluto is locked in the enhanced Jeans regime with λ ∼ (6–4). Finally, a systematic review of previous approximate hydrodynamic escape models is presented to compare the constraints which determine the escape rate in each model.

We introduce and present results from a new general circulation model for Mars adapted from the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM) version 3.1 terrestrial model. The radiative transfer has been replaced with a two-stream correlated-k scheme with carbon dioxide gas absorption coefficients suited for Mars. A time-invariant dust field is assumed with a Conrath (Conrath, B.J. [1975]. Icarus 24, 34–46) vertical distribution. Carbon dioxide is allowed to sublimate and condense at the surface, and the mass is removed from the atmosphere. The topography is averaged from MOLA data. The surface albedos and thermal inertias are derived from TES observations. The model is freely distributed to interested users. Comparisons between model temperatures, and spacecraft and Lander observations show agreement within ±10 K, depending on dust concentration. The annual pressure cycle is typically within 20 Pa of Viking Lander observations, however the model underestimates the surface pressure during southern summer, possibly due to increased dust activity that is not reflected in the model. Predicted model boundary layer depths are typically within a few hundred meters of observations, and tend to depend inversely on surface pressure, agreeing with observations.

This work focuses on determining the latitudinal structure of ammonia vapor in Saturn's cloud layer near 1.5 bars using the brightness temperature maps derived from the Cassini RADAR (Elachi et al., 2004) instrument, which works in a passive mode to measure thermal emission from Saturn at 2.2-cm wavelength. We perform an analysis of five brightness temperature maps that span epochs from 2005 to 2011, which are presented in a companion paper by Janssen et al. (2013a, this issue). The brightness temperature maps are representative of the spatial distribution of ammonia vapor, since ammonia gas is the only effective opacity source in Saturn's atmosphere at 2.2-cm wavelength. Relatively high brightness temperatures indicate relatively low ammonia relative humidity (RH), and vice versa. We compare the observed brightness temperatures to brightness temperatures computed using the Juno atmospheric microwave radiative transfer (JAMRT) program which includes both the means to calculate a tropospheric atmosphere model for Saturn and the means to carry out radiative transfer calculations at microwave frequencies. The reference atmosphere to which we compare has a 3x solar deep mixing ratio of ammonia (we use 1.352x10(exp -4) for the solar mixing ratio of ammonia vapor relative to H2; see Atreya, 2010) and is fully saturated above its cloud base. The maps are comprised of residual brightness temperatures-observed brightness temperature minus the model brightness temperature of the saturated atmosphere.

One significant difference between the atmospheres of stars and exoplanets is the presence of condensed particles (clouds or hazes) in the atmosphere of the latter. The main goal of this paper is to develop a self-consistent microphysical cloud model for 1D atmospheric codes, which can reproduce some observed properties of Earth, such as the average albedo, surface temperature, and global energy budget. The cloud model is designed to be computationally efficient, simple to implement, and applicable for a wide range of atmospheric parameters for planets in the habitable zone. We use a 1D, cloud-free, radiative-convective, and photochemical equilibrium code originally developed by Kasting, Pavlov, Segura, and collaborators as basis for our cloudy atmosphere model. The cloud model is based on models used by the meteorology community for Earth's clouds. The free parameters of the model are the relative humidity and number density of condensation nuclei, and the precipitation efficiency. In a 1D model, the cloud coverage cannot be self-consistently determined, thus we treat it as a free parameter. We apply this model to Earth (aerosol number density 100 cm^-3, relative humidity 77 %, liquid cloud fraction 40 %, and ice cloud fraction 25 %) and find that a precipitation efficiency of 0.8 is needed to reproduce the albedo, average surface temperature and global energy budget of Earth. We perform simulations to determine how the albedo and the climate of a planet is influenced by the free parameters of the cloud model. We find that the planetary climate is most sensitive to changes in the liquid water cloud fraction and precipitation efficiency. The advantage of our cloud model is that the cloud height and the droplet sizes are self-consistently calculated, both of which influence the climate and albedo of exoplanets., To appear in Icarus

We investigate the sensitivity of the circulation and thermal structure of the martian atmosphere to numerical model resolution in a general circulation model (GCM) using the martian implementation (MarsWRF) of the planetWRF atmospheric model. We provide a description of the MarsWRF GCM and use it to study the global atmosphere at horizontal resolutions from 7.5° × 9° to 0.5° × 0.5°, encompassing the range from standard Mars GCMs to global mesoscale modeling. We find that while most of the gross-scale features of the circulation (the rough location of jets, the qualitative thermal structure, and the major large-scale features of the surface level winds) are insensitive to horizontal resolution over this range, several major features of the circulation are sensitive in detail. The northern winter polar circulation shows the greatest sensitivity, showing a continuous transition from a smooth polar winter jet at low resolution, to a distinct vertically “split” jet as resolution increases. The separation of the lower and middle atmosphere polar jet occurs at roughly 10 Pa, with the split jet structure developing in concert with the intensification of meridional jets at roughly 10 Pa and above 0.1 Pa. These meridional jets appear to represent the separation of lower and middle atmosphere mean overturning circulations (with the former being consistent with the usual concept of the “Hadley cell”). Further, the transition in polar jet structure is more sensitive to changes in zonal than meridional horizontal resolution, suggesting that representation of small-scale wave-mean flow interactions is more important than fine-scale representation of the meridional thermal gradient across the polar front. Increasing the horizontal resolution improves the match between the modeled thermal structure and the Mars Climate Sounder retrievals for northern winter high latitudes. While increased horizontal resolution also improves the simulation of the northern high latitudes at equinox, even the lowest model resolution considered here appears to do a good job for the southern winter and southern equinoctial pole (although in detail some discrepancies remain). These results suggest that studies of the northern winter jet (e.g., transient waves and cyclogenesis) will be more sensitive to global model resolution that those of the south (e.g., the confining dynamics of the southern polar vortex relevant to studies of argon transport). For surface winds, the major effect of increased horizontal resolution is in the superposition of circulations forced by local-scale topography upon the large-scale surface wind patterns. While passive predictions of dust lifting are generally insensitive to model horizontal resolution when no lifting threshold is considered, increasing the stress threshold produces significantly more lifting in higher resolution simulations with the generation of finer-scale, higher-stress winds due primarily to better-resolved topography. Considering the positive feedbacks expected for radiatively active dust lifting, we expect this bias to increase when such feedbacks are permitted.

