Medusae Fossae-elysium Region, Mars: Depression in the Hend/odyssey Map of Mars Epithermal Neutrons

Introduction. The first data from the Gamma Ray Spectrometer (GRS) onboard Mars Odyssey spacecraft showed that the low neutron fluxes characterize both subpolar regions of Mars [1-3]. The low neutron fluxes mean the presence of hydrogen-rich soils and have been i nterpreted as an indication on abundant water ice in these areas. The equatorial region of Mars (equatorward of ~50 o) is characterized by higher fluxes of both epithermal (0.4 eV-100 keV, come from depth 1-2 m) and fast (3.4-7.3 MeV, come from depth 0.2-0.3 m) [3] neutrons meaning that this area is mostly dry. The pattern of distribution of the neutron fluxes is in a good agreement with the theoretical predictions on the stability of ground ice on present Mars [4,5]. The actual distribution of the ice, however, depends on variations of thermal inertia of soils and albedo of the surface [6]. The flux of the epithermal neutrons detected by the HEND instrument, which is part of GRS, has two noticeable depressions in the equatorial region, one in Arabia Terra and another in the Medusae Fossae-Elysium region (MFER) [1-3]. Here we present the initial results of analysis of characteristics of the neutron fluxes and regional geological setting of the epithermal neutron depression in this area. The main goal of our study was to put some constraints on the time of the anomaly formation and to assess possible form of hydrogen (ground ice vs. chemically bound water) there. Data analyzed. We analyzed data acquired by the HEND instrument during the first seven months of the experiment, from 19 February to 13 September 2002 [7,8]. The whole data set corrected for the Solar flares has been binned into 5 x 5 o (~300 km) pixels (2592 pixels) and grouped into five time series corresponding to mean areocentric longitudes from 339.9 (northern winter), through 1.2, 19.7, 35.4, to 55.4 (mid northern spring). This allows characterizing changes in the neutron fluxes as a function of the Martian seasons. For the purposes of our study, a data subset extending from 50 o S to 50 o N and from 120 o W (Arsia Mons) to 240 o W (western edge of Elysium Planitia), 480 pixels, was extracted from the global data set. Neutron fluxes in the Medusae Fossae-Elysium Region. In the area under study, pattern of areal distribution of the fast neutron flux does not display any consistent, positive or negative, signature that would …