Global Regolith Thermophysical Properties of the Moon From the Diviner Lunar Radiometer Experiment

We used infrared data from the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment to globally map thermophysical properties of the Moon's regolith fines layer. Thermal conductivity varies from 7.4x10(-4)Wm(-1)K(-1) at the surface to 3.4x10(-3)Wm(-1)K(-1) at depths of similar to 1m, given density values of 1,100kgm(-3) at the surface to 1,800kgm(-3) at 1m depth. On average, the scale height of these profiles is similar to 7cm, corresponding to a thermal inertia of 552Jm(-2)K(-1)s(-1/2) at 273K, relevant to the diurnally active near-surface layer, similar to 4-7cm. The temperature dependence of thermal conductivity and heat capacity leads to an similar to 2 times diurnal variation in thermal inertia at the equator. On global scales, the regolith fines are remarkably uniform, implying rapid homogenization by impact gardening of this layer on timescales 100Jm(-2)K(-1)s(-1/2)) in the interiors and ejecta of Copernican-aged impact craters and lower thermal inertia (<50Jm(-2)K(-1)s(-1/2)) within the lunar cold spots identified by Bandfield et al. (2014). Observed trends in ejecta thermal inertia provide a potential tool for age dating craters of previously unknown age, complementary to the approach suggested by Ghent et al. (2014). Several anomalous regions are identified in the global 128 pixels per degree maps presented here, including a high-thermal inertia deposit near the antipode of Tycho crater.

Plain Language Summary We measured the Moon's temperature cycles with the Lunar Reconnaissance Orbiter's Diviner instrument to make the first global maps of important physical properties of the dusty surface layer. These maps reveal a rich new view of the last billion years of impact processes and volcanism on the Moon. Impacts by meteorites cause the breakdown of rocks and accumulation of regoliththe granular surface materials. Our results show that regolith formation is a rapid process, which homogenizes and redistributes fine particles over large distances. These new observations provide a wealth of data for future study and also suggest a new technique for determining the ages of craters on the Moon and other planetary surfaces, using temperatures to infer the depth of accumulated regolith.