On its highly elliptical 24. h orbit around Venus, the Venus Express (VEX) spacecraft briefly reaches a periapsis altitude of nominally 250. km. Recently, however, dedicated and intense radio tracking campaigns have taken place in August 2008, October 2009, February and April 2010, for which the periapsis altitude was lowered to the 186-176. km altitude range in order to be able to probe the upper atmosphere of Venus above the North Pole for the first time ever in situ. As the spacecraft experiences atmospheric drag, its trajectory is measurably perturbed during the periapsis pass, allowing us to infer total atmospheric mass density at the periapsis altitude. A Precise Orbit Determination (POD) of the VEX motion is performed through an iterative least-squares fitting process to the Doppler tracking data, acquired by the VEX radioscience experiment (VeRa). The drag acceleration is modelled using an initial atmospheric density model (VTS3 model, Hedin, A.E., Niemann, H.B., Kasprzak, W.T., Seiff, A. [1983]. J. Geophys. Res. 88, 73-83). A scale factor of the drag acceleration is estimated for each periapsis pass, which scales Hedin's density model in order to best fit the radio tracking data. Reliable density scale factors have been obtained for 10 passes mainly from the second (October 2009) and third (April 2010) VExADE campaigns, which indicate a lower density by a factor of about 1.8 than Hedin's model predicts. These first ever in situ polar density measurements at solar minimum have allowed us to construct a diffusive equilibrium density model for Venus' thermosphere, constrained in the lower thermosphere primarily by SPICAV-SOIR measurements and above 175. km by the VExADE drag measurements (Muller-Wodarg et al., in preparation). The preliminary results of the VExADE campaigns show that it is possible to obtain with the POD technique reliable estimates of Venus' upper atmosphere densities at an altitude of around 175. km. Future VExADE campaigns will benefit from the planned further lowering of VEX pericenter altitude to below 170. km. © 2011 Elsevier Inc.

Molecular level Monte Carlo simulations have been performed with various model potentials for the CH4–N2 vapor–liquid equilibrium at conditions prevalent in the atmosphere of Saturn’s moon Titan. With a single potential parameter adjustment to reproduce the vapor–liquid equilibrium at a higher temperature, Monte Carlo simulations are in excellent agreement with available laboratory measurements. The results demonstrate the ability of simple pair potential models to describe phase equilibria with the requisite accuracy for atmospheric modeling, while keeping the number of adjustable parameters at a minimum. This allows for stable extrapolation beyond the range of available laboratory measurements into the supercooled region of the phase diagram, so that Monte Carlo simulations can serve as a reference to validate phenomenological models commonly used in atmospheric modeling. This is most important when the relevant region of the phase diagram lies outside the range of laboratory measurements as in the case of Titan. The present Monte Carlo simulations confirm the validity of phenomenological thermodynamic equations of state specifically designed for application to Titan. The validity extends well into the supercooled region of the phase diagram. The possible range of saturation levels of Titan’s troposphere above altitudes of 7 km is found to be completely determined by the remaining uncertainty of the most recent revision of the Cassini-Huygens data, yielding a saturation of 100 ± 6% with respect to CH4–N2 condensation up to an altitude of about 20 km.

First measurements of SO 2 and SO in the Venus mesosphere (70–100 km) are reported. This altitude range is distinctly above the ∼60–70 km range to which nadir-sounding IR and UV investigations are sensitive. Since July 2004, use of ground-based sub-mm spectroscopy has yielded multiple discoveries. Abundance of each molecule varies strongly on many timescales over the entire sub-Earth Venus hemisphere. Diurnal behavior is evident, with more SO 2 , and less SO, at night than during the day. Non-diurnal variability is also present, with measured SO 2 and SO abundances each changing by up to 2× or more between observations conducted on different dates, but at fixed phase, hence identical sub-Earth Venus local times. Change as large and rapid as a 5 σ doubling of SO on a one-week timescale is seen. The sum of SO 2 and SO abundances varies by an order of magnitude or more, indicating at least one additional sulfur reservoir must be present, and that it must function as both a sink and source for these molecules. The ratio SO 2 /SO varies by nearly two orders of magnitude, with both diurnal and non-diurnal components. In contrast to the strong time dependence of molecular abundances, their altitude distributions are temporally invariant, with far more SO 2 and SO at 85–100 km than at 70–85 km. The observed increase of SO 2 mixing ratio with altitude requires that the primary SO 2 source be upper mesospheric photochemistry, contrary to atmospheric models which assert upward transport as the only source of above-cloud SO 2 . Abundance of upper mesospheric aerosol, with assumption that it is composed primarily of sulfuric acid, is at least sufficient to provide the maximum gas phase (SO + SO 2 ) sulfur reported in this study. Sulfate aerosol is thus a plausible source of upper mesospheric SO 2 .

We analyze FUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus. We use a least-squares fit method to determine the brightness of the OI emissions at 130.4 and OI 135.6 nm, and of the bands of the CO fourth positive system which are dominated by fluorescence scattering. We compare the brightness observed along the UVIS foot track of the two OI multiplets with that deduced from a model of the excitation of these emissions by photoelectron impact on O atoms and resonance scattering of the solar 130.4 nm emission. The large optical thickness 130.4 nm emission is accounted for using a radiative transfer model. The airglow intensities are calculated along the foot track and found to agree with the observed 130.4 nm brightness within ∼10%. The modeled OI 135.6 nm brightness is also well reproduced by the model. The oxygen density profile of the VTS3 model is found to be consistent with the observations. We find that self-absorption of the (0, v ″) bands of the fourth positive emission of CO is important and we derive a CO vertical column of about 6.4 × 10 15 cm −2 in close agreement with the value provided by the VTS3 empirical atmospheric model.

Conflicting observations regarding the dominance of either sublimation or volcanism as the source of the atmosphere on Io and disparate reports on the extent of its spatial distribution and the absolute column abundance invite the development of detailed computational models capable of improving our understanding of Io’s unique atmospheric structure and origin. Improving upon previous models, Walker et al. (Walker, A.C., Gratiy, S.L., Levin, D.A., Goldstein, D.B., Varghese, P.L., Trafton, L.M., Moore, C.H., Stewart, B. [2009]. Icarus) developed a fully 3-D global rarefied gas dynamics model of Io’s atmosphere including both sublimation and volcanic sources of SO 2 gas. The fidelity of the model is tested by simulating remote observations at selected wavelength bands and comparing them to the corresponding astronomical observations of Io’s atmosphere. The simulations are performed with a new 3-D spherical-shell radiative transfer code utilizing a backward Monte Carlo method. We present: (1) simulations of the mid-infrared disk-integrated spectra of Io’s sunlit hemisphere at 19 μm, obtained with TEXES during 2001–2004; (2) simulations of disk-resolved images at Lyman- α obtained with the Hubble Space Telescope (HST), Space Telescope Imaging Spectrograph (STIS) during 1997–2001; and (3) disk-integrated simulations of emission line profiles in the millimeter wavelength range obtained with the IRAM-30 m telescope in October–November 1999. We found that the atmospheric model generally reproduces the longitudinal variation in band depth from the mid-infrared data; however, the best match is obtained when our simulation results are shifted ∼30° toward lower orbital longitudes. The simulations of Lyman- α images do not reproduce the mid-to-high latitude bright patches seen in the observations, suggesting that the model atmosphere sustains columns that are too high at those latitudes. The simulations of emission line profiles in the millimeter spectral region support the hypothesis that the atmospheric dynamics favorably explains the observed line widths, which are too wide to be formed by thermal Doppler broadening alone.

