Barney J. Conrath

An intercomparison of ground‐based millimeter, MGS TES, and Viking atmospheric temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the global Mars atmosphere

During the period October 1997 to September 1999 we obtained and analyzed over 100 millimeter-wave observations of Mars atmospheric CO line absorption for atmospheric temperature profiles. These measurements extend through one full Mars year (solar longitudes LS of 190° in 1997 to 180° in 1999) and coincide with atmospheric temperature profile and dust column measurements from the Thermal Emission Spectrometer (TES) experiment on board the Mars Global Surveyor (MGS) spacecraft. A comparison of Mars atmospheric temperatures retrieved by these distinct methods provides the first opportunity to place the long-term (1982–1999) millimeter retrievals of Mars atmospheric temperatures within the context of contemporaneous, spatially mapped spacecraft observations. Profile comparisons of 0–30 km altitude atmospheric temperatures retrieved with the two techniques agree typically to within the 5 K calibration accuracy of the millimeter observations. At the 0.5 mbar pressure level (∼25 km altitude) the 30°N/30°S average for TES infrared temperatures and the disk-averaged millimeter temperatures are also well correlated in their seasonal and dust-storm-related variations over the 1997–1999 period. This period includes the Noachis Terra regional dust storm, which led to very abrupt heating (∼15 K at 0.5 mbar) of the global Mars atmosphere at LS = 224° in 1997 [Christensen et al., 1998; Conrath et al., this issue; Smith et al., this issue]. Much colder (10–20 K) global atmospheric temperatures were observed during the 1997 versus 1977 perihelion periods (LS = 200°–330°), consistent with the much (2 to 8 times) lower global dust loading of the atmosphere during the 1997 perihelion dust storm season versus the Viking period of the 1977a,b storms. The 1998–1999 Mars atmosphere revealed by both the millimeter and TES observations is also 10–15 K colder than presented by the Viking climatology during the aphelion season (LS = 0°–180°, northern spring/summer) of Mars. We reassess the observational basis of the Viking dusty-warm climatology for this season to conclude that the global aphelion atmosphere of Mars is colder, less dusty, and cloudier than indicated by the established Viking climatology even for the Viking period. We also conclude that Mars atmospheric temperatures exhibit their most significant interannual variations during the perihelion dust storm season (10–20 K for LS = 200°–340°) and during the post-aphelion northern summer season (5–10 K for LS = 100°–200°).

Thermal Emission Spectrometer results: Mars atmospheric thermal structure and aerosol distribution

Infrared spectra returned by the Thermal Emission Spectrometer (TES) are well suited for retrieval of the thermal structure and the distribution of aerosols in the Martian atmosphere. Combined nadir- and limb-viewing spectra allow global monitoring of the atmosphere up to 0.01 mbar (65 km). We report here on the atmospheric thermal structure and the distribution of aerosols as observed thus far during the mapping phase of the Mars Global Surveyor mission. Zonal and temporal mean cross sections are used to examine the seasonal evolution of atmospheric temperatures and zonal winds during a period extending from northern hemisphere mid-summer through vernal equinox (L, = 104°-360°). Temperature maps at selected pressure levels provide a characterization of planetary-scale waves. Retrieved atmospheric infrared dust opacity maps show the formation and evolution of regional dust storms during southern hemisphere summer. Response of the atmospheric thermal structure to the changing dust loading is observed. Maps of water-ice clouds as viewed in the thermal infrared are presented along with seasonal trends of infrared water-ice opacity. Uses of these observations for diagnostic studies of the dynamics of the atmosphere are discussed.

Mars Global Surveyor Thermal Emission Spectrometer (TES) observations of dust opacity during aerobraking and science phasing

The Mars Global Surveyor (MGS) arrived at Mars in September 1997 near Marss southern spring equinox and has now provided monitoring of conditions in the Mars atmosphere for more than half a Mars year. The large majority of the spectra taken by the Thermal Emission Spectrometer (TES) are in a nadir geometry (downward looking mode) where Mars is observed through the atmosphere. Most of these contain the distinct spectral signature of atmospheric dust. For these nadirgeometry spectra we retrieve column-integrated infrared aerosol (dust) opacities. TES observations during the aerobraking and science-phasing portions of the MGS mission cover the seasonal range Ls = 184°–28°. Excellent spatial coverage was obtained in the southern hemisphere. Northern hemisphere coverage is generally limited to narrow strips taken during the periapsis pass but is still very valuable. At the beginning of the mission the 9-μm dust opacity at midsouthern latitudes was low (0.15–0.25). As the season advanced through southern spring and into summer, TES observed several regional dust storms (including the Noachis dust storm of November 1997) where peak 9-μm dust opacities approached or exceeded unity, as well as numerous smaller local storms. Both large and small dust storms exhibited significant changes in both spatial coverage and intensity over a timescale of a day. Throughout southern spring and summer the region at the edge of the retreating southern seasonal polar ice cap was observed to be consistently more dusty than other latitudes.

