LRO Diviner's Contributions to Understanding Lunar Polar Volatiles

The Diviner Lunar Radiometer Experiment (DLRE) on NASA's Lunar Reconnaissance Orbiter has mapped the temperature distribution at the lunar poles for over two years, capturing complete records of their diurnal and seasonal variability with a spatial resolution of ~200m and a temperature accuracy of 0.5 to 4K. These data are invaluable for understanding polar volatiles, since temperature is the dominant factor controlling their capture, evolution, retention, and three-dimensional distribution. While very cold temperatures had been theorized since the concept of high-latitude topographically shaded regions was first discussed in the mid-1900s, Diviner's observation of perennial sub-30K temperatures in patches with length scales of 10s of km was surprising (Paige et al., 2010). These regions, along with extensive areas never warming above ~100K, would permit the stability of water and other more exotic volatiles over solar system timescales (Zhang et al., 2009, 2010). The Diviner data have been interpreted using a numerical simulation of lunar polar surface and near-surface temperatures, including the effects of topographic scattering of infrared and visible light into shadowed regions, tied to the LRO-LOLA digital elevation model. This simulation, the UCLA Digital Moon, has been validated against the Diviner data and used to assess near-surface vertical temperature profiles (and their diurnal and seasonal variation) in the current era, and how the horizontal and vertical temperature distributions have evolved with changes in the lunar obliquity in the past. These quantities, in turn, can be used to constrain rates of volatile capture and loss, and to drive models of diffusion through the lunar regolith. The Diviner temperature maps were used to successfully target the LCROSS impact into an extremely cold region, and volatiles observed in the plume were consistent with Diviner's predictions (Paige et al., 2010). The data presently are being compared in detail with the distribution of water-equivalent hydrogen observed by the Lunar Prospector and LRO-LEND neutron spectrometers, and the polarization signatures observed by Chandrayaan-1. Given the widespread areas that are sufficiently cold to protect volatiles from thermal sublimation (and the abundance of water ice in even warmer patches near Mercury's poles), a key question is why the lunar poles apparently have retained only the minor frost deposits detected by the neutron spectrometers (the radar data could indicate small, meters-thick ice). Diviner shows no influence of surficial water ice in derived thermophysical properties, though this work is ongoing.