Analysis of Layering at Mars Near-surface Using Attenuation of Chlorine Gamma Rays

Introduction: Data collected by the Mars Gamma Ray Spectrometer (GRS) orbiting aboard the 2001 Mars Odyssey spacecraft indicates that the global distribution of chlorine on Mars is heterogeneous [1]. The GRS 50% footprint has a diameter of ~440 km for low energy Cl lines and ~540 km for high energy Cl lines, however smoothing with a 10-degree radius filter is required to improve signal to noise for the Cl signal [2]. Even with this smoothing, Cl concentration vary across the globe by a factor of ~3-4 , with highest Cl concentration centered over the Medusae Fossae Formation materials to the west of Tharsis Montes and low Cl values in the southern highlands and around Utopia Planitia. Keller et al. [1] describe this global distribution in further detail along with possible geologic mechanisms that may be responsible including Cl-rich fine deposits, dilution by Cl-poor rocks, volcanic acid-fog reactions [3, 4], alteration in hydrothermal systems [5], and aqueous leaching, transport, and deposition. Newsom et al. [6] also discusses the distribution of Cl based upon an investigation of regions with evidence of either extensive bedrock or proposed meter-thick mantling by airborne materials. While GRS detects gamma rays from the upper few tens of cm of the martian surface, lower energy gamma rays are more attenuated by the surface and atmosphere than higher energy gamma rays. Here, we use this difference in attenuation to search for evidence in the GRS data for significant regional layering of Cl within the upper few tens of cm of the surface. Evidence of layering would provide insight into the 3dimensional distribution of Cl and the distribution of materials at the martian near-surface. Chorine Layering Models: A high energy and low energy Cl gamma ray peak, 6111 and 1951 keV respectively, were used to look for evidence of attenuation that might indicated layering. Using a gamma ray and neutron transport code and models using various non-layered compositions, we determined that the thickness of the martian atmosphere has an effect on the flux of high and low energy Cl gamma ray lines generated at the surface. This is due to atmospheric effects on the the neutron energy distribution found at the surface. We found that the ratio of 6111/1951 gamma rays at the surface is a smooth function of atmospheric thickness and reasonably independent of surface composition. An atmospheric correction was determined by fitting a 3 order polynomial to non-layered model results. This factor has been applied to all 6111/1951 results presented below. Next, we determine the flux of 6111 and 1951 keV Cl gamma rays predicted at the surface of Mars from various two-layer models summarized in Table 1. For soils, we adjusted the Cl content of mean Pathfinder soil. The rock composition was representative of a medium iron basalt. For each model listed, we assigned top-layer depths of 0, 5, 10, 20, and 40 cm. Additionally, for Model A and B involving only soils, we adjusted the water composition to values of 1, 3, 5, and 7.5 wt% equivalent water. For Model C and D involving soil and rock, we used a soil water composition of 3 wt%. Table 1. Model Top Layer Bottom Layer A High Cl soil Low Cl soil B Low Cl soil High Cl soil C High Cl soil Low Cl rock D Low Cl rock High Cl soil