Io’s sublimation-driven atmosphere is modeled using the direct simulation Monte Carlo (DSMC) method. These rarefied gas dynamics simulations improve upon earlier models by using a three-dimensional domain encompassing the entire planet computed in parallel. The effects of plasma heating, planetary rotation, inhomogeneous surface frost, molecular residence time of SO 2 on the exposed (non-volatile) rocky surface, and surface temperature distribution are investigated. Circumplanetary flow is predicted to develop from the warm dayside toward the cooler nightside. Io’s rotation leads to a highly asymmetric frost surface temperature distribution (due to the frost’s high thermal inertia) which results in circumplanetary flow that is not axi-symmetric about the subsolar point. The non-equilibrium thermal structure of the atmosphere, specifically vibrational and rotational temperatures, is also examined. Plasma heating is found to significantly inflate the atmosphere on both the dayside and nightside. The plasma energy flux causes high temperatures at high altitudes but plasma energy depletion through the dense gas column above the warmest frost permits gas temperatures cooler than the surface at low altitudes. A frost map (Doute, S., Schmitt, B., Lopes-Gautier, R., Carlson, R., Soderblom, L., Shirley, J., and the Galileo NIMS Team [2001]. Icarus 149, 107–132) is used to control the sublimated flux of SO 2 which can result in inhomogeneous column densities that vary by nearly a factor of four for the same surface temperature. A short residence time for SO 2 molecules on the “rock” component is found to smooth lateral atmospheric inhomogeneities caused by variations in the surface frost distribution, creating an atmosphere that looks nearly identical to one with uniform frost coverage. A longer residence time is found to agree better with mid-infrared observations (Spencer, J.R., Lellouch, E., Richter, M.J., Lopez-Valverde, M.A., Jessup, K.L, Greathouse, T.K., Flaud, J. [2005]. Icarus 176, 283–304) and reproduce the observed anti-jovian/sub-jovian column density asymmetry. The computed peak dayside column density for Io assuming a surface frost temperature of 115 K agrees with those suggested by Lyman-α observations (Feaga, L.M., McGrath, M., Feldman, P.D. [2009]. Icarus 201, 570–584). On the other hand, the peak dayside column density at 120 K is a factor of five larger and is higher than the upper range of observations (Jessup, K.L., Spencer, J.R., Ballester, G.E., Howell, R.R., Roesler, F., Vigel, M., Yelle, R. [2004]. Icarus 169, 197–215; Spencer et al., 2005 ).

We identify mechanisms controlling the distribution of methane convection and large-scale circulation in a simplified, axisymmetric model atmosphere of Titan forced by gray radiation and moist (methane) convection. The large-scale overturning circulation, or Hadley cell, is global in latitudinal extent and provides fundamental control of precipitation and tropospheric winds. The precipitating, large-scale updraft regularly oscillates in latitude with seasons. The distance of greatest poleward excursion of the Hadley cell updraft is set by the mass of the convective layer of the atmosphere; convection efficiently communicates seasonal warming of the surface through the cold and dense lower atmosphere, increasing the heat capacity of the system. The presence of deep, precipitating convection introduces three effects relative to the case with no methane latent heating: (1) convection is narrowed and enhanced in the large-scale updraft of the Hadley cell; (2) the latitudinal amplitude of Hadley cell updraft oscillations is decreased; and (3) a time lag is introduced. These effects are observable in the location and timing of convective methane clouds in Titan’s atmosphere as a function of season. A comparison of simulations over a range of convective regimes with available observations suggest methane thermodynamic–dynamic feedback is important in the Titan climate.

Europa is bombarded by intense radiation that erodes the surface, launching molecules into a thin “atmosphere” representative of surface composition. In addition to atoms and molecules created in the mostly water ice surface such as H 2 O, O 2 , H 2 , the atmosphere is known to have species representative of trace surface materials. These trace species are carried off with the 10–10 4 H 2 O molecules ejected by each energetic heavy ion, a process we have simulated using molecular dynamics. Using the results of those simulations, we found that a neutral mass spectrometer orbiting ∼100 km above the surface could detect species with surface concentrations above ∼0.03%. We have also modeled the atmospheric spatial structure of the volatile species CO 2 and SO 2 under a variety of assumptions. Detections of these species with moderate time and space resolution would allow us to constrain surface composition, chemistry and to study space weathering processes.

This VIRTIS instrument on board Venus Express has collected spectrally resolved images of the Venus nightside limb that show the presence of the (0,0) band of the Δ g 1 → Σ g 3 infrared atmospheric system of O2 at 1.27 μm. The emission is produced by three-body recombination of oxygen atoms created by photodissociation of CO2 on the dayside. It is consistently bright so that emission limb profiles can be extracted from the images. The vertical distribution of O2( Δ g 1 ) may be derived following Abel inversion of the radiance limb profiles. Assuming photochemical equilibrium, it is combined with the CO2 vertical distribution to determine the atomic oxygen density. The uncertainties on the O density caused by the Abel inversion reach a few percent at the peak, increasing to about 50% near 120 km. We first analyze a case when the CO2 density was derived from a stellar occultation observed with the SPICAV spectrometer simultaneously with an image of the O2 limb airglow. In other cases, an average CO2 profile deduced from a series of ultraviolet stellar occultations is used to derive the O profile, leading to uncertainties on the O density less than 30%. It is found that the maximum O density is generally located between 94 and 115 km with a mean value of 104 km. It ranges from less than 1 × 10 11 to about 5 × 10 11 cm −3 with a global mean of 2.2 × 10 11 cm −3 . These values are in reasonable agreement with the VIRA midnight oxygen profile. The vertical O distribution is generally in good agreement with the oxygen profile calculated with a one-dimensional chemical-diffusive model. No statistical latitudinal dependence of the altitude of the oxygen peak is observed, but the maximum O density tends to decrease with increasing northern latitudes. The latitudinal distribution at a given time exhibits large variations in the O density profile and its vertical structure. The vertical oxygen distribution frequently shows multiple peaks possibly caused by waves or variations in the structure of turbulent transport. It is concluded that the O2 infrared night airglow is a powerful tool to map the distribution of atomic oxygen in the mesosphere between 90 and 115 km and improve future Venus reference atmosphere models.

