The Moon's Extremely Insulating Near-Surface: Diviner Infrared Observations of a Total Lunar Eclipse

We obtained targeted observations of ten locations on the lunar surface during the total eclipse of June 15, 2011, using the Diviner Lunar Radiometer on board the Lunar Reconnaissance Orbiter (LRO). The orbital vantage point provides unprecedented spatial resolution (~250 m) of eclipse cooling, revealing small scale variations in thermal properties due to rock abundance and regolith structure. The eclipse-driven thermal wave only penetrates a few millimeters, so the observed cooling effectively isolates the thermal properties of the very near surface of the Moon. Over much of the lunar surface, we observe unexpectedly large cooling relative to predictions based on typical lunar thermophysical parameters (e.g. Vasavada et al. 1999), implying a ~2x lower thermal inertia. This result suggests the lunar regolith is extremely insulating within millimeters of the surface, with thermal inertia perhaps approaching a theoretical lower limit for porous regolith. Depth profiles of thermophysical properties derived from the Diviner eclipse and lunation observations are consistent with a transition from the upper low density region to a constant higher density at depth. Based on eclipse and lunation cooling, we map global and local scale variations in the thermal inertia of the lunar surface. Anomalous thermal features surrounding some fresh impact craters (Bandfield et al. 2011) are not apparent in the eclipse observations, suggesting their unusual thermal properties arise from differences at depths of centimeters, rather than in the upper few millimeters. This would be the case if density and/or conductivity increase more slowly with depth in these regions relative to typical lunar regolith, possibly due to recent mechanical processing related to these impact events. In both the interior and exterior of a potentially icy polar crater (Spudis et al. 2010) eclipse cooling is comparable to other regions, implying any ice must be covered by at least a few millimeters of insulating regolith.