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 7.4 × 10−4 W m−1 K−1 at the surface to 3.4 × 10−3 W m−1 K−1 at depths of ~1 m, given density values of 1,100 kg m−3 at the surface to 1,800 kg m−3 at 1 m depth. On average, the scale height of these profiles is ~7 cm, corresponding to a thermal inertia of 55 ± 2 J m−2 K−1 s−1/2 at 273 K, relevant to the diurnally active near‐surface layer, ~4–7 cm. The temperature dependence of thermal conductivity and heat capacity leads to an ~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 <1 Gyr. Regional‐ and local‐scale variations show prominent impact features <1 Gyr old, including higher thermal inertia (> 100 J m−2 K−1 s−1/2) in the interiors and ejecta of Copernican‐aged impact craters and lower thermal inertia (< 50 J m−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.

[1] The Diviner Lunar Radiometer Experiment onboard the Lunar Reconnaissance Orbiter has measured solar reflectance and mid-infrared radiance globally, over four diurnal cycles, at unprecedented spatial and temporal resolution. These data are used to infer the radiative and bulk thermophysical properties of the near-surface regolith layer at all longitudes around the equator. Normal albedos are estimated from solar reflectance measurements. Normal spectral emissivities relative to the 8-mm Christiansen Feature are computed from brightness temperatures and used along with albedos as inputs to a numerical thermal model. Model fits to daytime temperatures require that the albedo increase with solar incidence angle. Measured nighttime cooling is remarkably similar across longitude and major geologic units, consistent with the scarcity of rock exposures and with the widespread presence of a near-surface layer whose physical structure and thermal response are determined by pulverization through micrometeoroid impacts. Nighttime temperatures are best fit using a graded regolith model, with a $40% increase in bulk density and an eightfold increase in thermal conductivity (adjusted for temperature) occurring within several centimeters of the surface.

Diviner Lunar Radiometer Experiment surface-temperature maps reveal the existence of widespread surface and near-surface cryogenic regions that extend beyond the boundaries of persistent shadow. The Lunar Crater Observation and Sensing Satellite (LCROSS) struck one of the coldest of these regions, where subsurface temperatures are estimated to be 38 kelvin. Large areas of the lunar polar regions are currently cold enough to cold-trap water ice as well as a range of both more volatile and less volatile species. The diverse mixture of water and high-volatility compounds detected in the LCROSS ejecta plume is strong evidence for the impact delivery and cold-trapping of volatiles derived from primitive outer solar system bodies.

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. CitationHamilton, V.E., et al., "Evidence for widespread hydrated minerals on asteroid (101955) Bennu." Nature astronomy 3, 4 (2019): p.