On August 11, 2004, we made adaptive optics observations of the Uranus and Neptune systems with the Keck II Near Infrared Camera. Uranus and Triton were observed in three broadband filters (J, H, and K-prime) and four narrowband filters [Hcont, FeII, He1_B, and H 2 ( v = 1 - 0 ) ]. Miranda, Ariel, Umbriel, and Oberon were observed in the four narrowband filters only. To achieve the highest possible photometric accuracy, and thus the tightest possible constraints on atmospheric aerosol models and gas mixing ratios, we used aperture photometry that accounted for the dependence of point-spread functions and growth curves on the adaptive optics reference object, and accounted for recently determined phase curves of each object. The satellite albedos we determined were compared with published relative spectra to verify the relative consistency of our observations, which were subsequently used to convert published relative spectra to absolute spectra. We used these absolute spectra and synthetic photometry methods to compare published values in dissimilar photometric systems to each other and to our observations. We find our satellite albedos in best agreement with photometry from Karkoschka [Karkoschka, E., 2001. Icarus 151, 51–68], which is the best characterized and most contemporaneous data set. Our estimated absolute accuracy of 5% to 8% is consistent with these intercomparisons. We obtained the following ring-subtracted and discrete feature-free albedos of Uranus: J: ( 1.66 ± 0.07 ) × 10 −2 , H: ( 1.09 ± 0.05 ) × 10 −2 , K ′ : ( 2.08 ± 0.15 ) × 10 −4 , Hcont: ( 3.71 ± 0.23 ) × 10 −2 , FeII: ( 1.14 ± 0.07 ) × 10 −3 , He1_B: ( 2.06 ± 0.16 ) × 10 −4 , and H2: ( 1.27 ± 0.10 ) × 10 −4 .

For impact craters with dimensions such as the Ries crater (corresponding to a 1 km meteorite) it has become a standard reference in textbooks on planetary science that under terrestrial conditions distal transfer of boulders may reach as far as 200 km. In order to test this assumption we simulated the impact-induced ballistic transfer of limestone boulders ejected out of the Ries crater and have come to the conclusion that “Reutersche Blocke” and “Ries-Brockhorizonte,” found at distances of up to 130 km away, are distal Ries ejecta. Boulders alleged to be Ries components found in Northern Switzerland at distances of up to 200 km away can be related to the Ries event, if the parameters of our numerical simulation are stretched to its limits. Our simulation includes the following assumptions and variables: (1) boulders are ejected from the interference zone at a very early stage of impact; (2) starting conditions may range between velocities of 1 and 4 km/s and 35° to 65° for the flight path angle; (3) drag-free and transitional conditions at the impact site have been incorporated into the density model of the atmosphere; (4) a typical boulder is represented by an suitable aerodynamic drag model; (5) an aerothermal heat model was used to determine heat load.

In order to evaluate the obliquity-driven atmospheric-density path length effect on nuclide production rate on Mars, we performed a Monte-Carlo simulation to produce the number of secondary particles such as muons, neutrons and protons in the martian atmosphere and to simulate that production of 10 Be and 36 Cl in the martian regolith by muons and neutrons depends on how much atmosphere had been present for the past 10 million years. The vertical profile of the present martian atmosphere to generate secondary particles has been determined based on the data provided by the Viking missions. For other thickness profiles, we scaled Linsley's atmospheric model. Atmospheric shower has been generated with the SIBYLL 2.1 for high-energy hadronic interactions and EHSA for low energy photonuclear interactions. With increasing atmospheric thickness, more primary interactions occur in the atmosphere. Consequently the proton flux is reduced and the secondary cosmic ray flux, such as muons or energetic neutrons increases at surface. The result indicates that the muon production is more sensitive to obliquity-driven atmospheric variations than proton reduction. A thicker atmosphere would result in enhanced nuclide production at a place deeper than 5 m below the surface and the nuclides present in detectable concentrations. Application to the polar deposit is described.

Near-infrared observations of the nightside of Venus reveal regions of high brightness temperatures. These regions of high brightness temperatures are caused by the localized evaporation of the middle and lower cloud decks, which are about 50 to 60 km above the surface of the planet. We simulate the Venus condensational middle and lower cloud deck with the University of Colorado/NASA Ames Community Aerosol and Radiation Model for Atmospheres (CARMA). Our simulated clouds have similar characteristics to the observed Venus clouds. Our radiative transfer model reproduces the observed temperature structure and atmospheric stability structure within the middle cloud region. A radiative-dynamical feedback occurs which generates mixing due to increased absorption of upwelling infrared radiation within the lower cloud region, as previously suggested by others. We find that localized variations in temperature structure or in sub-grid scale mixing cannot directly explain the longevity and optical depth of the clouds. However, vertical motions are capable of altering the cloud optical depth by a sufficient magnitude in a short enough timescale to be responsible for the observed clearings.

We report the detection of H13CN and HC15N in mid-infrared spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard Cassini, along with the determination of the 12C/13C and 14N/15N isotopic ratios. We analyzed two sets of limb spectra recorded near 13–15° S (Tb flyby) and 83° N (T4 flyby) at 0.5 cm−1 resolution. The spectral range 1210–1310 cm−1 was used to retrieve the temperature profile in the range 145–490 km at 13° S and 165–300 km at 83° N. These two temperature profiles were then incorporated in the atmospheric model to retrieve the abundance profile of H12C14N, H13CN and HC15N from their bands at 713, 706 and 711 cm−1, respectively. The HCN abundance profile was retrieved in the range 90–460 km at 15° S and 165–305 km at 83° N. There is no evidence for vertical variations of the isotopic ratios. Constraining the isotopic abundance profiles to be proportional to the HCN one, we find C 12 / C 13 = 89 −18 +22 at 15° S, and 68 −12 +16 at 83° N, two values that are statistically consistent. A combination of these results yields a 12C/13C value equal to 75 ± 12 . This global result, as well as the 15° S one, envelop the value in Titan's methane ( 82.3 ± 1 ) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779–784] measured at 10° S and is slightly lower than the terrestrial inorganic standard value (89). The 14N/15N isotopic ratio is found equal to 56 −13 +16 at 15° S and 56 −9 +10 at 83° N. Combining the two values yields 14N/15N = 56 ± 8, which corresponds to an enrichment in 15N of about 4.9 compared with the terrestrial ratio. These results agree with the values obtained from previous ground-based millimeter observations [Hidayat, T., Marten, A., Bezard, B., Gautier, D., Owen, T., Matthews, H.E., Paubert, G., 1997. Icarus 126, 170–182; Marten, A., Hidayat, T., Biraud, Y., Moreno, R., 2002. Icarus 158, 532–544]. The 15N/14N ratio found in HCN is ∼3 times higher than in N2 [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779–784], which implies a large fractionation process in the HCN photochemistry.

