The effect of craters on the lunar neutron flux

Abstract

The variation of remotely sensed neutron count rates is measured as a function of cratercentric distance using data from the Lunar Prospector Neutron Spectrometer. The count rate, stacked over many craters, peaks over the crater center, has a minimum near the crater rim, and at larger distances, it increases to a mean value that is up to 1% lower than the mean count rate observed over the crater. A simple model is presented, based upon an analytical topographical profile for the stacked craters fitted to data from the Lunar Orbiter Laser Altimeter. The effect of topography coupled with neutron beaming from the surface largely reproduces the observed count rate profiles. However, a model that better fits the observations can be found by including the additional freedom to increase the neutron emissivity of the crater area by ∼0.35% relative to the unperturbed surface. It is unclear what might give rise to this effect, but it may relate to additional surface roughness in the vicinities of craters. The amplitude of the crater-related signal in the neutron count rate is small, but not too small to demand consideration when inferring water-equivalent hydrogen (WEH) weight percentages in polar permanently shaded regions (PSRs). If the small crater-wide count rate excess is concentrated into a much smaller PSR, then it can lead to a large bias in the inferred WEH weight percentage. For instance, it may increase the inferred WEH for Cabeus crater at the Moon's south pole from ∼1% to ∼4%.

Bistatic radar observations of the Moon using Mini-RF on LRO and the Arecibo Observatory

“…The south pole of the Moon, in general, was a target of the Mini-RF bistatic campaign because of various lines of evidence consistent with the presence of H, OH, water or water ice in the near surface [Feldman et al, 1998;Feldman et al, 2001;Colaprete et al, 2010;Mitrofanov et al, 2010;Gladstone et al, 2012;Spudis et al, 2010;Thomson et al, 2012;Zuber et al, 2012;Lucey et al, 2014;Eke et al, 2015]. Much of the research into water ice at the poles has focused on permanently shadowed regions (PSRs).…”

A tale of two poles: Toward understanding the presence, distribution, and origin of volatiles at the polar regions of the Moon and Mercury

“…Finally, Eke et al . [] carried out a global analysis of LP epithermal neutron data and found that the geometry of bowl‐shaped craters in the size range of 20 to 60 km could cause a subtle enhancement of epithermal neutrons. Based on this analysis, Eke et al .…”

“…Based on this analysis, Eke et al . [] suggested that the inferred hydrogen abundances within the relatively deep Cabeus crater could have been underestimated at 1 wt % WEH and may be as high as ~4 wt % WEH.…”

“…Based on the knowledge that these different neutron energy ranges are sensitive to different hydrogen burial depths, Miller et al [2014] concluded that hydrogen enhancements of~1 wt % WEH at Shackleton crater were possibly located at the surface, whereas the hydrogen enhancements at other south polar locations were likely buried by tens of centimeters of drier material. Finally, Eke et al [2015] carried out a global analysis of LP epithermal neutron data and found that the geometry of bowl-shaped craters in the size range of 20 to 60 km could cause a subtle enhancement of epithermal neutrons. Based on this analysis, Eke et al [2015] suggested that the inferred hydrogen abundances within the relatively deep Cabeus crater could have been underestimated at 1 wt % WEH and may be as high as~4 wt % WEH.…”

How thick are Mercury’s polar water ice deposits?

“…While the LCROSS experiment (Colaprete et al 2010) found a few per cent by mass of the material in Cabeus crater on the Moon was water ice, which is at a level consistent with neutron spectroscopy results (Eke et al 2015), there is evidence for substantially purer water ice deposits near Mercury's poles.…”