Convective instability in the martian middle atmosphere

Dry convective instabilities in Mars’s middle atmosphere are detected and mapped using temperature retrievals from Mars Climate Sounder observations spanning 1.5 martian years. The instabilities are moderately frequent in the winter extratropics. The frequency and strength of middle atmospheric convective instability in the northern extratropics is significantly higher in MY 28 than in MY 29. This may have coupled with changes to the northern hemisphere mid-latitude and tropical middle atmospheric temperatures and contributed to the development of the 2007 global dust storm. We interpret these instabilities to be the result of gravity waves saturating within regions of low stability created by the thermal tides. Gravity wave saturation in the winter extratropics has been proposed to provide the momentum lacking in general circulation models to produce the strong dynamically-maintained temperature maximum at 1-2 Pa over the winter pole, so these observations could be a partial control on modeling experiments.     

Clouds on Titan during the Cassini prime mission: A complete analysis of the VIMS data

We use data from the VIMS instrument on board the Cassini spacecraft to construct high sensitivity and high spatial-resolution maps of the locations of tropospheric clouds on Titan in the late northern winter season during which the Cassini prime mission took place. These observations show that, in this season, clouds on Titan are strongly hemispherically asymmetric. Mid-latitude clouds, in particular, occur only in the southern hemisphere and have not ever been observed in the north. Such an asymmetry is in general agreement with circulation models where sub-solar surface heating controls the locations of clouds and appears in conflict with models where perennial polar hazes prevent significant summertime polar heating from affecting the circulation. The southern mid-latitude clouds appear to be distributed uniformly in longitude, in contrast to some previous observations. Southern high-latitude clouds exhibit a significant concentration, however, between about 180° and 270°E longitude. A spatially and temporally uniform cloud always appears northward of ∼50°N latitude. This cloud appears unchanged over the course of the observations, consistent with the interpretation that it is caused by continuous ethane condensation as air subsides and radiatively cools through the tropopause. The location of this cloud likely provides a direct tracer of elements of north polar atmospheric circulation, potentially allowing continuous monitoring of circulation changes as Titan passes through equinox into north polar spring and summer. We show that a similar analysis of this dataset by Rodriguez et al. (2009) contains substantial errors and should not be used.     

Traveling waves in the martian atmosphere from MGS TES Nadir data

We have characterized the annual behavior of martian atmospheric traveling waves in the MGS TES data set from the first two martian years of mapping. There is a high degree of repeatability between the two years. They are dominated by strong low zonal wavenumber waves with high amplitudes near the polar jets, strongest in late northern fall and early northern winter. The m=1 waves have amplitudes up to about 20 K, are vertically extended, and occasionally extend even into the tropics. Periods for m=1 range from 2.5 to 30 sols. Much weaker waves were identified in the south, with amplitudes less than about 3.5 K. Traveling waves with m=2 and m=3 are also seen, but their amplitudes are typically limited to less than 4 K, and are generally more confined near the surface. In the north, they are more evident in fall and spring rather than winter solstice, which is clearly dominated by m=1 waves. Some evidence of storm tracks has been identified in the data, with accentuated weather-related temperature perturbations near longitudes 200° to 320° E for both the southern and northern hemispheres near latitude ±65° at the surface. Some evidence was also found for a sharpening of longitudinal gradients into what may be frontal systems. EP flux divergences show the waves extracting energy from the zonal mean winds. When the m=1 waves were strongest, decelerations of the zonal jet of order 30 m/(s sol) were measured. Above 1 scale height, the waves extract energy from the jet predominately through barotropic processes, but their character is overall mixed barotropic/baroclinic. Inertial instabilities may exist at altitude on the equatorward flanks of the polar jets, and marginal stability extends through to the tropics. This may explain the coordination of the tropical behavior of the waves with that centered along the polar jet, consistent with the ideas expressed in Wilson et al. (2002, Geophys. Res. Lett. 29, #1684) and similar to those in Barnes et al. (1993, J. Geophys. Res. 98, 3125-3148). Throughout the year, there exist large regions with the meridional gradient of PV less than zero, but they are strongest near winter solstice. Poleward of the winter jet, the regions of instability reach the surface, equatorward they do not. These regions, satisfying a necessary criterion for instability, likely explain the genesis of the waves, and perhaps also their bimodal character between surface (faster waves) and altitude (slow m=1 waves).     

