THE UNUSUAL ATLAS/HERCULES REGION OF THE MOON

Additional enigmatic deposits have emerged since the Lunar Reconnaissance Orbiter (LRO) entered orbit that contrast with the lunar regolith at large. Unlike optical anomalies, such as swirls, this new class of deposits are unusual for their thermophysical characteristics. One ubiquitous class consists of lunar cold-spot craters, so named because they are characterized with extensive regions, relative to their cavity diameter and depth, of anomalously cold temperatures in nighttime Diviner data [1]. These extensive cold regions are not easily explained by conventional impact mechanics. Hayne et al., [2] further characterized these ejecta features with high H-parameter values suggesting their thermophysical properties are consistent with a ‘fluffed up’ regolith in the upper 10 to 30 cm created by some aspect of the impact process.

However, also reported by Hayne et al., [2] were cold spots associated with pyroclastics as well as the thermophysically unique Atlas crater region (~45N, ~45E). With thermophysical similarities to cold-spot regoliths initially it seems to fit the definition of a cold-spot. But, the Atlas region is without similar geomorphological characteristics and bears dramatic differences in spatial scale. The ‘Atlas thermophysical anomaly’ also consists of high H-parameter values relative to its surroundings, similar to lunar cold-spots, and suggests it consists of finer regolith materials, with higher porosities, and with lower thermal inertias than typical regolith. But, unlike typical cold-spots, the mechanism for formation is even less clear.

The Atlas region may provide a unique insight into how the lunar regolith is being mechanically and chemically weathered that contrasts with cold-spots in general. For example, unlike Atlas, cold-spots are not typically observed in radar wavelengths. Here we focus our investigation on the physical properties of the Atlas region and how they may or may not be synergistic with its thermophysical properties with increasing depth. And we do this with a combination of monostatic and bistatic observations at multiple wavelengths of the region. We also begin to postulate potential formation mechanisms of this region relative to lunar cold-spots and radar-dark halo craters (e.g., new craters, pyroclastic deposits, etc.).

References: [1] Bandfield et al. (2014) Icarus, 231, 221. [2] Hayne et al. (2017) JGR, 122, 2371.