Hubble Space Telescope observations revealed that Saturn’s equatorial jet at the cloud level blows at ∼275 m s −1 today, approximately half the ∼470 m s −1 wind during the Voyager flybys in 1980–1981. Radiative transfer calculations estimate the clouds to be significantly higher today than in 1980. The higher clouds make it difficult to observationally isolate any true slowdown from the vertical wind shear because Voyager and Cassini observations show that the winds become slower with altitude. Here, we test the hypothesis that the large equatorial storm in 1990 called the Great White Spot (GWS) decelerated the equatorial jet. We first use order of magnitude estimates to show: (1) if the GWS triggers vertical momentum redistribution, a minor speed change in the troposphere can lead to a substantial stratospheric wind speed change; (2) stormtriggered turbulent mixing slows a prograde equatorial jet; and (3) a prograde equatorial jet inhibits turbulent mixing in latitude. To test whether a GWS-like large storm decelerates the equatorial jet, we perform numerical experiments using the Explicit Planetary Isentropic Coordinate (EPIC) atmosphere model. Our simulation results are consistent with our order of magnitude predictions. We show that the storm excites waves, and the waves transport westward momentum from the troposphere to the stratosphere and decelerate the equatorial jet by as much as ∼40 m s −1 at the 10-mbar level. However, our results show that the storm’s effect is too weak at the cloud levels to halve the jet’s speed from ∼470 m s −1 .O ur results suggest that a combination of higher clouds and a true slowdown is necessary to explain the apparent equatorial jet slowdown. We also analyze the effect of waves on the apparent cloud motions, and show that waves can influence cloud-tracking wind speed measurements.

The dynamics of mergers of large circulations in Jupiter's atmosphere may permit different models of the atmosphere to be tested. We report well-resolved observations of such events at visible wavelengths: three anticyclonic and three cyclonic events. A merger of anticyclonic white ovals in the South South Temperate domain (2002 March) is compared with the previously reported merger of ovals BE and FA in the South Temperate domain (2000 March). In each case, the two similar-sized ovals converged rapidly once they were separated by less than the sum of their diameters; they orbited around each other anticyclonically during the merger; the merged oval initially had the same rapid drift as the western parent; and, in an unexpected similarity, a cyclonic oval emerged westward from the point of merger. Evidence suggests that a merger of smaller ovals in the North North Temperate domain (2002 February) had similar dynamics. In contrast, mergers of cyclonic ovals in the North Equatorial Belt (‘barges’: 2001 November, 2005 May) proceeded in a different manner. The two parent barges showed no consistent acceleration towards each other as they converged; on contact there was no obvious sign of mutual circulation, and the low-albedo regions had almost passed each other before they finally merged; and the resulting barge had a drift rate intermediate between the two parents, and a length that was greater than either parent. Again, a third such event involving a smaller barge (2002 December) showed many of the same characteristics. These observations define different dynamical behaviour during anticyclonic and cyclonic mergers.

The explicit planetary isentropic coordinate (EPIC) atmospheric model has been upgraded to use a hybrid vertical coordinate, ζ, that transitions continuously from potential temperature, θ, aloft to a function of a pressure coordinate, σ, that is terrain following near topography. The result is a model that simulates terrestrial and gas-giant atmospheres equally well. Considering that surface pressure varies by orders of magnitude from one planet to the next whereas topography has a roughly similar geometric scale everywhere, we define σ in terms of log p rather than the traditional p. We include a pure-sigma region at the bottom that allows for accurate modeling of the planetary boundary layer (PBL) for terrestrial applications and the deep atmosphere for gas-giant applications. We describe the functional form for ζ ( θ , σ ) , the method used to calculate θ, and the method used to calculate the hybrid vertical velocity, ζ ˙ , all of which are new. Potential temperature is only predicted in the pure-sigma region while in the hybrid region it is found diagnostically; in a complementary manner, pressure is predicted in the hybrid region and at the surface but found diagnostically in the pure-sigma region. The hybrid vertical velocity, ζ ˙ , is calculated directly near the beginning of each timestep rather than iteratively at the end. A brief description of the model's new turbulence scheme is included. To compare with previous models and to illustrate the flexibility of the hybrid coordinate, we run the Held–Suarez benchmark for Earth and a published Great Dark Spot simulation for Neptune.

Calculations of column intensities are performed for a number of infrared transitions of the H 3 + molecular ion, using a model atmosphere recently produced by Grodent et al. [2001. A self-consistent model of the jovian auroral thermal structure. J. Geophys. Res. 106, 12933–12952]. The line intensities integrated along the line of sight through the model atmosphere are first computed assuming that all of the emitting energy levels are in local thermodynamic equilibrium (LTE). These results are compared with those derived from a detailed balance calculation using a method recently proposed by Oka and Epp [2004. Non-thermal rotational distribution of H 3 + . Astrophys. J. In press]. It is shown that the population of excited vibrational levels starts to depart from that derived from LTE at altitudes higher than 500 km (above the jovian cloud tops). This effect has been noted previously by Kim et al. [1992. Densities and vibrational distribution of H 3 + in the jovian auroral atmosphere. J. Geophys. Res. 97, 6093–6101]. By 2000 km, all of the excited vibrational levels are populated at less than 10% of the expected LTE value. This has important implications for the jovian upper atmosphere. In particular, the H 3 + cooling effect will be greatly reduced high in the atmosphere. Modelled LTE line emission is greater than that derived from non-LTE modelling. Comparison of the non-LTE modelling with recent spectral measurements of the jovian auroral/polar regions in the L- and K-infrared windows shows that the Grodent et al. [2001. A self-consistent model of the jovian auroral thermal structure. J. Geophys. Res. 106, 12933–12952] profile overestimates the measured line intensity by ∼3. Allowing for this, the non-LTE modelling shows that the column densities derived from (quasi-)LTE treatment of the measured line intensities may underestimate the real H 3 + abundance by a factor of between 6 and 200. This means that attempts to derive important ionospheric properties, such as conductivity and related energy inputs due to magnetosphere–ionosphere coupling, from observed spectra will need to take this into account, if they are not to be seriously in error.