Forced waves in the martian atmosphere from MGS TES nadir data

We have analyzed the temperature retrievals from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) nadir spectra to yield latitude-height resolved maps of various atmospheric forced wave modes as a function of season for a full Mars year. Among the isolated wave modes is the zonal mean, time mean temperature, which we used to derive zonal mean zonal winds and stationary wave quasi-geostrophic indices of refraction, diagnostic of their propagation. The diurnal Kelvin wave was isolated in the data, with results roughly consistent with models (Wilson and Hamilton, 1996, J. Atmos. Sci. 33, 1290-1326). The s = 1 and s = 2 stationary waves were found to have significant amplitude in ducts extending up the winter polar jets, while the s = 3 stationary wave was found to be confined to near the surface. The s = 1 stationary wave was found to have little phase tilt with height during northern winter, but significant westward phase tilt with height in the southern winter. This indicates that the wave carries heat poleward, slightly more than that found in Barnes et al. (1996; J. Geophys. Res. 101, 12,753-12,776). The s = 1 stationary wave is likely the dominant mechanism for eddy meridional heat transport for the southern winter. We noted that the phase of the s = 2 stationary wave is nearly constant with time, but that the s = 1 stationary wave moved 90° of longitude from fall to winter and back in spring in the North. While interannual variability is not yet addressed, overall, these results provide the first comprehensive benchmark for forced waves in Mars’s atmosphere against which future atmospheric models of Mars can be compared.     

Comparison of PFS and TES observations of temperature and water vapor in the martian atmosphere

The interval from L_s = 330° in Mars Year (MY) 26 until L_s = 84° in MY 27 has been used to compare and validate measurements from the Mars Global Surveyor Thermal Emission Spectrometer (TES) and the Mars Express Planetary Fourier Spectrometer (PFS). We studied differences between atmospheric temperatures observed by the two instruments. The best agreement between atmospheric temperatures was found at 50 Pa between 40°S and 40°N latitude, where differences were within ±5 K. For other atmospheric levels, differences as large as ∼25 K were observed between the two instruments at some locations. The largest temperature differences occurred mainly over the Hellas Planitia, Argyre Planitia, Tharsis and Valles Marineris regions.On this basis we report on the variability of the martian atmosphere during the 5.5 martian years of Mars climatology obtained by combining the two data sets from TES and PFS. Atmospheric temperatures at 50 Pa responded to the global-scale dust storms of MY 25 and in MY 28 raising temperatures from ∼220 K to ∼250 K during the daytime. An atmospheric temperature of ∼140 K at 50 Pa was observed poleward of 70°N during northern winter and poleward of 60°S during southern winter each year in both the PFS and TES results. Water vapor observed by the two spectrometers showed consistent seasonal and latitudinal variations.     

Propagation of tropospheric gravity waves into the upper atmosphere of Mars

We study the propagation of gravity waves in the martian atmosphere using a linearized one-dimensional full-wave model. Calculations are carried out for atmospheric parameters characteristic of Mars Orbiter Laser Altimeter (on Mars Global Surveyor MGS) observations of apparent gravity waves in high latitude clouds and MGS radio occultation measurements of temperature variations with height suggestive of gravity wave activity. Waves that reach the thermosphere produce fluctuations in density comparable in amplitude with the density variations detected in Mars Odyssey aerobraking data. Gravity waves of modest amplitude are found to deposit momentum and generate significant heating and cooling in the martian atmosphere. The largest heating and cooling effects occur in the thermosphere, at altitudes between about 130 and 150 km, with heating occurring at the lower altitudes and cooling taking place above.     

