THE THERMOPHYSICAL PROPERTIES OF LUNAR COLD SPOTS: EXTENSIVE REGOLITH MODIFICATION AROUND YOUNG IMPACT CRATERS

Thermal infrared mapping by the Diviner instrument on the Lunar Reconnaissance Orbiter (LRO) has identified a new class of nighttime temperature anomalies associated with fresh lunar craters. Termed “cold spots”, these are ray-like regions extending 10-100 crater radii which are several K cooler at night than the surrounding regolith [1]. Previous work shows that cold spots persist for ~100 ka to ~1 Ma, demonstrating that these features fade rapidly in the lunar environment and mark one of the youngest populations of impact craters [2]. Cold spots can be explained by a “fluffing-up” of the upper centimeters of regolith by still poorly understood impact processes, resulting in a decrease in regolith packing density and lower thermal inertia.

The Diviner derived H-parameter is a scale height describing the thickness of “fluffed” material overlying compacted regolith [3]. We evaluate the H-parameter of a large population of cold spots and present evidence that the depth and extent of regolith modification scales with crater size. Several new cold spots formed during the LRO mission [2, 4] have H-parameters which increase with crater size, with the largest 70 m new crater producing a cold spot with a maximum H-parameter of ~16 cm. The size-frequency distribution of the equatorial cold spot population predicts that larger cold spots take longer to fade than smaller cold spots, possibly as a result of greater initial H-parameter. Additionally, we show that the fading rate of cold spots can be used to estimate the age of individual cold spot craters. These results inform our understanding the cratering processes responsible for cold spot formation, and the regolith processes responsible for their fading.

References: [1] Bandfield et al. (2014), Icarus, 231, 221-231. [2] Williams et al. (2018), J. Geophys. Res.: Planets, 123, 2380-2392. [3] Hayne et al (2017), J. Geophys. Res., 122, 2371-2400. [4] Speyerer et al. (2016), Nature, 538(7624), 215.