An Impact Melt Origin for Tycho Antipodal Deposits

Abstract:

Recent spacecraft measurements from the Lunar Reconnaissance Orbiter’s (LRO) radar, thermal infrared, and high-resolution imagery have been used to identify a region on the lunar far side that exhibits unusually high rock abundance and is covered by smooth deposits (Bandfield et al., 2011, 2015; Robinson et al., 2011, 2015). Further investigation of the region, centered at 42.5ºN, 167.5ºE, reveals three distinct regional morphologies. The first, flat deposits referred to as “ponds,” occur in depressions such as crater bottoms; the second, a veneer with apparent viscous flow-like features, is regionally widespread but thickest in the interior of craters on the slopes above ponds; the third, which is most apparent in the Diviner Rock Abundance dataset, is rubble or rocky material that tends to occur preferentially on the upper slopes of craters within a specific range of azimuths. Although the region is host to a variety of features that suggest the presence of previously fluid material (e.g., equipotential pond surfaces, lobate-like flows), there is no clear source for volcanic activity in the region. Previous studies have suggested these deposits are from relatively young impact melts, with Tycho crater (located at the antipode to the deposits of interest) being the most likely source (Robinson et al., 2015). Here, we use the Diviner Rock Abundance data paired with NAC DEMs to evaluate the local extent of the three rock morphologies observed in the region. We then use this data to create an impact-heating model, taking into account the efficiency of heating for a range of slopes, which describes the distribution of materials in a regional context. Our model results provide sufficient evidence that melting and subsequent fluid flow was possible over the timescales required for material to arrive from the Tycho impact. Furthermore, the model predicts flow to have occurred over discrete areas within the Tycho antipode region, which is confirmed by observations.