The evolution of a large-amplitude disturbance at cloud level in Jupiter's 24° N jet stream in 1990 is used to constrain the vertical structure of a realistic atmospheric model down to the 6 bar pressure level. We use the EPIC model (Dowling et al., 1998, The explicit planetary isentropic-coordinate (EPIC) atmospheric model, Icarus 132, 221–238) to perform long-term, three-dimensional, nonlinear simulations with a series of systematic variations in vertical structure and find that the details of the 1990 disturbance combine with the characteristics of the 24° N jet, the fastest on Jupiter, to yield a tight constraint on the solution space. The most important free parameters are the vertical dependence of the zonal-wind profile, and the thermal structure, below the cloud tops ( p > 0.7 bar ) at the jet's central latitude. The temporal evolution of the disturbed cloud patterns, which spans more than 2 years, can be reproduced if the jet peak reaches ∼180 ms−1 at the cloud level and increases to ∼210 ms−1 at 1 bar and up to ∼240 ms−1 at 6 bar; the observations were not reproduced for other configurations investigated. This trend is consistent with that measured by the Galileo Probe at 7° N; the implication is that this jovian jet extends well below the solar radiation penetration level situated near the 2 bar level.

Analysis of Hubble Space Telescope images of Uranus taken between 1994 and 2002 shows evidence for temporal changes in zonal brightness patterns in the south polar region. Between 1994 and 2002, a relatively bright ring developed near 70° S. The pole itself, which was the brightest area of the southern hemisphere in 1994, has become relatively dark. The polar collar at 45° S has also become brighter relative to the rest of the southern polar region. Comparison of images through different filters suggests that the change is occurring at pressures of 2–4 bars in the atmosphere. A change at this depth is consistent with radio measurements which indicate seasonal variability in Uranus' deep atmosphere. Disk-integrated photometry at visible wavelengths also exhibits variability on seasonal (∼ decades) timescales. The observed changes are not predicted by existing dynamical models of Uranus' atmosphere.

A new model of albedo and emissivity of planetary surfaces covered by volatile ices in the form of porous slab-like deposits is described. In the model, a radiative transfer model is coupled with a microphysical model in order to link changes in albedo and emissivity to changes in porosity caused by ice metamorphism. Preliminary results for Triton, Pluto, and Io are presented (the martian CO 2 caps will be the subject of a separate publication). The coupled model will aid in the interpretation of ground-based and spacecraft observations and should lead to advances in surface and atmospheric modeling.

We present the analysis and modeling of the emission spectra of the jovian northern auroral region taken from May 28 to June 3, 1993, with the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. They extend from 1204 to 1240 Â covering H Lyman α and part of the Werner and the Lyman bands of H2. We used the 2×2 arcsec large science aperture combined with the G160M grating (spectral resolution of 570 mÂ) centred on Jupiter's central meridian. The auroral region studied extends from 50 to 60° north latitudes and from 130 to 220° System III longitudes. Within the 1 arcsec pointing uncertainty, most of the region delineated by the theoretical ovals at 5.9 and 30 jovian radii (RJ) in the VIP4 model of J. E. C. Connerney, M. H. Acuna, N. F. Ness, and T. Satoh (1998, New models of Jupiter's magnetic field constrained by the Io flux tube footprint. J. Geophys. Res. 103, 11,929–11,939), including the auroral oval derived from the Wide Field Planetary Camera 2 images by J. T. Clarke, G. E. Ballester, J. T. Trauger, R. Evans, J. E. P. Connerney, K. Stapelfeld, D. Crisp, P. D. Feldman, C. J. Burrows, S. Casertano, J. S. Gallagher, R. E. Griffiths, J. J. Hester, J. G. Hoessel, J. A. Holtzman, J. E. Krist, V. Meadows, J. R. Mould, P. A. Scowen, A. M. Watson, and J. A. Westphal (1996, Far-ultraviolet imaging of Jupiter's aurora and the Io “footprint”. Science274, 404–409), was sampled. We derive auroral brightnesses (averaged over the slit) ranging from 20 kR in the 190–220° longitude region to 30 kR in the 130–160° longitude range. We use the theoretical model developed by D. Rego, R. Prange, and L. Ben Jaffel (1999, Auroral Lyman α and H2 bands from the giant planets. 3. Lyman α intensity and spectral profile including radiative effects and H2 color ratios. J. Geophys. Res. Planets.104, 5939–5954), which calculates self-consistently the auroral Lyman α line profile for electron and proton precipitations and a given frequency redistribution function. This model shows that Lyman α profiles are not dependent upon the identity of the particles for a given penetration depth. These profiles only constrain the atmospheric H column density above the emitting layer, Hcol. Best agreement with the data is found with the complete frequency redistribution (CR). We derive an auroral Hcol of 1.3×1016 cm−2 for all spectra with an upper limit of 5×1016 cm−2. Precipitating protons also produce fast H atoms by charge exchange which can also be excited, but the resulting Doppler-shifted profile was not detected. Present data thus rule out protons as being the only precipitating particles. In the auroral zones, theoretical models predict that chemical reactions induced by particle precipitations often result in the production of atomic hydrogen, and thus an enhancement of the H density is expected. However, the value of Hcol we derive, between ∼8 and 23 times less than the equatorial values of L. Ben Jaffel, J. T. Clarke, R. Prange, R. Gladstone, and A. Vidal-Madjar (1993, The Lyman alpha bulge of Jupiter: Effects of a non-thermal velocity field. J. Geophys. Res. Lett.20(8), 747–750), is in good agreement with the auroral values of R. Prange, D. Rego, L. Pallier, L. Ben Jaffel, C. Emerich, J. T. Clarke, G. E. Ballester, and J. Ajello (1997, Detection of self-reversed Lyman α lines from the jovian aurorae with the Hubble space Telescope, Astrophys. J.484, L169–L173). This suggests that the H abundance predicted with standard atmospheric model is unrealistically large in the auroral region.

