Impact melts on the Moon: How far will they go?

Physics

Scientific paper

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[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties

Scientific paper

Smooth deposits are widespread on the Moon and are attributed to three main modes of formation: basaltic flood volcanism, (fluidized) basin impact ejecta, and impact melt [1]. The first cover about 30% of the Moon and the latter third are localized within and near relatively recent (Copernican, < 1by) impact craters. Basin ejecta are globally distributed and form relatively smooth surfaces in topographic lows. Lunar Reconnaissance Orbiter Narrow Angle Camera (NAC) meter scale images reveal a large region (>3,000 km2, at 41°N, 167°E) containing hundreds of discrete smooth deposits. These deposits are manifested as ponded materials, with associated viscid flows, and thin veneers on surrounding slopes suggesting widespread emplacement of impact melt. Observations of impact crater densities show the ponds’ age to be Copernican (<1Gy), and possibly younger than 100 Ma. Their occurrence is far from any large Copernican-aged craters, thus their source as impact melt is not obvious. Alternatively, they may have resulted from a series of small magmatic extrusions. However, their young age and spatial occurrence are inconsistent with any known style of lunar volcanism and their UV/VIS (320 nm to 690 nm) color ratios indicate no compositional difference from their substrate (feldspathic highlands). The NAC images reveal meter and decameter scale textures that indicate the viscid material was emplaced with velocities high enough that allowed uphill movement. Textures include flows with festoons of boulders along fronts, flows with bulbous margins, fractures (cooling cracks?) in ponds, and a rough texture on steep slopes. These features suggest unknown mechanisms or ranges of impact melt emplacement on the Moon and indicate that smooth plains deposits may originate as impact melt even when they are far from any apparent source crater. [1] D. Wilhelms (1987) U.S. Geol. Survey Prof. Paper 1348.

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