More than 3 Mars' years (MY) of atmospheric argon (Ar) measurements are used to study annual and seasonal variations in atmospheric transport and mixing. Data are obtained over the period 20 May 2002 to 4 May 2008 by the Gamma Subsystem (GS) of the Gamma Ray Spectrometer (GRS) on the Mars Odyssey spacecraft in orbit around Mars. Here we augment previous studies of Mars' Ar in which strong seasonal variations were observed and horizontal meridional mixing coefficients for the southern hemisphere were computed. Comparison of year‐to‐year seasonal abundance shows strong similarity but also some short‐period (∼15°–30° L s ) and interannual variations. Evidence for short periods of strong eddy transport is exhibited during autumn and winter. The seasonal change in Ar concentration for southern latitudes is relatively gradual and well defined, but seasonal changes at high northern latitudes are chaotic and indicate that atmospheric disturbance is ubiquitous. Major topographic landforms (Elysium, Tharsis, Noachis Terra, Hellas) apparently have little control over seasonal Ar concentration at the spatial resolution of the GRS data set. Some indication of local enhanced Ar concentration is present from 30°N to 60°N for the Hellas and Tharsis sectors in late winter and early spring. The data show some significant (3 σ ) differences between MY 26 and MY 27 in geographical sectors that are likely produced by local weather. The GS data do not show seasonal variation of Ar at equatorial and low‐latitude zones, in contrast to those from the Alpha Particle X‐ray Spectrometer (APXS) measurements from the Mars Exploration Rovers.

SPECTIVE OF USE AS A GEOPHYSICAL TRACER. P.-Y. Meslin, W.V. Boynton, J.-C. Sabroux , F. Forget, E. Chassefière , O. Gasnault , J.-F. Pineau , A.E. Metzger , B. Janes 2 and the GRS team, Laboratoire de Météorologie Dynamique, IPSL, Université Paris 6, Fra nce (pierre-yves.meslin@lmd.jussieu.fr), Lunar and Planetary Laboratory (Tucson, AZ), Institut de Radioprotection et de Sûreté Nucléaire (Gif-sur-Yvette, France), Service d'Aéronomie, IPSL (Paris, France), 5 Centre d’Etude Spatiale des Rayonnements (Toulouse , France), Albedo Technologies (Razès, France), Jet Propulsion Laboratory, Pasadena, CA.

MODEL ATMOSPHERES ON CONCENTRATION COMPUTATIONS. A. L. Sprague, W. V. Boynton, A. Colaprete, D. M. Janes, A.E. Metzger, K.E. Kerry, F. Forget, R. Starr, R. M. Haberle, Lunar and Planetary Laboratory, Tucson, AZ, 85745, (sprague@lpl.arizona.edu) NASA Ames Research Center, Moffett Field, Mountain View, CA 94035-1000 (Anthony.Colaprete-1@nasa.gov), Jet Propulsion Laboratory, Pasadena, CA, 91109, Institut Pierre Simon Laplace Laboratoire de Météorologie Dynamique, UMR 8539 Université Paris 6, BP 99, 4 place Jussieu, 75252 Paris, France, Department of Physics, Catholic University of America, Washington, DC.

One and one half Mars years, from 8 June 2002 to 2 April 2005, of atmospheric argon (Ar) measurements are described and studied in the context of understanding how Ar, a minor constituent of the Martian atmosphere that does not condense at temperatures found on Mars, can be used to study Martian circulation and dynamics. The Ar data are from the gamma subsystem of the gamma ray spectrometer currently operating on the Mars Odyssey spacecraft in orbit around Mars. A comprehensive data analysis including γ ray production and attenuation by the atmosphere is presented. Four discoveries are discussed. (1) There is a factor of 6 enhancement of Ar measured over south polar latitudes (75°S to 90°S) for 2 consecutive Mars years occurring near the onset of southern winter. (2) There is no similar strong enhancement of Ar over north polar regions during northern winter. Part of this difference is explained by the global topographic dichotomy and the fact that the duration of northern autumn and winter is only ∼80% as long as that of southern autumn and winter. (3) Rapid seasonal fluctuations in Ar abundance from 60°S to 90°S may be evidence for wave activity at the perimeter of the southern seasonal polar cap. (4) The magnitude of the Ar enhancement over the south polar region lacks spatial coincidence with either the relatively dark “cryptic terrain” or the relatively bright albedo regions; however, a divergence in Ar abundance near Ls = 267.5° may be an expression of later CO2 sublimation in the bright terrain rather than in the cryptic region.

