Endogenic modification of impact craters on Mercury

Abstract

Surface resolution of Mercury from Mariner-10 images is insufficient for identifying volcanic forms analogous to most of those on the moon. Consequently, other criteria must be used in order to assess the possibility of Mercurian volcanism. One criterion is the presence of internally modified impact craters similar to floor-fractured and mare-filled craters on the moon. Such craters typically occur near or within the lunar maria and exhibit features (fractures, shallow floors, dark-haloed craters, mare units) that either are resolvable at Mariner-10 resolutions or are detectable by indirect means, e.g., albedo and color contrasts. Mariner-10 images reveal several plausible examples, which, as on the moon, most commonly occur near plains-filled basins. The color-ratio images by Hapke, Danielson, Klaasen and Wilson indicate that some of the Mercurian craters exhibit red plains materials on their floors. Such distinctive units may correspond either to lavas analogous to mare basalts within certain lunar craters or to compositionally distinct subsurface material retained in the impact crater. Indicators used to recognize crater modification also can be used to recognize basin modification. Specifically, several basins exhibit photometric contrasts between basin-filling plains and basin exterior; moreover, several impact craters superposed on the interior plains exhibit dark haloes suggesting excavation of compositionally distinct material. Despite the generally poor surface resolution, possible endogenic features are recognized, including irregular rimless depressions. Mercurian volcanism may have occurred and in certain regions may resemble that of the Mare Australe region on the moon.