Global distribution of near‐surface hydrogen on Mars

TLDR

Neutron spectroscopy indicates that some equatorial Martian regolith may contain up to ~11 % water‑equivalent hydrogen in the upper 10 g cm⁻², implying significant subsurface water reservoirs. Neutron data from Mars Odyssey reveal hydrogen‑rich deposits ranging from 2–10 % water‑equivalent near the equator to 20–100 % poleward of ±50°, with a global near‑surface hydrogen inventory equivalent to a ~14‑cm water layer assuming a 1‑m reservoir.

Abstract

Neutron data observed using the Neutron Spectrometer aboard 2001 Mars Odyssey provide a lower limit to the global inventory of Martian water‐equivalent hydrogen. Hydrogen‐rich deposits ranging between about 20% and 100% water‐equivalent by mass are found poleward of ±50° latitude, and less rich, but significant, deposits are found at near‐equatorial latitudes. The equatorial deposits between ±45° latitude range between 2% and 10% water‐equivalent hydrogen by mass and reach their maximum in two regions that straddle the 0‐km elevation contour. Higher water abundances, up to ∼11%, are required in subsurface regolith of some equatorial regions if the upper 10 g/cm 2 of regolith is desiccated, as suggested on average by comparison of epithermal and fast neutron data. The hydrogen contents of surface soils in the latitude range between 50° and 80° north and south are equal within data uncertainties. A lower‐limit estimate of the global inventory of near surface hydrogen amounts to a global water layer about 14 cm thick if the reservoir sampled from orbit is assumed to be 1 m thick.