The global distribution of near-surface hydrogen on Mars
- William C.
- Thomas H.
- Sylvestre
- David L.
- David T.
- William V.
- Richard C.
- Herbert O.
- Robert L.
- Kurt R.
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, massive 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.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 977413
- Report Number(s):
- LA-UR-04-0355; LA-UR-04-355; TRN: US201009%%747
- Resource Relation:
- Journal Volume: 109; Journal Issue: E9; Conference: Submitted to: Sixth International Conference on Mars, Pasadena, California, July 20-25, 2003
- Country of Publication:
- United States
- Language:
- English
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