THEMIS-VIS observations of clouds in the martian mesosphere: Altitudes, wind speeds, and decameter-scale morphology

We present measurements of the altitude and eastward velocity component of mesospheric clouds in 35 imaging sequences acquired by the Mars Odyssey (ODY) spacecraft’s Thermal Emission Imaging System visible imaging subsystem (THEMIS-VIS). We measure altitude by using the parallax drift of high-altitude features, and the velocity by exploiting the time delay in the THEMIS-VIS imaging sequence.We observe two distinct classes of mesospheric clouds: equatorial mesospheric clouds observed between 0° and 180° L_s; and northern mid-latitude clouds observed only in twilight in the 200–300° L_s period. The equatorial mesospheric clouds are quite rare in the THEMIS-VIS data set. We have detected them in only five imaging sequences, out of a total of 2048 multi-band equatorial imaging sequences. All five fall between 20° south and 0° latitude, and between 260° and 295° east longitude. The mid-latitude mesospheric clouds are apparently much more common; for these we find 30 examples out of 210 northern winter mid-latitude twilight imaging sequences. The observed mid-latitude clouds are found, with only one exception, in the Acidalia region, but this is quite likely an artifact of the pattern of THEMIS-VIS image targeting. Comparing our THEMIS-VIS images with daily global maps generated from Mars Orbiter Camera Wide Angle (MOC-WA) images, we find some evidence that some mid-latitude mesospheric cloud features correspond to cloud features commonly observed by MOC-WA. Comparing the velocity of our mesospheric clouds with a GCM, we find good agreement for the northern mid-latitude class, but also find that the GCM fails to match the strong easterly winds measured for the equatorial clouds.Applying a simple radiative transfer model to some of the equatorial mesospheric clouds, we find good model fits in two different imaging sequences. By using the observed radiance contrast between cloud and cloud-free regions at multiple visible-band wavelengths, these fits simultaneously constrain the optical depths and particles sizes of the clouds. The particle sizes are constrained primarily by the relative contrasts at the available wavelengths, and are found to be quite different in the two imaging sequences: r_eff = 0.1 μm and r_eff = 1.5 μm. The optical depths (constrained by the absolute contrasts) are substantial: 0.22 and 0.5, respectively. These optical depths imply a mass density that greatly exceeds the saturated mass density of water vapor at mesospheric temperatures, and so the aerosol particles are probably composed mainly of CO₂ ice. Our simple radiative transfer model is not applicable to twilight, when the mid-latitude mesospheric clouds were observed, and so we leave the properties of these clouds as a question for further work.     

The 10 October 1999 HIP 9369 occultation by the northern polar region of Jupiter: ingress and egress lightcurves analysis

The occultation of bright star HIP9369 by the northern polar region of Jupiter was observed from four locations in North and South America, providing four data sets for ingress and egress. The inversion of the eight occultation lightcurves provides temperature profiles at different latitudes ranging from 55°N to 73.2°N. We estimate the errors on the profiles due to the uncertainties of the inversion method and compare the value of the temperature at the deepest level probed (∼ 50 μbar) with previous observations. The shape of the temperature gradient profile is found similar to previous investigations of planetary atmospheres with propagating and breaking gravity waves. We analyze the small scale structures in both lightcurves and temperature profiles using the continuous wavelet transform. The calculated power spectra of localized fluctuations in the temperature profiles show slopes close to −3 for all eight profiles. We also isolate and reconstruct the three-dimensional geometry of a single wave mode with vertical and horizontal wavelengths of respectively 3 and 70 km. The identified wave is consistent with the gravity wave regime, with a horizontal phase speed nearly parallel to the planetary meridian. Nevertheless, the dissipation of the corresponding wave in Jupiter’s stratosphere should preclude its detection at the observed levels and an acoustic wave cannot be ruled out.     

