Searching the Moon’s Shadows
Initial LAMP observations of the faint nightside ultraviolet brightness of the Moon are shown here as tracks overlaid on a shaded topography map of the south pole region. A permanently shadowed crater, Faustini, shows a decrease in brightness compared to surrounding regions, while the crater selected for the October 9, 2009, LCROSS impact, Cabeus A, has a more modest contrast.
Maps produced by the Lyman- Alpha Mapping Project (LAMP) aboard NASA’s Lunar Reconnaissance Orbiter reveal newly discovered features at the Moon’s northern and southern poles in regions that lie in perpetual darkness, such as the bottoms of craters that never receive direct sunlight.
These permanently shadowed regions, or PSRs, have long been recognized as excellent cold traps, potentially capable of storing large quantities of volatiles in their soils for billions of years. Obtaining useful observations of PSRs is a challenging and difficult process, however, although it has been accomplished through passive remote sensing methods as well as the analysis of ejecta thrown up as a result of the intentional impact of the LCROSS spacecraft into such a crater in October 2009.
LAMP, developed by scientists at Southwest Research Institute (SwRI), uses a novel method to peer into PSRs, making visible what had been invisible. The instrument’s ultraviolet spectrograph detects far-ultraviolet light from two faint sources in the night sky: the all-sky Lyman-alpha glow produced when hydrogen atoms from the interplanetary medium scatter the Sun’s Lyman-alpha emissions, and the much fainter emissions of ultraviolet-bright stars. The reflected light from these two sources together produces only a few hundred events per second in the photon-counting LAMP instrument, so to build maps with useful signal-tonoise ratios required the careful accumulation of the observations from thousands of individual orbits by the lunar reconnaissance spacecraft. Fortunately, the LRO’s polar orbit around the Moon provides for repeated observations of the permanently shadowed areas.
Darker, redder, wetter
The LAMP maps have shown that many PSRs are darker at farultraviolet wavelengths and redder than nearby surface areas that receive sunlight. The darker regions are consistent with large surface porosities — indicating “fluffy” soils — while the reddening is consistent with the presence of water frost on the surface.
While not conclusive, the LAMP results suggest that high porosity and the presence of as much as 1 to 2 percent water frost in the permanently shadowed soil of several south polar craters was consistent with their observed far ultraviolet albedos. This was unexpected, as earlier research had indicated that interplanetary medium-originated Lyman-alpha would destroy any water frost at the surface faster than it can accumulate.
The LAMP team estimates that the loss of water frost is about 16 times slower than previously had been believed. In addition, the accumulation of water frost is also likely to be highly dependent on local conditions, such as temperature, thermal cycling and even geologically recent “impact gardening” in which micrometeoroid impacts redistribute the location and depth of volatile compounds.
Finding water frost at these new locations adds to a rapidly improving understanding of the Moon’s water and related species, as discovered by three other space missions through near-infrared emissions observations and found buried within the Cabeus crater by the LCROSS impactor. LAMP added to the LCROSS results by measuring hydrogen, mercury and other volatile gases ejected along with the water from the permanently shaded soils of the Moon’s Cabeus crater.
These images produced by the Lyman Alpha Mapping Project (LAMP) aboard NASA’s Lunar Reconnaissance Orbiter reveal features at the Moon’s northern and southern poles in the regions that lie in perpetual darkness. They show that many permanently shadowed regions, or PSRs, are darker at far-ultraviolet wavelengths (top inset) and redder than nearby surface areas that receive sunlight (bottom inset). The darker PSR regions are consistent with having large surface porosities — indicating “fluffy” soils — while the reddening is consistent with the presence of water frost on the surface.
An even more unexpected finding by members of the SwRI team is that LAMP’s technique for measuring the lunar Lyman-alpha albedo indicates higher surface porosities within PSRs, and supports the long-postulated presence of tenuous “fairy-castle”-like arrangements of surface grains in the PSR soils.
Comparisons with future LAMP maps created using data gathered from the Moon’s day side will prove helpful for revealing more about the presence of water frost, as well as the surface porosities of the darker surface features observed. The LAMP team will also apply the Lyman-alpha technique elsewhere on the Moon and on other solar system objects such as Mercury.
The LRO’s findings are expected to be valuable to the future consideration of a permanent Moon base. The permanently shadowed regions of the Moon are revealing themselves to be some of the most exotic places in the solar system, well worthy of future exploration. Any discovery of water frost and other resources in the area also could reduce the need to transport resources from Earth to a base at the pole.
Results of this study appear in the paper, “Far-Ultraviolet Reflectance Properties of the Moon’s Permanently Shadowed Regions,” by G.R. Gladstone, K.D. Retherford, A.F. Egan, D.E. Kaufmann, P.F. Miles, et al., published in the Jan. 7, 2012 issue of the Journal of Geophysical Research.
NASA Goddard Space Flight Center in Greenbelt, Md., developed and manages the LRO mission. The LRO’s current Science Mission is implemented for NASA’s Science Mission Directorate. NASA’s Exploration Systems Mission Directorate sponsored the LRO’s initial one-year Exploration Mission, which concluded in September 2010.