[1] 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/cm2 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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JE002160

Global distribution of near-surface hydrogen on Mars

Prime objectives of the neutron spectrometer (NS) component of the Gamma-Ray Spectrometer suite of instruments aboard Mars Odyssey are to identify the major reservoirs of hydrogen on Mars, determine their relative contributions to its total water inventory, and estimate the portion of the current inventory that is near the surface. Although more information is required than is currently available, epithermal neutron currents alone can provide a significant lower bound of hydrogen abundances on Mars. Observations from Viking 1, Viking 2, and Mars Pathfinder positively identified two of these reservoirs. By far the largest near-surface reservoir is comprised of the two residual polar caps, which together are sufficient to cover Mars with a global ocean about 30 m deep. The second is contained in the atmosphere, which if deposited on the surface, would cover Mars with a thin film of water about 10{sup -5} m deep. Although negligible in comparison, the fact that an atmospheric reservoir exists shows that it can provide a conduit that couples transient reservoirs of near-surface water ice. It has long been speculated that Mars has had, and may still retain, a far larger reservoir of water. Topographic features such as rampart craters, collapsed chaotic terrain, massivemore » outflow channels, and valley networks provide strong support for the past existence of large bodies of surface water. Measurements of the areal size and depth of all paleo-water and volcanic features led to an estimate of a total water inventory equivalent to a global ocean that was between 100 and 500 m thick. Measurements of the D/H ratio have allowed predictions that between 5 and 50 m of this inventory was lost to space. Altogether, these estimates lead to between 20 and 465 m of water from the juvenile Martian inventory that is not accounted for. First analyses of Mars Odyssey neutron and gamma-ray data showed that reservoirs of hydrogen do indeed exist poleward of about {+-}50{sup o} latitude. Mars Odyssey neutron observations also revealed a near equatorial hydrogen reservoir that maximizes in Arabia Terra and its antipode. Initial quantitative estimates of hydrogen abundances in these investigations were normalized to an assumed 1% H{sub 2}O content by mass for the Viking 1 landing site. However, a recent analysis of the seasonal variation of the CO{sub 2} frost cover at the north pole has allowed an independent absolute calibration of the three neutron energy bands measured using the NS aboard Mars Odyssey. This calibration allows a reinterpretation of neutron fluxes measured globally to provide a lower bound of the hydrogen abundance within about 1 m of the Martian surface. A determination of true hydrogen abundances requires knowledge of the stratigraphy of hydrogen-bearing layers because the presence of an overlying relatively desiccated layer would mask enhanced abundances of a lower layer.« less

http://geology.indiana.edu/bish/Feldman.pdf

The Global Distribution of Near-Surface Hydrogen on Mars

[1] We report maps of the concentrations of H, Si, Cl, K, Fe, and Th as determined by the Gamma Ray Spectrometer (GRS) on board the 2001 Mars Odyssey Mission for ±∼45° latitudes. The procedures by which the spectra are processed to yield quantitative concentrations are described in detail. The concentrations of elements determined over the locations of the various Mars landers generally agree well with the lander values except for Fe, although the mean of the GRS Fe data agrees well with that of Martian meteorites. The water-equivalent concentration of hydrogen by mass varies from about 1.5% to 7.5% (by mass) with the most enriched areas being near Apollinaris Patera and Arabia Terra. Cl shows a distribution similar to H over the surface except that the Cl content over Medusae Fossae is much greater than elsewhere. The map of Fe shows enrichment in the northern lowlands versus the southern highlands. Silicon shows only very modest variation over the surface with mass fractions ranging from 19% to 22% over most of the planet, though a significant depletion in Si is noted in a region west of Tharsis Montes and Olympus Mons where the Si content is as low as 18%. K and Th show a very similar pattern with depletions associated with young volcanic deposits and enrichments associated with the TES Surface Type-2 material. It is noted that there appears to be no evidence of significant globally distributed thick dust deposits of uniform composition.

Concentration of H, Si, Cl, K, Fe, and Th in the low- and mid-latitude regions of Mars

[1] We use Thermal Emission Imaging System (THEMIS) data to identify and characterize the global distribution of distinct materials interpreted to contain chloride salts on the Martian surface. Previously mapped global geochemical and physical properties are used in concert with thermophysical and morphological observations to assess the materials' local and regional characteristics. The results of our survey have expanded the characterization of the materials from ∼200 to ∼640 distinct sites dispersed throughout low-albedo Noachian- and Hesperian-aged terrains. Our survey also shows that the materials are detected in locally thermophysically distinct terrains and display a range of morphologies. Topography indicates that the majority of the materials occur in local lows, although crosscutting relationships indicate that some sites are located in “geologic windows” implying that the materials may be older than the terrains in which they are situated. Once exposed, the materials appear to undergo erosion, which may be the reason we do not observe large laterally extensive materials at the surface. The materials are predominantly local in nature, yet their prevalence across the southern highlands suggests that they represent one or more globally ubiquitous processes. We consider a number of formation hypotheses but find that most observations are consistent with formation via ponding of surface runoff or groundwater upwelling, although efflorescence and hydrothermal activity may also be possible in some locales. The materials' inferred ages suggest that the conditions that enabled the deposition of the materials persisted for up to 1 billion years.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JE003613

Geologic context of proposed chloride‐bearing materials on Mars

Using Dawn’s Gamma Ray and Neutron Detector, we tested models of Vesta’s evolution based on studies of howardite, eucrite, and diogenite (HED) meteorites. Global Fe/O and Fe/Si ratios are consistent with HED compositions. Neutron measurements confirm that a thick, diogenitic lower crust is exposed in the Rheasilvia basin, which is consistent with global magmatic differentiation. Vesta’s regolith contains substantial amounts of hydrogen. The highest hydrogen concentrations coincide with older, low-albedo regions near the equator, where water ice is unstable. The young, Rheasilvia basin contains the lowest concentrations. These observations are consistent with gradual accumulation of hydrogen by infall of carbonaceous chondrites—observed as clasts in some howardites—and subsequent removal or burial of this material by large impacts.

https://science.sciencemag.org/content/sci/338/6104/242.full.pdf

Elemental mapping by Dawn reveals exogenic H in Vesta's regolith.

The Mars Odyssey Gamma-Ray Spectrometer is a suite of three different instruments, a gamma subsystem (GS), a neutron spectrometer, and a high-energy neutron detector, working together to collect data that will permit the mapping of elemental concentrations on the surface of Mars. The instruments are complimentary in that the neutron instruments have greater sensitivity to low amounts of hydrogen, but their signals saturate as the hydrogen content gets high. The hydrogen signal in the GS, on the other hand, does not saturate at high hydrogen contents and is sensitive to small differences in hydrogen content even when hydrogen is very abundant. The hydrogen signal in the neutron instruments and the GS have a different dependence on depth, and thus by combining both data sets we can infer not only the amount of hydrogen, but constrain its distribution with depth. In addition to hydrogen, the GS determines the abundances of several other elements. The instruments, the basis of the technique, and the data processing requirements are described as are some expected applications of the data to scientific problems.

A. K. Ton'chev

Papers

The Mars Odyssey Gamma-Ray Spectrometer Instrument Suite