Titan’s cloud seasonal activity from winter to spring with Cassini/VIMS

Since Saturn orbital insertion in July 2004, the Cassini orbiter has been observing Titan throughout most of the northern winter season (October 2002–August 2009) and the beginning of spring, allowing a detailed monitoring of Titan’s cloud coverage at high spatial resolution with close flybys on a monthly basis. This study reports on the analysis of all the near-infrared images of Titan’s clouds acquired by the Visual and Infrared Mapping Spectrometer (VIMS) during 67 targeted flybys of Titan between July 2004 and April 2010.The VIMS observations show numerous sporadic clouds at southern high and mid-latitudes, rare clouds in the equatorial region, and reveal a long-lived cloud cap above the north pole, ubiquitous poleward of 60°N. These observations allow us to follow the evolution of the cloud coverage during almost a 6-year period including the equinox, and greatly help to further constrain global circulation models (GCMs). After 4 years of regular outbursts observed by Cassini between 2004 and 2008, southern polar cloud activity started declining, and completely ceased 1 year before spring equinox. The extensive cloud system over the north pole, stable between 2004 and 2008, progressively fractionated and vanished as Titan entered into northern spring. At southern mid-latitudes, clouds were continuously observed throughout the VIMS observing period, even after equinox, in a latitude band between 30°S and 60°S. During the whole period of observation, only a dozen clouds were observed closer to the equator, though they were slightly more frequent as equinox approached.We also investigated the distribution of clouds with longitude. We found that southern polar clouds, before disappearing in mid-2008, were systematically concentrated in the leading hemisphere of Titan, in particular above and to the east of Ontario Lacus, the largest reservoir of hydrocarbons in the area. Clouds are also non-homogeneously distributed with longitude at southern mid-latitudes. The n = 2-mode wave pattern of the distribution, observed since 2003 by Earth-based telescopes and confirmed by our Cassini observations, may be attributed to Saturn’s tides.Although the latitudinal distribution of clouds is now relatively well reproduced and understood by the GCMs, the non-homogeneous longitudinal distributions and the evolution of the cloud coverage with seasons still need investigation. If the observation of a few single clouds at the tropics and at northern mid-latitudes late in winter and at the start of spring cannot be further interpreted for the moment, the obvious shutdown of the cloud activity at Titan’s poles provides clear signs of the onset of the general circulation turnover that is expected to accompany the beginning of Titan’s northern spring. According to our GCM, the persistence of clouds at certain latitudes rather suggests a ‘sudden’ shift in near future of the meteorology into the more illuminated hemisphere. Finally, the observed seasonal change in cloud activity occurred with a significant time lag that is not predicted by our model. This may be due to an overall methane humidity at Titan’s surface higher than previously expected.     

Stratospheric superrotation in the TitanWRF model

TitanWRF general circulation model simulations performed without sub-grid-scale horizontal diffusion of momentum produce roughly the observed amount of superrotation in Titan’s stratosphere. We compare these results to Cassini-Huygens measurements of Titan’s winds and temperatures, and predict temperature and winds at future seasons. We use angular momentum and transformed Eulerian mean diagnostics to show that equatorial superrotation is generated during episodic angular momentum ‘transfer events’ during model spin-up, and maintained by similar (yet shorter) events once the model has reached steady state. We then use wave and barotropic instability analysis to suggest that these transfer events are produced by barotropic waves, generated at low latitudes then propagating poleward through a critical layer, thus accelerating low latitudes while decelerating the mid-to-high latitude jet in the late fall through early spring hemisphere. Finally, we identify the dominant waves responsible for the transfers of angular momentum close to northern winter solstice during spin-up and at steady state. Problems with our simulations include peak latitudinal temperature gradients and zonal winds occurring ∼60 km lower than observed by Cassini CIRS, and no reduction in zonal wind speed around 80 km, as was observed by Huygens. While the latter may have been due to transient effects (e.g. gravity waves), the former suggests that our low (∼420 km) model top is adversely affecting the circulation near the jet peak, and/or that we require active haze transport in order to correctly model heating rates and thus the circulation. Future work will include running the model with a higher top, and including advection of a haze particle size distribution.     

