IDENTIFYING AND CHARACTERIZING IMPACT-MELT OUTCROPS IN THE NECTARIS BASIN

Barbara Cohen, NASA Marshall Space Flight Center (barbara.a.cohen @nas a. gov), Sam Lawrence & Julie Stopar, Arizona State University, Noah Petro, NASA Goddard Space Flight Center, Gwen Bart, University of Idaho, Ryan Clegg-Watkins, Washington University in St. Louis, Brett Denevi & Rachel Klima, Johns Hopkins University Applied Physics Laboratory, Becky Ghent, University of Toronto, Gareth Morgan, Smithsonian Institute, Paul Spudis, Lunar and Planetary Institute

Motivation v The formation of the Nectaris basin is a key event defining the stratigraphy of the Moon (the Nectarian epoch) and the onset of the Lunar Cataclysm and solar-system-wide late heavy bombardment. v There is little agreement on whether current samples represent Nectaris; sample age estimates range from 3.87-4.2 Ga, or we may not have any samples at all. v No known age for Nectaris = no constraint on Late Heavy Bombardment = LRO NAC visible morphology image of the Nectaris mul ti ring ba sin no boundary condition for dynamical models of solar system formation!

� What if we could date Nectaris directly via sample return or in situ dating*?� *See Poster 2046 in this session! Identifying Nectaris impact melt Identifying safe landing sites v Small plains near inner basin ring massifs and intermassif “draped” v Nearside, mid-latitude location makes a good deposits are mapped as Nectaris target for a landed mission. basin impact melt sheet remnants v Outcrop areas are broad and very mature (Spudis and Smith, 2013), see inset. (safe), reflecting regolith development since v Impact mixing models (e.g. Petro Nectaris formation. and Pieters, 2006) indicate no more v We are using radar-based surface and than 40% of materials should be subsurface unit mapping and thermal inertia to derived from other large, nearside understand geologic setting. basins – unlike A16 site. v NAC DEM-based small-crater and boulder v Multiple datasets, including abundance are being used to better elemental abundances, derived understand where spacecraft could land in mineral maps, and UV and albedo these areas and sample surface and estimates of glassy materials, can be subsurface materials. used to understand the composition of these areas.

←Detail of the Spudis & ←Example spectral variability in Smith (2013) geologic map of the area from Moon Mineralogy 3 the Nectaris basin and its Mapper (M ) data (different color surrounding terrain. The stretches) shows a diversity of mafic dark-orange material mineralogy. between Nectaris basin rings are similar in location and appearance to the Maunder →The mean iron content of the Fm in Orientale and may be Nectaris basin outcrops (5.6 wt.%) remnants of the Nectaris suggests a slightly mafic target and Basin im pact-melt sheet. variability results from regolith for m ation including for eign material (Spudis & Smith, 2013).

←The possible ←LRO WAC UV color composites impact-melt can help distinguish between fresh remnants are high- crystalline plagioclase vs shocked or standing areas melted (maskelynite or quenched embayed by Mare glass) plagioclase (Deneviet al. Nectaris and may 2014). Sm all cr ater s expose both under lie crystalline (yellow) and glassy the Montes materials (red). Pyrenaeus range. →The confinement of basaltic material to the basin interior is shown in the TiO2 map, while Th variability is likely to be correlated with Imbrium ejecta, but is low over the sites of interest. ←LROC Wide-Angle Cam er a (WAC) and Narrow-angle Camera (NAC) images provide albedo differences ←Rock abundance data and color ratios. When fr om LR O Diviner (e.g., acquired from multiple Bandfield et al., 2011) viewing angles, NAC maps rocks with photometr y can be used to sufficient thermal inertia infer the composition and to remain warm relative physical properties of surface to the regolith featur es (e.g. Clegg et al. throughout the lunar 2015). night. Fresh blocks exposed by small impacts provide clear rocky signatures.