Observations of Martian ice clouds by the Mars Global Surveyor Thermal Emission Spectrometer: The first Martian year

Successful operation of the Mars Global Surveyor spacecraft, beginning in September 1997 (Ls = 184°), has permitted extensive observations over more than a Martian year. Initially, thin (normal optical depth <0.06 at 825 cm−1) ice clouds and hazes were widespread, showing a distinct latitudinal gradient. With the onset of a regional dust storm at Ls = 224°, ice clouds vanished in the southern hemisphere, to reappear gradually after the decay of the storm. The zonally averaged cloud opacities show little difference between the beginning and end of the first Martian year. A broad low-latitude cloud belt with considerable longitudinal structure was present in early northern summer. Apparently characteristic of the northern summer season, it vanished between Ls = 140° and 150°. The latitudinal extent of this feature is apparently controlled by the ascending branch of the Hadley circulation. The most opaque clouds (optical depth ∼0.6) were found above the summits of major volcanic features; these showed spatial structure possibly associated with wave activity. Variety among low-lying late morning clouds suggests localized differences in circulation and microclimates. Limb observations showed extensive optically thin (optical depth <0.04) stratiform clouds at altitudes up to 55 km. Considerable latitude and altitude variations were evident in ice clouds in early northern spring (Ls = 25°); near 30 km, thin clouds extended from just north of the equator to ∼45°N, nearly to the north polar vortex. A water ice haze was present in the north polar night (Ls = 30°) at altitudes up to 40 km. Because little dust was present this probably provided heterogeneous nucleation sites for the formation of CO2 clouds and snowfall at altitudes below ∼20 km, where atmospheric temperatures dropped to the CO2 condensation point. The relatively invariant spectral shape of the water ice cloud feature over space and time indicates that ice particle radii are generally between 1 and 4 μm.

Forced waves in the martian atmosphere from MGS TES nadir data

Abstract 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.

The helium abundance of Saturn from Voyager measurements

Voyager radio-occultation and IR spectroscopy measurements are combined to infer an He mole fraction in the upper troposphere of Uranus of 0.152 + or - 0.033; the corresponding mass fraction is Y = 0.262 + or - 0.048. This value is in agreement with recent estimates of the solar He abundance, suggesting that He differentiation has not occurred on Uranus. Comparisons with values previously obtained for Jupiter and Saturn imply that migration of He toward the core began long ago on Saturn and may also have recently begun on Jupiter. The protosolar He abundance inferred from the Uranus measurements and from recent solar evolutionary models is used along with an assumed primordial He mass fraction of 0.23-0.24 to estimate a 3-4-percent enrichment of He in the interstellar medium between the big bang and the origin of the solar system. The result is in agreement with galactic chemical evolution models which include a substantial decrease in D during the evolutionary process.

One Martian year of atmospheric observations by the thermal emission spectrometer

The Mars Global Surveyor has completed one full Martian year of mapping. Infrared spectra returned by the Thermal Emission Spectrometer (TES) are very well suited for monitoring the thermal structure and the distribution of aerosols and water vapor in the Mars atmosphere. Nadir-viewing spectra allow a global picture of the state of the Mars atmosphere on a daily basis. We report here on the observed annual cycle of the latitudinal dependence of atmospheric temperature, dust aerosols, water-ice clouds, and water vapor.

Thermal tides and stationary waves on Mars as revealed by Mars Global Surveyor thermal emission spectrometer

Atmospheric temperature retrievals from thermal emission spectrometer (TES) observed radiances make possible the most complete separation of the constituent wave modes evident in Mars atmosphere to date. We use all of the data from the first aerobraking period as well as the science phasing orbits, which affords good sampling of the diurnal tides and stationary waves. TES retrievals of atmospheric temperature on a grid of pressure levels are the fundamental data set in this study. We then fit this data to selected Fourier modes in longitude and time for altitude, latitude, and Ls bins. From this we have identified the amplitudes and phases of the diurnal and semidiurnal tides, the first few (gravest) stationary waves, and a few modes which arise because of couplings between sun-fixed tides and topography. We also retrieve estimates of the zonal and time of day mean temperature meridional cross sections and their rates of change. The zonal and time of day mean temperature meridional cross sections agree with those of Conrath et al. [this issue] to within 1 K where we can reliably retrieve this mode (90°S to ∼20°S). Heating rates of up to 2.4 K/sol were observed around three scale heights above 60°S-90°S during the Ls = 310°–320° dust storm. Diurnal tide amplitudes of greater than 8 K were observed during the Noachis and Ls = 310°–320° dust storms. From Ls = 255°–285° an unexplained phase reversal at two scale heights was observed in the diurnal tide from 60°S-80°S. Convective penetration above the unstable boundary layer may explain anomalous (180° out of phase with the sun) diurnal tide phases between 0.5 and one scale height above the subsolar point. Semidiurnal tides are of order 2 K throughout the southern extratropics. A stationary mode of wavenumber one was observed with amplitude 1–4 K in the southern extratropics. Topographically coupled tidal modes were also quantified.

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

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

THERMAL STRUCTURE AND DYNAMICS OF NEPTUNE'S ATMOSPHERE FROM VOYAGER MEASUREMENTS

Spatially resolved Voyager infrared spectra from a global mapping sequence are used to characterize the thermal structure of the atmosphere of Neptune between approximately 50 and 100 mbar. A zonal mean meridional temperature cross section is obtained, which shows a minimum in the upper tropospheric and lower stratospheric temperatures at southern mid latitudes with maxima at the equator and high southern latitudes. This is qualitatively similar to the structure observed on Uranus, even though the obliquities and internal heat fluxes of the two planets are quite different. Thermal wind calculations show zonal wind speeds decreasing with altitude at all latitudes. Within the framework of a linear radiative-dynamical model, the thermal structure and cloud top winds together imply a frictional damping time comparable in magnitude to the radiative relaxation time; similar results have previously been obtained for the other three giant planets. Periodogram techniques for used to search the observed longitude variations of temperature for possible planetary-scale wave features. Data from two latitude bins are analyzed; one of the bins includes the Great Dark Spot, and the other is centered on the southern mid-latitude minimum in the zonal mean temperature. A weakly significant feature near planetary wavenumber 4 is identified at the latitude of the spot. The rest of the variance can be attributed to instrument noise and possibly a broadband meteorological spectrum indistinguishable from white noise.