Permanently shadowed regions (PSRs) persist at the lunar poles due to the Moon’s low axial tilt and may offer conditions suitable for long-term stability for volatiles. Evidence from recent observations of the lunar poles by instruments aboard NASA’s Lunar Reconnaissance Orbiter is focused on isolating potential signatures of such volatiles, following the detection of water and other volatiles after the LCROSS impact into Cabeus crater. However, the body of results to date yields a relatively inconclusive answer to the question of volatile abundance at the surface, with most techniques indicating either a very thin veneer of water ice frost or signatures that may also represent freshly exposed/weathered regolith.
We present new temperature maps constructed from more than three years of observations (July 5th 2009 – February 25th 2013) by LRO’s Diviner Lunar Radiometer Experiment. We calculate bolometric temperature, the wavelength-integrated radiance in all seven thermal Diviner channels expressed as the temperature of an equivalent blackbody. Maximum and minimum bolometric temperatures, and the total thermal range are calculated for areas between 90°S and 82.5°S. Diviner channels 8 (50 – 100 μm) and 9 (100 – 400 μm) are most sensitive to surface temperatures of 43 – 69K and < 43K, respectively. The coldest PSRs exhibit temperatures close to the sensitivity threshold of channel 9 and thus bolometric temperatures become dependant predominantly on the brightness temperature of the longest wavelength channels.
A variety of thermal behaviors are observed for south pole PSRs, indicating a range of thermophysical properties for surface materials. Some of the coldest maximum bolometric temperatures observed reside in Haworth crater and appear never to exceed ~40K. This is also the region with the lowest temperature range, indicating a persistently stable thermal regime. Doubly shadowed regions also exhibit similar cold and stable thermal regimes. They typically lie within small diameter craters inside PSRs and are shielded from minor thermal emissions from PSR surfaces by crater rims.
Recent 1064nm albedo measurements by LOLA indicate that there is some correlation between high albedo and low temperatures. However, not all cold places are bright, and many bright places are also warm. Plausible causes of high 1064nm albedo (as well as low ultraviolet albedo as measured by the Lyman-Alpha Mapping Project, LAMP) include regolith brightening by mass-wasting, the effects of space weathering at low temperatures or minor amounts of surface ice. Comparison of bright LOLA albedo measured in Shackleton Crater with albedos from a broad crater population sampled from PSRs and equatorial regions indicates that surface volatiles are not necessarily required to explain high albedo PSRs. Further investigation requires isolation of super cold areas and correlation of bolometric temperature with albedos from both LOLA and LAMP albedo. Consideration of sub-pixel effects is important since limited surface expression of volatiles i.e. a sparse surface or sub-surface distribution, may have little effects on apparent albedo at the scale of measurement footprints.