Soil water content on Mars as estimated from neutron measurements by the HEND instrument onboard the 2001 Mars Odyssey spacecraft

We present the results of 20 months of observations of Mars by the Russian HEND instrument onboard the NASA 2001 Mars Odyssey spacecraft. We show that there are two extended subpolar regions with a soil water content of several tens of percent in the northern and southern hemispheres of Mars. The southern subpolar region is well described by a two-layer model, according to which a soil with a water content of up to 55% by mass lies under a relatively dry soil with a water mass fraction of 2% and a thickness of 15-20 g/cm(2). The distribution of water in Martian regolith northern subpolar region is in good agreement with the homogeneous model and does not require invoking the more complex two-layer soil model. The water-ice content in the subsurface layer of the northern subpolar region reaches 53% by mass. We show that there are two regions with a relatively high water content near the Martian equator. These are Arabia Terra and the Medusae Fossae formation region southwest of Olympus Mons. In these regions, a lower layer with 9-10% of water by mass may underlie the upper layer of relatively dry material similar to30 g/cm(2) in thickness. The moistest spot near the equator is at about 30degrees E and 10degrees N. Its lower-layer soil may contain more than 16% of water by mass.