Fluxes of Fast and Epithermal Neutrons from Lunar Prospector: Evidence for Water Ice at the Lunar Poles

Abstract

Maps of epithermal- and fast-neutron fluxes measured by Lunar Prospector were used to search for deposits enriched in hydrogen at both lunar poles. Depressions in epithermal fluxes were observed close to permanently shaded areas at both poles. The peak depression at the North Pole is 4.6 percent below the average epithermal flux intensity at lower latitudes, and that at the South Pole is 3.0 percent below the low-latitude average. No measurable depression in fast neutrons is seen at either pole. These data are consistent with deposits of hydrogen in the form of water ice that are covered by as much as 40 centimeters of desiccated regolith within permanently shaded craters near both poles.

Evidence for water ice on the Moon: Results for anomalous polar craters from the LRO Mini‐RF imaging radar

“…[30] The neutron spectrometer experiment flown on Lunar Prospector [Feldman et al, 1998[Feldman et al, , 2000Lawrence et al, 2006] documented the occurrence of excess hydrogen (roughly a factor of three times more than the global mean concentration) at both poles. Because this instrument had low resolution (pixels of 45 km or larger), it could not resolve the much smaller interiors of many of the permanently shadowed craters near the poles (Figure 11) [Bussey et al, 2005[Bussey et al, , 2010.…”

“…The Clementine team interpreted this result as indicating the presence of water ice on the upper wall of the crater Shackleton, near the pole. Subsequently, the Lunar Prospector neutron spectrometer found low levels of epithermal neutron flux near both poles, interpreted as indication of elevated hydrogen concentration there [Feldman et al, 1998[Feldman et al, , 2000.…”

Water detection on atmosphereless celestial bodies: Alternative explanations of the observations

“…The Effect of Surface Material Except surface temperature trends, two other kinds of regional variations can be explained in the context of the hypothesis of ion implantation origin of the hydrogen detected by neutron spectrometer on Lunar Prospector: (1) presence of excess hydrogen in some low-latitude regions [Feldman et al, 1998b] and (2) Note that all results discussed in this section (the order of the upper limit of hydrogen concentration, excess hydrogen in polar regions, and, probably, spots of excess hydrogen at lower latitudes) are expected to be obtained also for Mercury, if the neutron spectrometric measurements are performed there. Permanently shaded craters of Mercury are not exposed to direct keV proton irradiation, and the flux of Mev to Gev galactic protons is too small (a few protons per cm 2 in a second) to achieve considerable values of hydrogen concentration in a reasonable time.…”

“…Thus neutron spectrometry used 9 n Lunar Prospector to discover excess hydrogen in the lunar polar regions [Feldman et al, 1998a[Feldman et al, , 1998b[Feldman et al, , 2000 However, the only thing proved by radar reflection of such kind is that dielectric loss of the surface material is low, so that multipie scattering of the radar signal takes place. This is the case for water ice, but low dielectric loss can be characteristic of other materials.…”

Impacting Lunar Prospector in a cold trap to detect water ice

“…The existence of smaller scale enhancements that overlay polar regions likely to be in permanent shadow appears to provide a strong case for the presence of water ice near both poles [Feldman et al, 1998]. The ice appears to be distributed widely but in a patchy manner and may have been gardened into the regolith at about a 1% mixing ratio.…”