Characterising Saturn's vertical temperature structure from Cassini/CIRS

Thermal infrared spectra of Saturn from 10-1400 cm⁻¹ at 15 cm⁻¹ spectral resolution and a spatial resolution of 1°-2° latitude have been obtained by the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297]. Many thousands of spectra, acquired over eighteen-months of observations, are analysed using an optimal estimation retrieval code [Irwin, P.G.J., Parrish, P., Fouchet, T., Calcutt, S.B., Taylor, F.W., Simon-Miller, A.A., Nixon, C.A., 2004. Icarus 172, 37-49] to retrieve the temperature structure and para-hydrogen distribution over Saturn's northern (winter) and southern (summer) hemispheres. The vertical temperature structure is analysed in detail to study seasonal asymmetries in the tropopause height (65-90 mbar), the location of the radiative-convective boundary (350-500 mbar), and the variation with latitude of a temperature knee (between 150 and 300 mbar) which was first observed in inversions of Voyager/IRIS spectra [Hanel, R., and 15 colleagues, 1981. Science 212, 192-200; Hanel, R., Conrath, B., Flasar, F.M., Kunde, V., Maguire, W., Pearl, J.C., Pirraglia, J., Samuelson, R., Cruikshank, D.P., Gautier, D., Gierasch, P.J., Horn, L., Ponnamperuma, C., 1982. Science 215, 544-548]. Uncertainties due to both the modelling of spectral absorptions (collision-induced absorption coefficients, tropospheric hazes, helium abundance) and the nature of our retrieval algorithm are quantified.Temperatures in the stratosphere near 1 mbar show a 25-30 K temperature difference between the north pole and south pole. This asymmetry becomes less pronounced with depth as the radiative time constant for the atmospheric response increases at deeper pressure levels. Hemispherically-symmetric small-scale temperature structures associated with zonal winds are superimposed onto the temperature asymmetry for pressures greater than 100 mbar. The para-hydrogen fraction in the 100-400 mbar range is greater than equilibrium predictions for the southern hemisphere and parts of the northern hemisphere, and less than equilibrium predictions polewards of 40° N.The temperature knee between 150-300 mbar is larger in the summer hemisphere than in the winter, smaller and higher at the equator, deeper and larger in the equatorial belts and small at the poles. Solar heating on tropospheric haze is proposed as a possible mechanism for this effect; the increased efficiency of ortho- to para-hydrogen conversion in the southern hemisphere is consistent with the presence of larger aerosols in the summer hemisphere, which we demonstrate to be qualitatively consistent with previous studies of Saturn's tropospheric aerosol distribution.     

Investigation of water vapor on Mars with PFS/SW of Mars Express

New insight into the seasonal, diurnal and spatial distribution of water vapor on Mars has been obtained from analyzing the spectra of the short-wavelength channel (SW) of the Planetary Fourier Spectrometer (PFS) onboard Mars Express. The processed dataset, recorded between January 2004 and April 2005, covers the seasons from LS=331° of Mars Year 26 to LS=196° of the following year. In this period the mean column density around vernal equinox was 8.2 pr. μm. The maximum values during northern summer were about 65 pr. μm, located around 75° N latitude with a longitudinally inhomogeneous distribution. Regarding the atmospheric transport, the majority of polar water vapor remains in the north polar region while only about a quarter is transported southward. Geographically there are two water vapor maxima visible, over Arabia Terra and the Tharsis plateau, that are most likely caused both by atmosphere-ground interaction and by atmospheric circulation. A comparison with other instruments generally shows a good agreement, only the SPICAM results are systematically lower. Compared to the results from the PFS long-wavelength channel the results of this work are slightly higher. A strong discrepancy is visible northward of about 50° N during the northern summer that is possibly explained by a non-uniform vertical H₂O mixing. In particular, a confinement of the water to the lower few kilometers yields a much better agreement between the retrieved column densities of the two PFS channels.     

