Enhanced Hydrogen Abundances Near Both Lunar Poles

Details Enhanced Hydrogen Abundances Near Both Lunar Poles Enhanced Hydrogen Abundances Near Both Lunar Poles

Jan 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...14f&link_type=abstract

Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 14

Computer Science

Hydrogen, Ice, Lunar Surface, Polar Caps, Moon, Water, Lunar Craters, Lunar Composition, Selenology, Lunar Prospector, Neutron Spectrometers

Scientific paper

Chemical analyses of all samples of the Moon returned to Earth show that the lunar surface is highly depleted in volatiles. Specifically, the H content of lunar soils averages only 50 ppm, which can be explained in terms of surface implantation of solar-wind H. We note that all returned samples come from near-equatorial latitudes where daytime temperatures are sufficiently high that water is not stable to evaporation, photo dissociation, ionization, and eventual loss to space through pickup by the solar wind. However, it has long been postulated that a significant fraction of water delivered to the Moon by comets, meteoroids, and interplanetary dust can be stably trapped within the permanently shaded floors of polar craters where temperatures are sufficiently low so that sublimation times can be longer than several billion years. Recent results from analysis of the high-altitude (100 +/- 20 km) portion of the Lunar Prospector Neutron Spectrometer (LPNS) dataset have revealed that H abundances near both lunar poles are enhanced relative to that which exist at equatorial latitudes. Because this average enhancement is not much larger than the near-equatorial average of 50 ppm, it is reasonable to ask how much of the polar-H enhancement comes from the solar wind and how much comes from lunar impacts by solid interplanetary materials. Perhaps the low temperatures at polar latitudes could reduce loss rates of solar-wind-implanted H sufficiently to account for the inferred difference between average polar and equatorial H abundances. Although the foregoing suggestion is plausible, neither laboratory simulations on returned soil samples nor numerical simulations of H loss rates from the radiation-damaged surfaces of soil grains have been performed to prove its feasibility. We try to address this question by analyzing the low-altitude (30 +/- 15 km) portion of LPNS data to search for relatively small spatial-scale enhancements in H data to search for relatively small spatial-scale enhancements in H abundances at both lunar poles. Maps were constructed of epithermal-neutrons corrected for elemental abundance variations by subtracting 7% of measured thermal-neutron counting rates. Although the spatial resolution of the LPNS at 30-km altitude is about 55 km FWHM, we binned all the data in 0.5 deg x 0.5 deg spatial pixels and then applied a 30-km FWHM Gaussian smoothing algorithm. Resultant polar maps of corrected epithermal counting rates are shown. Inspection reveals discrete depressions in counting rates that are superimposed on more generally distributed depressions that surround both poles. Comparison with the radar-measured polar topography shows that the areas of most depressed epithermal counts rates in the south overlie craters that have floors in permanent shade. Furthermore, these depressions are neither cylindrically symmetric about either pole nor do they minimize at the poles. Similar maps of fast neutrons (not shown here) reveal a single, statistically significant depression centered on the maximum depression of the epithermal neutrons at about 88 S, 20 E. Comparison between high-altitude and low-altitude epithermal maps reveals a larger depression at low altitudes in the south but the same magnitude of depression in the north. Hydrogen enhancements in the south must therefore have spatial scales comparable to the spatial resolution of LPNS, 55-km FWHM, yet consist of smaller clumps more uniformly distributed over the LPNS field of view in the north. A quantitative comparison between measured epithermal counting rates and numerical simulations using the measured polar topography yields the following: (1) H abundances within the permanently shaded craters near the south pole are equivalent to a water-ice mass fraction of 1.5 +/- 0.8%, (2) the enhanced H within these craters must not be buried beneath the surface by more than -log /square cm (5 cm at a density of 2g / cubic centimeter (3) the H abundance near both poles averages 100 ppm above that known from near-equatorial returned soil samples (50 ppm,), and (4) the total mass content of H poleward of about 75 deg is about 200 x 106 t, and that poleward of +75deg is about 150 x 106 t. If the enhanced H within all regions of permanent shade near both poles is in the form of water ice, use of the area estimates given by Margot et al yields estimates of 135 (240) x 106 t in the 2250 (4000) square km of shaded areas poleward of -87.5 deg, and 62 x 10 6 t in the 1030 square km of shaded areas poleward of +87.5 deg. We note, however, that neutron observations by themselves cannot uniquely identify the chemical form of H in lunar regolith. That is, LPNS data cannot discriminate solar-wind-implanted H from OH, H20, etc. In the absence of a temperature map of the lunar poles and a knowledge of the retentivity of H by lunar soil grains as a function of temperature, sufficient information is not yet available to uniquely identify water-ice deposits at the lunar poles. Nevertheless, the floors of permanently shaded polar craters are predicted to have temperatures sufficiently low that water ice should be stable for billions of years, yet uncratered surfaces near the polar cap are too high in temperature to retain water ice. This suggests that at least some of the enhanced deposits of H identified by LPNS are in the form of water ice. (Additional information contained in the original)

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