Young lunar volcanic features: Thermophysical properties and formation

Irregular mare patches (IMPs) are small volcanic features on the lunar nearside with young model ages. Several formation mechanisms have been proposed including: caldera collapse, explosive outgassing, lava flow inflation, pyroclastic eruption, and regolith drainage. We present new observations of the four largest IMPs (Sosigenes, In a, Cauchy-5, and Maskelyne) using the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer (Diviner) and evaluate the formation hypotheses in the context of both previous results and the results presented here. We find that the IMPs have a rock abundance slightly higher than their surrounding terrain. Comparison of the Diviner data with thermal models excludes the possibility of extensive competent rocks within ∼15 cm of the surface at the IMPs. We also derive the thermal inertia at the four largest IMPs. Three appear to have thermal inertias slightly higher than typical regolith due to alteration by nearby craters or mass wasting from surrounding steep slopes, but Ina has a thermal inertia lower than the surrounding terrain. In particular, the largest smooth mound in Ina is the area with the lowest thermal inertia, suggesting that the material on the mound is less consolidated than typical regolith and/or contains fewer small rocks ( < 1 m). Formation by lava flows or regolith drainage is not expected to result in material with a lower thermal inertia than pre-existing regolith, so some other process such as explosive outgassing or pyroclastic eruptions must have occurred

Young lunar volcanic features: Thermophysical properties and formation

Irregular mare patches (IMPs) are small volcanic features on the lunar nearside with young model ages. Several formation mechanisms have been proposed including: caldera collapse, explosive outgassing, lava flow inflation, pyroclastic eruption, and regolith drainage. We present new observations of the four largest IMPs (Sosigenes, In a, Cauchy-5, and Maskelyne) using the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer (Diviner) and evaluate the formation hypotheses in the context of both previous results and the results presented here. We find that the IMPs have a rock abundance slightly higher than their surrounding terrain. Comparison of the Diviner data with thermal models excludes the possibility of extensive competent rocks within ∼15 cm of the surface at the IMPs. We also derive the thermal inertia at the four largest IMPs. Three appear to have thermal inertias slightly higher than typical regolith due to alteration by nearby craters or mass wasting from surrounding steep slopes, but Ina has a thermal inertia lower than the surrounding terrain. In particular, the largest smooth mound in Ina is the area with the lowest thermal inertia, suggesting that the material on the mound is less consolidated than typical regolith and/or contains fewer small rocks ( < 1 m). Formation by lava flows or regolith drainage is not expected to result in material with a lower thermal inertia than pre-existing regolith, so some other process such as explosive outgassing or pyroclastic eruptions must have occurred

The Effects of Terrain Properties Upon the Small Crater Population Distribution at Giordano Bruno: Implications for Lunar Chronology

The distribution of impact craters on the ejecta of Giordano Bruno, a recent (&lt;10&nbsp;Ma) 22-km diameter crater within the lunar highlands, exhibits substantial variations. We surveyed craters D&nbsp;≥&nbsp;10&nbsp;m across a 1,323&nbsp;km2 area of Giordano Bruno's ejecta and compared the distribution of craters with variations in thermophysical properties derived from the Lunar Reconnaissance Orbiter Diviner instrument. We used Diviner-derived rock abundance and nighttime regolith temperatures along with thermal model-predicted surface temperatures for a diversity of terrains to identify and isolate areas of the ejecta based on thermophysical properties such as bulk density and thermal conductivity. We found that thermophysical properties of the ejecta vary considerably both laterally and vertically, and consistently differ from typical regolith, indicating the presence of higher thermal inertia materials. Crater-size frequencies are significantly lower in areas with terrain properties exhibiting higher: rock abundance, nighttime temperatures, and/or modeled thermal inertia. This discrepancy in crater distribution increases for craters smaller than ∼25&nbsp;m. These thermophysical variations indicate changes in the mechanical properties of the target materials. We suggest that these variations-specifically, terrain-dependent crater scaling variations and impactor-scale heterogeneities in material properties such as the presence or absence of large boulders-may influence crater diameters or inhibit crater production altogether in Giordano Bruno's ejecta; furthermore, these factors are size-dependent