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

Abstract

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.

ALMA Thermal Observations of Europa

“…However, we do not see any obvious correlation between residuals, derived thermal inertias, and albedo. Furthermore, this map does produce a locally elevated thermal inertia surrounding the crater Pwyll, which is consistent with nighttime PPR data of the same region (Spencer et al 1999;Trumbo et al 2017) and the tendency of crater ejecta to exhibit elevated thermal inertia elsewhere in the solar system (e.g., Mellon et al 2000;Hayne et al 2017).…”

“…It is also interesting to note that the anomaly is almost directly antipodal to Pwyll, the largest fresh crater on Europa at 271°W and 25°S. Antipodal focusing of impact ejecta has been suggested as a potential explanation for a high thermal inertia deposit and corresponding geologic features on the Moon that are approximately antipodal to Tycho crater Hayne et al 2017). However, Tycho was likely the result of a more powerful impact and is roughly three times larger than Pwyll in diameter.…”

“…For instance, most of the positive temperature anomalies in the sub-Jovian to leading hemisphere image of Figure 2 can be equivalently explained by a decrease in albedo of ∼8-20% or an elevated subsurface heat flux of ∼1.2-2 W m −2 (consistent with liquid water a few hundred meters below the surface). However, as we provide the model with spatially varying albedos derived from concrete measurements and because other objects in the solar system exhibit significant localized differences in thermal inertia (e.g., Putzig et al 2005;Howett et al 2010Howett et al , 2011Howett et al , 2012Hayne et al 2017), we experiment with the idea that the residuals may be caused by thermal inertia variations.…”

An Examination of Several Discrete Lunar Nearside Photometric Anomalies Observed in Lyman‐α Maps

“…If these anomalies are the product of impact scouring, this would suggest that they are at least <81 kyr. However, most new features created by impact, and this young, have also shown thermophysical properties consistent with lunar cold spots (Bandfield et al, ; Hayne et al, ). Perhaps, an argument against a younger age of origin (or at least a contemporary origin) is that none of these anomalies exhibit characteristics of cold spots recently discovered and predominantly associated with young, small craters and their ejecta (at least in part, the area of cold spots have been suggested to present a volume of material more than could realistically fit back into the associated crater; Hayne et al, ; Speyerer et al, ).…”

Thermophysical Properties of the North Polar Residual Cap using Mars Global Surveyor Thermal Emission Spectrometer

“…The three cases are as follows: Linearly increasing density with depth until solid ice is reached. This best resembles profiles of depth and density present in Arthern et al (). Exponentially increasing density with depth that approaches solid ice (Hayne et al, ). An abrupt increase in density to that of solid ice at a discrete depth (akin to a solid ice table, e.g., Mellon et al, ; Schorghofer, ). …”