EVIDENCE FOR GLASS-RICH SURFACES ON LUNAR IMPACT MELT DEPOSITS (Invited Presentation)

Deposits of smooth, low albedo material are observed around many fresh impact craters on the Moon. These deposits are interpreted to be solidified silicate melts produced from the impact event, and emplaced during the late stages of impact crater formation. Despite their resemblance to lava flows in optical images, lunar impact melt deposits have a surface texture unlike any measured terrestrial lava flow. They are incredibly rough at decimeter scales, with radar returns at S-Band (12.6 cm) similar to blocky lava flows on Earth. However, in high-resolution, submeter scale optical images, they appear quite smooth, more similar to pahoehoe flows than blocky flows. The cause of this unusual surface texture is unknown, but may relate to the unique cooling conditions experienced by lunar impact melts. Lunar impact melt deposits cool under vacuum with initial temperatures far in excess of their liquidus, while terrestrial lava flows cool under a convective atmosphere with initial temperatures close to their liquidus. In this work, we propose that the unique cooling conditions experienced by lunar impact melt deposits cause them to form with a glassy surficial layer. This layer is then disrupted after formation to produce decimeter sized blocks covering an otherwise ‘smooth’ flow. To test this hypothesis, we use data from the Lunar Reconnaissance Orbiter’s (LRO) Mini-RF instrument to characterize the small-scale texture of the deposits, and data from Chandrayaan-1’s Moon Mineralogy Mapper (M³) to characterize their composition. We look for evidence in the M³ data for a glass-rich composition using laboratory spectra acquired of a range of glass-bearing materials as inputs for a customized spectral unmixing model. We find that glass-bearing materials with microcrystalline inclusions of pyroxene are consistent with observations by both Mini-RF and M³ of impact melt deposits on the Moon. This suggests that their rapid cooling history can explain the discrepancy between the optical and radar observations of impact melts on the Moon.