We describe a new general circulation model (GCM) designed for planetary atmospheric studies called the EPIC model. This is a finite-difference model based on the isentropic-coordinate scheme of Hsu and Arakawa (1990. Mon. Wea. Rev. 118, 1933–1959). We report on previously undocumented modifications, additions, and key practical issues that experience running the model has revealed to be important. The model integrates the hydrostatic primitive equations, which are valid for large-scale atmospheric dynamics and include gravity waves (buoyancy waves), planetary waves (Rossby waves), and horizontally propagating sound waves (Lamb waves), but not vertically propagating sound waves because of the hydrostatic approximation. The vertical coordinate is entropy in the form of potential temperature, which coincides with material surfaces for adiabatic motion. This means that there is no vertical velocity except where there is heating, which improves accuracy and helps the model maintain conservation properties over long integrations. An isentropic vertical coordinate is natural for the atmospheres of Jupiter, Saturn, Uranus, and Neptune, which are believed to have essentially adiabatic interiors that match up with the bottom of the model and is also excellent for middle-atmosphere studies on any planet. Radiative processes are parameterized by Newtonian cooling, and the latent heat of ortho–para hydrogen conversion is included when appropriate, with a suitably defined mean potential temperature. The model is written with general map factors that make it easy to configure in oblate spherical, cylindrical, or Cartesian coordinates. The code includes optional Message Passing Interface (MPI) library calls and hence runs on any Unix-based parallel computer or network cluster. An optional graphical user interface to commercial visualization software facilitates control of the model and analysis of output. Memory is allocated dynamically such that the user does not recompile to change horizontal or vertical resolution or range. Applications to date include comet impact forecasts and hindcasts for Jupiter, meridional circulation studies of Uranus and Neptune, and the accompanying paper on three-dimensional simulations of Neptune's Great Dark Spot (1998. Icarus 132, 239–265).

We analyze an extensive data set of immersion and emersion lightcurves of the occultation of 28 Sgr by Saturn's atmosphere on 3 July 1989. The data give profiles of number density as a function of altitude at a variety of latitudes, at pressures ranging from about 0.5 to about 20 μbar. The atmosphere is essentially isothermal in this range, with a temperature close to 140 K for an assumed mean molecular weight of 2.135. Owing to favorable ring geometry, an accurate radial scale is available for all observations, and we confirm the substantial equatorial bulge produced by zonal winds of ∼450 m/s first observed in the Voyager radio-occultation experiments. The fact that the bulge is still present at microbar pressures suggests that the equatorial winds persist to high altitudes. According to our radial scale, the 2.4-μbar level, which corresponds to half-flux in the stellar occultations, is at an equatorial radius of 60,960 km. This radial scale is in good agreement with the Voyager radio-occultation data at mbar pressures and allows smooth interpolation of the isothermal structure between the stellar-occultation and radio-occultation regions. We do not have such a smooth interpolation between our data and Voyager ultraviolet occultation data, unless we discard the lowest 200 km of Voyager ultraviolet data. When this is done, we obtain a complete atmospheric model from an equatorial radius of 61,500 km down to an equatorial radius of 60,500 km. This model gives excellent agreement between all 28 Sgr, Voyager, and Pioneer 11 data.

We report measurements of the Jupiter brightness spectrum in the 850-μm and 1100-μm atmospheric windows with a spectral resolution of 125 MHz, obtained with a Fourier transform spectrometer on the James Clerk Maxwell Telescope. Three results were obtained. First, the predicted absorption features due to the rotational lines of HCN at 266 and 354 GHz were not detected within our error limits of less than 1%. We establish new upper limits for the HCN abundance in the jovian troposphere for five assumed abundance distributions and for two assumed NH3abundances. The upper limits are 1.7 to 13 times smaller than the abundance value obtained in the only reported detection of HCN in Jupiter prior to the impact of Shoemaker–Levy 9. Second, the continuum brightness temperature spectrum at 850 μm was determined and is in agreement with previous measurements, but has large error bars due to uncertainties in the photometric calibration. We estimate the ammonia abundance in the 1–2 bar region to be 1.7 times solar, but this result is tentative since scattering by NH3cloud particles and absorption by gaseous H2S were neglected in our atmospheric model. Finally, the first rotational line of PH3at 267 GHz was not detected, a result which we demonstrate is consistent with the statistical noise level in these measurements, with current values of the spectroscopic parameters, and with phosphine measurements at other wavelengths.

Spatially resolved photometric observations of Uranus at 2650 A obtained with the Voyager 2 Photopolarimeter Subsystem (PPS) are presented and analyzed with model atmospheres. A vertically homogenous atmospheric model with Rayleigh scattering and absorption fits Uranus well at 2650 A at phase angles from 16° to 157°; no strong forward scattering typical of abundant large particles is observed. A microphysical model developed for Uranus to fit Voyager imaging system data (K. Rages, J. B. Pollack, M. G. Tomasko, and L. R. Doose, 1991, Icarus 89, 359–376) predicts the vertical distribution and properties of stratospheric haze aerosols at 22.5° and 65°S. This microphysical model also fits the 2650-A PPS data at low latitudes. Increasing the aerosol sizes in the Rages et al. size distribution by a factor of 1.2 creates an unobserved forward scattering peak and can be excluded. The low-latitude upper stratospheric aerosols on Uranus are smaller (

CO emission lines in the 4.7-μm fundamental vibrational band were detected from Jupiter at the Shoemaker–Levy 9 fragment L impact site on July 20, 1994 UT, 4 to 5 hr after impact. For an atmospheric model with a single temperature for the emitting CO, which is assumed to be in local thermodynamic equilibrium (LTE), the CO temperature is estimated to beT(CO) = 280 ± 10 K. For this case, the CO column density isN(CO) = 1.2 × 1017cm−2and the estimated mass of CO in the L site is 1.6 × 1013g, with uncertainties of a factor five. The oxygen in this mass of CO can be plausibly explained as coming from material originally in the impactor. Larger amounts of cool CO below the emitting CO could have been present, however. The possible departure of the CO vibrational level populations from LTE and the effect on abundance estimates are discussed qualitatively. Spectra of other impact sites taken at times on the order of days after impact show no detectable changes in the CO absorption lines of impact sites vs nonimpact sites.

The initial results of a NIR spectral imaging study conducted from Mauna Kea Observatory during the 1990 opposition are presented. The study's main goals were to determine whether it is possible to map and monitor subtle variations in surface and atmospheric components on Mars from ground-based observations and to constrain Mars surface and airborne dust composition. Small variations near 2.3 microns include both atmospheric (CO) and surface/dust mineral absorption, although the data cannot yet accurately discriminate these components or assess their relative contributions. The present data set, even after preliminary calibration steps, demonstrates the potential of the Mars imaging spectroscopic observations in the near-IR. Focused ground-based telescopic studies in selected wavelength regions accompanied by detailed high spectral resolution atmospheric modeling will allow the mapping and monitoring of surface minerals, atmospheric aerosols, and atmospheric trace gases.

