Abstract | On the Moon, there are geologically young ponded melt deposits in a ~100 km area approximately antipodal to young crater Tycho (diameter ~ 86 km) of Copernican age. The deposits have distinctive morphology apparent in high-resolution images obtained by LROC NAC camera (LRO mission to the Moon). This morphology is very similar to morphology of impact melt ponds observed on walls and proximal ejecta of Tycho itself and other large Copernican-age lunar craters. The ponded deposits are also characterized by relatively high night-time temperature consistent with a large number of rocks observed in the images; they also have a distinctive microwave radar signature. These ponded melt deposits have been interpreted as a result of ejecta focusing in the Tycho antipode. The majority of large lunar Copernican-age craters do not have associated antipodal melt deposits; this is explained by the fact that to reach the antipode, ejecta should be launched at a grazing ejection angle, which only occurs, if the primary impact is sufficiently oblique.
I developed an analytical theory of impact ejecta focusing in the vicinity of the primary impact antipode. The theoretical results confirm that melt formation in the antipodal region likely occurs due to in-situ conversion of ejecta kinetic energy into heat (aka “frictional melting”) and does not require delivery of primary melt. The analytical theory enables effective quantitative comparison of antipode ejecta focusing between different planetary bodies. On Mercury, reaching the antipode requires a higher launch velocity, therefore, less ejecta get there, however, the kinetic energy per unit mass is higher than on the Moon, which in combination results in more effective melt production. A few (2 – 5) antipodal impact melt deposits are anticipated on Mercury.
Some ponded deposits in small depressions on Mercury are seen in high-resolution images obtained by MDIS NAC camera (MESSENGER mission) near antipode of large (~85 km) young crater Debussy of Kuiperian age. I suggest that they are analogous to the lunar Tycho antipodal deposits and formed by melting due to focusing of Debussy ejecta. BepiColombo data will enable testing this hypothesis through analysis of their fine-scale morphology and thermal signature. Limited resolution of MDIS NAC images in the equatorial zone and Southern hemisphere preclude systematic search for such deposits on Mercury. BepiColombo mission data will enable exhaustive search for antipodal melt deposits.
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