Curvilinear features in the southern hemisphere observed by Mars Global Surveyor Mars Orbiter Camera

We have used the complete set of Mars Global Surveyor (MGS) Mars Daily Global Maps (MDGMs) to study martian weather in the southern hemisphere, focusing on curvilinear features, including frontal events and streaks. “Frontal events” refer to visible events that are morphologically analogous to terrestrial baroclinic storms. MDGMs show that visible frontal events were mainly concentrated in the 210-300°E (60-150°W) sector and the 0-60°E sector around the southern polar cap during L_s = 140-250° and L_s = 340-60°. The non-uniform spatial and temporal distributions of activity were also shown by MGS Thermal Emission Spectrometer transient temperature variations near the surface. “Streaks” refer to long curvilinear features in the polar hood or over the polar cap. They are an indicator of the shape of the polar vortex. Streaks in late winter usually show wavy segments between the 180° meridian and Argyre. Model results suggest that the zonal wave number m = 3 eastward traveling waves are important for their formation.     

Water vapor variability in the north polar region of Mars from Viking MAWD and MGS TES datasets

Atmospheric water vapor abundances in Mars’ north polar region (NPR, from 60° to 90°N) are mapped as function of latitude and longitude for spring and summer seasons, and their spatial, seasonal, and interannual variability is discussed. Water vapor data are from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and the Viking Orbiter (VO) Mars Atmospheric Water Detector (MAWD). The data cover three complete northern spring-summer seasons in 1977-1978, 2000-2001 and 2002-2003, and shorter periods of spring-summer seasons during 1975, 1999 and 2004. Long term interannual variability in the averaged NPR abundances may exist, with Viking MAWD observations showing twice as much water vapor during summer as the MGS TES observations more than 10 martian years (MY) later. While the averaged abundances are very similar in TES observations for the same season in different years, the spatial distributions in the early summer season do vary significantly year over year. Spatial and temporal variabilities increase between L_s ∼ 80-140°, which may be related to vapor sublimation from the North Polar Residual Cap (NPRC), or to changes in circulation. Spatial variability is observed on scales of ∼100 km and temporal variability is observed on scales of <10 sols during summer. During late spring the TES water vapor spatial distribution is seen to correlate with the low topography/low albedo region of northern Acidalia Planitia (270-360°E), and with the dust spatial distribution across the NPR during late spring-early summer. Non-uniform vertical distribution of water vapor, a regolith source or atmospheric circulation ‘pooling’ of water vapor from the NPRC into the topographic depression may be behind the correlation with low topography/low albedo. Sublimation winds carrying water vapor off the NPRC and lifting surface dust in the areas surrounding the NPRC may explain the correlation between the water vapor and dust spatial distributions. Correlation between water vapor and dust in MAWD data are only observed over low topography/low albedo area. Maximum water vapor abundances are observed at L_s = 105-115° and outside of the NPRC at 75-80°N; the TES data, however, do not extend over the NPRC and thus, this conclusion may be biased. Some water vapor appears to be released in plumes or ‘outbursts’ in the MAWD and TES datasets during late spring and early summer. We propose that the sublimation rate of ice varies across the NPRC with varying surface winds, giving rise to the observed ‘outbursts’ at some seasons.     

