Mapping Mafic Mineralogy in Ladon Basin, Mars, Using CRISM Multispectral Data.

Ladon basin is a Pre-Noachian multiring impact basin located in southwest Margaritifer Terra (centered at 330° E, 17° S). Since its initial formation 4.17 Ga (Robbins et al., 2013), Ladon has been heavily modified by subsequent impact events as well as the extensive Late Noachian to Middle Hesperian Uzboi-Ladon-Morava outflow system (Grant et al., 2011) that connects Argyre basin in the south to Margaritifer basin and Chryse Planitia in the northern plains. Topographic or morphologic remnants of the basin rings are therefore severely degraded; for example, the number of rings is uncertain with estimates ranging from 3 (Lillis et al., 2008) to 6 (Schultz & Frey, 1990). In this study, we utilized summary parameter maps (Viviano-Beck et al., 2014) derived from Mars Reconnaissance Orbiter (MRO) Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) multispectral data along with topographic and morphologic contextual datasets to explore the use of mafic mineralogy (i.e., outcrops of low-calcium pyroxene and olivine) to better constrain the number and placement of Ladon ring structures as well as to understand overall basin history. To do this, we systematically analyzed the CRISM data to delineate spatially coherent mineral outcrops while assigning attributes that document mineral type, confidence level, and morphologic association(s). We found that low-calcium pyroxene (LCP) outcrops, in particular, are common and located not only in uplifted crater materials like central peaks consistent with Skok et al. (2012), but also in more shallow deposits like erosional massifs or crater rim crests, in non-crater escarpments, and even in some depositional landforms. Mapped LCP deposits along with morphologic structures circumferential to Ladon suggest fragments of 4 ring structures at approximately 470, 570, 670, and 1025 km diameters. Interestingly, however, the innermost series of massifs at 470 km diameter (Ladon Montes) host remarkably few LCP signatures; this distinction may imply that these materials were uplifted from a greater depth and/or sampled a different crustal composition than the outer Ladon ring structures. In addition to better constraining the ring structure of Ladon, this type of mapping may offer new insight on large basin formation and stratigraphy that translates to other regions on Mars.