The atmospheric processes underlying the observed spatial and temporal enhancement of noncondensing gases in Mars' atmosphere are investigated. The Gamma Ray Spectrometer (GRS) on board Mars Odyssey has obtained measurements indicating that the absolute and relative column abundance of noncondensing gases (primarily argon and nitrogen) maximizes at high latitudes in both hemispheres during winter as CO2 gas condenses and forms the seasonal polar ice cap. This condensing CO2 "leaves behind" noncondensing gases whose local absolute and relative column abundances can increase at a rate controlled by mixing with less-enhanced air from lower latitudes. Understanding the processes responsible for the magnitude and seasonal variations of these enhancement values is an aid in understanding atmospheric transport processes. The NASA Ames Mars General Circulation Model is employed to help understand the atmospheric thermodynamical mechanisms that give rise to the observed temporal and magnitude variations in the polar enhancement values. The model produces a threefold noncondensable gas enhancement in the south polar region and an approximate 1.4-fold increase in noncondensables in the north polar region. These model results are temporally consistent with observed values, but the observed enhancement magnitudes exceed those modeled by up to a factor of two. The difference in strength and the season of formation between transient eddies in the southern and northern hemispheres may play a large role in determining the different character of the two polar enhancements. Model simulations also illuminate the effect that topography, orbital eccentricity, and atmospheric dust opacity have on producing the north versus south polar enhancement dichotomy.

Quantifying secular variations in the chemical composition of the Martian crust provides unique insights into the processes that have guided the evolution of the Martian crust‐mantle system. Using global abundances for a suite of elements determined by the Gamma Ray Spectrometer (GRS) on board the Mars Odyssey spacecraft and global mapping of apparent surface age adapted from existing geologic maps in the USGS Martian Geologic Investigation series, we report the average abundance of K, Th, Fe, Cl, H, and Si for the major Martian geologic epochs (Noachian, Hesperian, and Amazonian). Average GRS‐determined K and Th abundances generally decrease by 9% and 7%, respectively, between the Hesperian and the Amazonian, possibly implying evolving magma chemistry throughout major resurfacing events (although the effects of surficial alteration processes cannot be entirely discounted). GRS‐determined Fe and Cl averages increase by 12% and 19%, respectively, with younger apparent relative surface age, suggesting the possible mobilization and transport of these elements through aqueous processes (although an igneous origin for the variation in Fe also cannot be excluded). While H abundance does vary with surface age, the relationship is likely not governed by geologic processes. No statistically reliable apparent surface age relation was found for Si.

Northern auroral regions of Earth were imaged with energetic photons in the 0.1-10 keV range using the High-Resolution Camera (HRC-I) aboard the Chandra X-ray Observatory at 10 epochs (each ~20 min duration) between mid-December 2003 and mid-April 2004. These observations aimed at searching for Earth's soft (<2 keV) X-ray aurora in a comparative study with Jupiter's X-ray aurora, where a pulsating X-ray "hot-spot" has been previously observed by Chandra. The first Chandra soft X-ray observations of Earth's aurora show that it is highly variable (intense arcs, multiple arcs, diffuse patches, at times absent). In at least one of the observations an isolated blob of emission is observed near the expected cusp location. A fortuitous overflight of DMSP satellite F13 provided SSJ/4 energetic particle measurements above a bright arc seen by Chandra on 24 January 2004, 20:01-20:22 UT. A model of the emissions expected strongly suggests that the observed soft X-ray signal is bremsstrahlung and characteristic K-shell line emissions of nitrogen and oxygen in the atmosphere produced by electrons.

On November 25 and 26, 1999, the Chandra X-ray spacecraft conducted a set of four 19,000 sec observations of Jupiter. The ACIS-S instrument configuration was used for its good low energy efficiency and spatial resolution. An anomalous response was obtained which was subsequently attributed to strong jovian infrared radiation penetrating the detector and piling up spurious events across the entire X-ray range. However, the pre-observation establishment of an offsetting bias field has allowed the recovery of data from that portion of Jupiter's disc which remained within the elevated portion of the bias field during the observation. This ranges from fewer than 3000 sec to the entire observing time for about 10% of the planet. Auroral emission is seen near both poles in each observation. The northern aurora ia overall more intense than the southern, consistent with prior Einstein and ROSAT Observatory results. The southern aurora shows more modulation with Jupiter's rotation than the northern. Spatial resolution has been improved by at least a factor of two over prior measurements but convincing evidence of structure has not been seen. Lower latitude emission, first observed by ROSAT, is confirmed with flux levels averaging more than a factor of five below peak auroral values. Pronounced variation in the observed emission has occurred over the observing period. The spectral response extends from 0.24 keV, below which noise dominates, to about 1.2 keV. For all four observations the spectrum is clearly enhanced between 0.45 and 0.85 keV. This is apparently unequivocal evidence that Jupiter's X-ray emission is the result of oxygen and perhaps sulfur ions precipitating into the planet's atmosphere, where they undergo charge exchange interactions. The identification of specific transitions lines in the spectrum is among the ongoing efforts. A bremsstrahlung component has not yet been identified.