Interannual variability in TES atmospheric observations of Mars during 1999-2003

We use infrared spectra returned by the Mars Global Surveyor Thermal Emission Spectrometer (TES) to retrieve atmospheric and surface temperature, dust and water ice aerosol optical depth, and water vapor column abundance. The data presented here span more than two martian years (Mars Year 24, L_s=104°, 1 March 1999 to Mars Year 26, L_s=180°, 4 May 2003). We present an overview of the seasonal (L_s), latitudinal, and longitudinal dependence of atmospheric quantities during this period, as well as an initial assessment of the interannual variability in the current martian climate. We find that the perihelion season (L_s=180°-360°) is relatively warm, dusty, free of water ice clouds, and shows a relatively high degree of interannual variability in dust optical depth and atmospheric temperature. On the other hand, the aphelion season (L_s=0°-180°) is relatively cool, cloudy, free of dust, and shows a low degree of interannual variability. Water vapor abundance shows a moderate amount of interannual variability at all seasons, but the most in the perihelion season. Much of the small amount of interannual variability that is observed in the aphelion season appears to be caused by perihelion-season planet-encircling dust storms. These dust storms increase albedo through deposition of bright dust on the surface causing cooler daytime surface and atmospheric temperatures well after dust optical depth returns to prestorm values.     

Cloud brightness distribution and turbulence in Venus using Galileo violet images

Venus cloud covered atmosphere offers a well-suited framework to study the coupling between the atmospheric dynamics and the structure of the cloud field. Violet images obtained during the Galileo flyby from 12 to 17 February 1990 have been analyzed to retrieve the zonal power spectra of the cloud brightness distribution field between latitudes 70° N and 50° S. The brightness distribution spectra serve as a diagnostic of the eddy kinetic energy spectrum providing indirect information about the distribution of energy along different spatial scales. We composed images covering a full rotation of the atmosphere at the level of the UV contrasted clouds obtaining maps of almost 360° that allowed us to obtain the brightness power spectra from wavenumbers k=1 to 50. A full analysis of the spectrum slope for different latitude bands and ranges of wave numbers is presented. The power spectra follow a classical law k−n with exponent n ranging from −1.7 to −2.9 depending on latitude and the wavenumber range. For the whole planet, the average of this parameter is −2.1 intermediate between those predicted by the classical turbulence theories for three- and two-dimensional motions (n=−5/3 and n=−3). A comparison with previous analysis of Mariner 10 (in 1974) and Pioneer Venus (in 1979) shows significant temporal changes in the cloud global structure and in the turbulence characteristics of the atmosphere.     

THEMIS observations of Mars aerosol optical depth from 2002-2008

We use infrared images obtained by the Thermal Emission Imaging System (THEMIS) instrument on-board Mars Odyssey to retrieve the optical depth of dust and water ice aerosols over more than 3.5 martian years between February 2002 (MY 25, Ls=330°) and December 2008 (MY 29, Ls=183°). These data provide an important bridge between earlier TES observations and recent observations from Mars Express and Mars Reconnaissance Orbiter. An improvement to our earlier retrieval [Smith, M.D., Bandfield, J.L., Christensen, P.R., Richardson, M.I., 2003. J. Geophys. Res. 108, doi:10.1029/2003JE002114] to include atmospheric temperature information from THEMIS Band 10 observations leads to much improved retrievals during the largest dust storms. The new retrievals show moderate dust storm activity during Mars Years 26 and 27, although details of the strength and timing of dust storms is different from year to year. A planet-encircling dust storm event was observed during Mars Year 28 near Southern Hemisphere Summer solstice. A belt of low-latitude water ice clouds was observed during the aphelion season during each year, Mars Years 26 through 29. The optical depth of water ice clouds is somewhat higher in the THEMIS retrievals at ∼5:00 PM local time than in the TES retrievals at ∼2:00 PM, suggestive of possible local time variation of clouds.     

