The Medusae Fossae-Elysium region,Mars: geologic characteristics of the depression of epithermal-neutron flux based on HEND measurements onboard the Mars Odyssey spacecraft

We studied the geological structure and characteristics of fluxes of fast and epithermal neutrons from the area of Mars within 50°S–50° N and 120°–240° W, the Medusae Fossae-Elysium region. The study area does not show any anomalies in the energy spectrum of fast neutrons, indicating that a thin (10–20 cm) subsurface layer in the equatorial region is characterized by quite a uniform distribution of hydrogen-containing phases. At the same time, the studied region is characterized by a distinct depression in the epithermal-neutron flux, which is outlined by a 0.150–0.170 counts/s contour line and does not show any seasonal changes in its shape, depth, and latitudinal position. This suggests that the depression is not associated with the short-term (seasonal) variations of the temperature regime and that it represents an aggregation of hydrogen-containing phases with longer lifetimes. The position of the epithermal-neutron-flux depression does not correlate with regional geology, and none of the geological units can be responsible for its appearance. To a rather greater degree, the flux of epithermal neutrons correlates with the amount of dust material on the surface. This indicates that the dust material presumably played a major role in the formation of the neutron-flux depression within the Medusae Fossae-Elysium region. The depression of the epithermal-neutron flux presumably has a polygenic nature. It can be related both to the primary accumulation of water-bearing material and to the later blowing of this material by wind and transportation of the dust fraction with its simultaneous enrichment in hydrogen-containing phases (most likely, in water-bearing minerals).Translated from Astronomicheskii Vestnik, Vol. 39, No. 1, 2005, pp. 3–25. Original Russian Text Copyright © 2005 by Ivanov, Kozyrev, Litvak, Mitrofanov.