Mars' south polar Ar enhancement: a tracer for south polar seasonal meridional mixing.

The gamma ray spectrometer on the Mars Odyssey spacecraft measured an enhancement of atmospheric argon over southern high latitudes during autumn followed by dissipation during winter and spring. Argon does not freeze at temperatures normal for southern winter (approximately 145 kelvin) and is left in the atmosphere, enriched relative to carbon dioxide (CO2), as the southern seasonal cap of CO2 frost accumulates. Calculations of seasonal transport of argon into and out of southern high latitudes point to meridional (north-south) mixing throughout southern winter and spring.

Distribution of hydrogen in the near surface of Mars: evidence for subsurface ice deposits.

Using the Gamma-Ray Spectrometer on the Mars Odyssey, we have identified two regions near the poles that are enriched in hydrogen. The data indicate the presence of a subsurface layer enriched in hydrogen overlain by a hydrogen-poor layer. The thickness of the upper layer decreases with decreasing distance to the pole, ranging from a column density of about 150 grams per square centimeter at -42 degrees latitude to about 40 grams per square centimeter at -77 degrees. The hydrogen-rich regions correlate with regions of predicted ice stability. We suggest that the host of the hydrogen in the subsurface layer is ice, which constitutes 35 +/- 15% of the layer by weight.

Lunar surface radioactivity: preliminary results of the apollo 15 and apollo 16 gamma-ray spectrometer experiments.

Gamma-ray spectrometers on the Apollo 15 and Apollo 16 missions have been used to map the moon's radioactivity over 20 percent of its surface. The highest levels of natural radioactivity are found in Mare Imbrium and Oceanus Procellarum with contrastingly lower enhancements in the eastern maria. The ratio of potassium to uranium is higher on the far side than on the near side, although it is everywhere lower than commonly found on the earth.

Gamma ray spectroscopic measurements of Mars.

A gamma ray spectrometer placed in orbit around Mars is expected to yield significant compositional data which can be related to the evolution of that planet. Components of the observable gamma ray flux come from the Martian surface, galactic and intergalactic space, and the spacecraft itself. The flux can be detected by a scintillation crystal or solid state detector, either of which combines efficiency of detection with energy resolution, and returns information to the earth as a pulse height distribution in order to detect characteristic energy line structure. The data will be evaluated for evidence of elemental differentiation with reference to terrestrial, meteoritic, solar, and lunar abundances. A lengthy mission will allow the surface of Mars to be mapped in a search for possible correlations between composition and topography or albedo.