Quasi-periodic dust events in the summertime south polar region of Mars

A Hovmöller diagram analysis of the dust optical depth measured by the Mars Global Surveyor Thermal Emission Spectrometer shows the occurrence of quasi-periodic westwardly-propagating disturbances with timescales of 10-20 sols during summer in the south polar region of Mars. Dust clouds emerge repeatedly around the region with a latitude of around 70-80°S and a longitude of 240-300°E, move westward at speeds of 3-6 m s⁻¹, reach the region with a longitude of 60-120°E, and finally disappear. This longitude range coincides with elevated terrains in the south polar region, and in this region an increase of dust optical depth encircling the south pole is also observed. This implies that the quasi-periodic dust events will contribute to the enhancement of the atmospheric dust loading in this region. These dust events might be related to baroclinic instability caused by the thermal contrast across the CO₂ cap edge, or the horizontal advection or vertical convection with radiative-dynamical feedback. The westward movement of the dust clouds suggests steady westward winds blowing in the near-surface layer, where the quasi-periodic dust lifting is expected to occur. Such a westward cap-edge flow will be created by the Coriolis force acting on the flow from the ice side to the regolith side.     

Saturn’s tropospheric composition and clouds from Cassini/VIMS 4.6-5.1 μm nightside spectroscopy

The latitudinal variation of Saturn’s tropospheric composition (NH₃, PH₃ and AsH₃) and aerosol properties (cloud altitudes and opacities) are derived from Cassini/VIMS 4.6-5.1 μm thermal emission spectroscopy on the planet’s nightside (April 22, 2006). The gaseous and aerosol distributions are used to trace atmospheric circulation and chemistry within and below Saturn’s cloud decks (in the 1- to 4-bar region). Extensive testing of VIMS spectral models is used to assess and minimise the effects of degeneracies between retrieved variables and sensitivity to the choice of aerosol properties. Best fits indicate cloud opacity in two regimes: (a) a compact cloud deck centred in the 2.5-2.8 bar region, symmetric between the northern and southern hemispheres, with small-scale opacity variations responsible for numerous narrow light/dark axisymmetric lanes; and (b) a hemispherically asymmetric population of aerosols at pressures less than 1.4 bar (whose exact altitude and vertical structure is not constrained by nightside spectra) which is 1.5-2.0× more opaque in the summer hemisphere than in the north and shows an equatorial maximum between ±10° (planetocentric).Saturn’s NH₃ spatial variability shows significant enhancement by vertical advection within ±5° of the equator and in axisymmetric bands at 23-25°S and 42-47°N. The latter is consistent with extratropical upwelling in a dark band on the poleward side of the prograde jet at 41°N (planetocentric). PH₃ dominates the morphology of the VIMS spectrum, and high-altitude PH₃ at p < 1.3 bar has an equatorial maximum and a mid-latitude asymmetry (elevated in the summer hemisphere), whereas deep PH₃ is latitudinally-uniform with off-equatorial maxima near ±10°. The spatial distribution of AsH₃ shows similar off-equatorial maxima at ±7° with a global abundance of 2-3 ppb. VIMS appears to be sensitive to both (i) an upper tropospheric circulation (sensed by NH₃ and upper-tropospheric PH₃ and hazes) and (ii) a lower tropospheric circulation (sensed by deep PH₃, AsH₃ and the lower cloud deck).     

Uranus in 2003: Zonal winds, banded structure, and discrete features

Imaging of Uranus in 2003 with the Keck 10-m telescope reveals banded zonal structure and dozens of discrete cloud features at J and H bands; several features in the northern hemisphere are also detectable at K′. By tracking features over four days, we extend the zonal wind profile well into the northern hemisphere. We report the first measurements of wind velocities at latitudes −13°, +19°, and northward of +43°, the first direct wind measurements near the equator, and the highest wind velocity seen yet on Uranus (+218 m/s). At northern mid-latitudes (+20° to +40°), the winds appear to have accelerated when compared to earlier HST and Keck observations; southern wind speeds (−20° to −43°) have not changed since Voyager measurements in 1986. The equator of Uranus exhibits a subtle wave structure, indicated by diffuse patches roughly every 30° in longitude. The largest discrete cloud features on Uranus show complex structure extending over tens of degrees, reminiscent of activity seen around Neptune's Great Dark Spot during the Voyager encounter with that planet. There is no sign of a northern “polar collar” as is seen in the south, but a number of discrete features seen at the “expected” latitudes may signal the early stages of development of a northern collar.