We have developed a coupled atmosphere and ocean model of Titan's surface. The atmospheric model is a 1-D spectrally-resolved radiative-convective model. The ocean thermodynamics are based upon solution theory. The ocean, initially composed of CH4, becomes progressively enriched in ethane over time. The partial pressures of N2 and CH4 in the atmosphere are dependent on the ocean temperature and composition. We find that the resulting system is stable against a runaway greenhouse. Accounting for the decreased solar luminosity, we find that Titan's surface temperature was about 20 K colder 4 Gyr ago. Without an ocean, but only small CH4 lakes, the temperature change is 12 K. In both cases we find that the surface of Titan may have been ice covered about 3 Gyr ago. In the lakes case condensation of N2 provides the ice, whereas in the ocean case the ocean freezes. The dominant factor influencing the evolution of Titan's surface temperature is the change in the solar constant--amplified, if an ocean is present, by the temperature dependence of the solubility of N2. Accretional heating can dramatically alter the surface temperature; a surface thermal flux of 500 erg cm-2 sec-1, representative of small levels of accretional heating, results in a approximately 20 K change in surface temperatures.

Aspects of the time-variable nature of the Jovian atmosphere are addressed using high-resolution photometrically calibrated multicolored imaging data obtained over two Jovian apparitions. During the period of observations, Jupiter's South Equatorial Belts (SEB) underwent a drastic brightening and its Equatorial Zone gradually darkened throughout the period. Based on the data, vertically inhomogeneous atmospheric structure models are constructed and used to make direct quantitative comparisons between different latitudinal regions and different epochs. The drastic brightening of the SEB is explained by an increase in both the optical thickness and the single-scattering albedo of the upper tropospheric cloud.

Heat source information is derived here from the Voyager 2 Ultraviolet Spectrometer occultation data of Uranus. Analytic functions for the local heat dependence on altitude are used to obtain a temperature profile by solving the heat equation. The stellar entrance and exit occultation and a solar occultation are used to infer the thermal and density structure of the atmosphere. The least squares fit solution to the solar occultation data gives one source located at 1.8 x 10 exp -5 microbar with a strength of 0.056 +/- 0.01 erg/sq cm/s. Latitudinal temperature gradients are obtained.

An analysis is conducted of the composition and thermal structure of Titan's upper atmosphere, and the relative importance of airglow-excitation processes, on the basis of Voyager 1 UV spectrometer data for solar occultation and airglow. Attention is also given to the Smith et al. (1982) optical depth profiles for the evening terminator, entrance occultation, and asymptotic temperature. A magnetospheric power input figure is obtained that is 25 times smaller than the initial estimate by Strobel and Shemansky (1982).

The barotropic, orographically-forced intraseasonal oscillations of the Martian atmosphere are presently analyzed by means of a single-layer shallow-water model. An oscillation with 85-Martian day mean period is noted in a time-series of the model global atmospheric angular momentum, and its robustness is verified. The oscillation has a standing zonal component whose centers of action are located upstream and downstream of salient orographic features. A resemblance arises between the oscillation's northern-hemisphere features and those of the terrestrial 40-day oscillation.

The idea of an early greenhouse atmosphere for Triton is presented and the conditions under which it may have been sustained are quantified. The volatile content of primordial Triton is modeled, and tidal heating rates are assessed to set bounds on the available energy. The atmospheric model formalism is presented, and it is shown how a massive atmosphere could have been raised by modest tidal heating fluxes. The implications of the model atmospheres for the atmospheric escape rates, the chemical evolution, and the cratering record are addressed.

Results are presented from a detailed analysis of Voyager images of Triton obtained at the highest solar phase angles; these have been fit to Mie scattering models in order to obtain the mean particle sizes, number densities, and the vertical extent of the two different scattering components of the Triton atmosphere. The 0.001-0.01 optical depths of about 0.17 micron particles are vertically distributed with scale heights of about 10 km throughout Triton. A number of properties of the haze particles in question suggest that they are composed of photochemically produced gases which have condensed in the cold lower atmosphere of Triton.

Experiments in which C2H2, C2H4, and HCN were photolyzed separately and as a mixture in UV light have been conducted in order to ascertain the physical properties of model Titan atmosphere aerosols. Aerosols formed from photolysis of C2H4 were physically similar to those formed from C2H2; protolysis of HCN rapidly generated particles that did not grow to sizes greater than 0.09 microns. While the formation of particles from C4H2 was observed within minutes, formation was slowed by a factor of 4 when C2H2 and HCN were added.

The formation and evolution of convective H 2 O clouds in the deep Jovian troposphere are investigated using an axisymmetric model with parameterized microphysics to calculate the vertical and radial wind components and the liquid and ice contents in the clouds. A constant upwelling velocity from within the deep atmosphere was assumed, and the model was initiated with a slightly superadiabatic lapse rate below the lifting condensation level (LCL), which was located at 5160 mbar. The abundance of water in Jupiter's atmosphere was assumed to be solar, and several relative humidity profiles were evaluated. The initial temperature profile above the LCL was assumed to be dry-adiabatic. The role of condensate mass-loading in the dynamic evolution of the cloud was considered. Results show that for plausible values of superadiabatic deviations and upwelling velocities, the Jovian water clouds are 25 km deep and extend up to the 3-bar pressure level. These cumulus clouds contain 2.5–5 g/m 3 of liquid water and ice, with updrafts at the axis reaching 30 m/sec and a cloud top temperature of 235 K. Clouds could retain their vertical structure for long periods of time. According to the model the water clouds reach the ammonia condensation level only when the temperature and relative humidity profiles are coupled in such a way that instability (or at least neutrality) to moist convection is ensured over a large depth of the troposphere. Under intermediate conditions, at which the upper layers were assumed to be stable, clouds were 40–50 km deep with updrafts of 60–70 m/sec, but did not reach the ammonia cloud deck. Thus, they were probably not connected to the observed equatorial “plumes”.

New observations of Titan's near-infrared spectrum (4000-5000 cm-1) combined with points taken from Fink and Larson's (1979) spectrum (4000-12500 cm-1) provide information on Titan's haze, possible clouds, surface albedo, and atmospheric abundance of H2. In the near-infrared, the main features in Titan's spectrum result from absorption of solar radiation by CH4. The strength of this absorption varies considerably with wavelength, allowing us to probe various atmospheric levels down to the surface itself by choosing specific wavelengths for analysis. At 4715 cm-1, the pressure-induced S(1) fundamental band of H2 lies in the wings of CH4 bands. Based on current values for the CH4 line parameters, Titan's spectrum can be best interpreted with a volume mixing ratio of H2 between 0.5 and 1.0%. Our observations suggest the existence of an optically thin CH4 cloud layer. The optical depths that we derive for Titan's haze and clouds are small enough to allow us to sense the surface of Titan at 4900, 6250, and 7700 cm-1. The most plausible interpretation of the albedos determined at these wavenumbers suggests a surface dominated by "dirty" water ice. A global ethane ocean is not compatible with these albedos.