Lunar equatorial surface temperatures and regolith properties from the Diviner Lunar Radiometer Experiment

Abstract

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-m 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.