Spectral and photogeologic mapping of Schrödinger Basin and implications for post-South Pole-Aitken impact deep subsurface stratigraphy

Abstract

Schrödinger Basin provides a window into the stratigraphy of the lunar crust adjacent to the South Pole-Aitken Basin region that we have probed with Lunar Reconnaissance Orbiter, Moon Mineralogy Mapper (M³), and crater-scaling relationships. The composition of materials that make up the basin wall, impact melt, and peak ring provide a cross-section of the lunar crust, which reveals products of the lunar magma ocean, subsequent magmatism, and reworking of those components into a megaregolith. Large hectometer- to kilometer-size areas of anorthite-rich material (anorthosite), low-Ca pyroxene material (a noritic unit), and olivine-rich material (troctolite or dunite) are exposed, with a few areas of intermediate composition. The Schrödinger impact excavated ∼20 km into an orthopyroxene + plagioclase (noritic) lunar crust, which is exposed in the basin walls, rim, and proximal ejecta, and dominates the composition of materials that make up the basin floor. Substantially later in lunar history, two spatially and chronologically isolated volcanic eruptions occurred on the basin floor. Two large craters east of Schrödinger excavated a compositionally gabbroic subsurface unit that was not tapped by Schrödinger. This indicates a compositional crustal facies change, which may be from SPA ejecta, but could reflect heterogeneity in the original lunar crust.

Introduction

Schrödinger Basin is the best-preserved impact basin of its size on the Moon (Wilhelms, 1987) and, thus, provides a unique opportunity to study the basin-forming process. Schrödinger Basin is also the second youngest impact basin, and sits adjacent to the oldest and largest impact basin, South Pole-Aitken (SPA, Fig. 1). In addition to well-preserved impact-generated materials, the basin floor hosts two younger volcanic deposits (Shoemaker et al., 1994, Gaddis et al., 2003). These facts and others make Schrödinger an ideal location to determine the duration of the basin-forming epoch and test the lunar cataclysm hypothesis (O’Sullivan et al., 2011). Thus, at a single location on the lunar surface, one can address the two highest science priorities for lunar exploration (NRC, 2007). A detailed geological analysis of the materials in and around Schrödinger using remote sensing data and impact crater modeling are the subject of this paper.

Schrödinger Basin has a mean diameter of 315 km, average depth of 4.5 km, and is centered at 75°S, 132.5°E, which places it on the western rim of SPA. Schrödinger has a prominent inner peak ring, which is 125 km in diameter and rises 1–2.5 km above the basin floor. Schrödinger’s ejecta blanket has an irregular margin with extensive ray-like lobes (Fig. 1) that have been mapped to distances of ∼450 km from the basin rim (Wilhelms et al., 1979). Three long, deep grooves. or chasms, created by ejected debris extend radially from the basin in three directions (where due north is 0°): an unnamed chasm, approximately 200 km long, with a heading of 105°; Vallis Schrödinger, approximately 315 km long, has a radial direction of 315°; and Vallis Planck, which is ∼600 km long and has a radial direction of 350° (Wilhelms et al., 1979). Based on principles of superposition and relative crater densities, Schrödinger is one of the youngest lunar basins, only marginally older than Orientale (Wilhelms, 1987, Shoemaker et al., 1994). It has been targeted as an ideal location for a future landing site (O’Sullivan et al., 2011) because it is thought to have tapped deep crustal lithologies associated with the SPA basin-forming event, contain ejected lithologies from the Orientale basin-forming event, and has volcanic materials in later mare and/or pyroclastic eruptions on its floor (Shoemaker et al., 1994, Mest, 2011).

Wilhelms et al. (1979) first mapped the basin as part of the geologic map of the south side of the Moon. The map describes Schrödinger’s walls, peak ring, and floor as Nectarian, and distinguishes Schrödinger’s original basin materials from places within the basin later modified by Nectarian and Imbrian basin ejecta. They also mapped the volcanic units, which they identified as Imbrian pyroclastic deposits.

A previous analysis using Clementine Ultraviolet Visible (UVVIS) multispectral data (Shoemaker et al., 1994) indicated that the floor of the basin has two impact melt lithologies: a rough plains unit (with gentle hummocks, swales, and low knobs) and a smooth plains unit (with no detectable relief), the latter of which is interpreted to contain the most melt. A ridge representing either a buckling of the melt sheet or later extrusion rises slightly above the basin floor. The data also indicated a mare basalt erupted about 600 million years later, with a calibrated crater counting age of ∼3.2 Ga (Shoemaker et al., 1994). A large pyroclastic deposit elsewhere on the basin floor is estimated to be even younger, probably <2 Ga and possibly ⩽1 Ga (Shoemaker et al., 1994). More recent analyses of Kaguya data suggested the peak ring contains olivine and crystalline plagioclase (Ohtake et al., 2009, Yamamoto et al., 2010, Yamamoto et al., 2012), which those studies interpreted to be from the upper mantle or Mg-rich crustal intrusions (for olivine) and the uppermost unit of the lunar magma ocean (for plagioclase).

Mest (2011) recently completed a geologic map of Schrödinger Basin that integrated Lunar Reconnaissance Orbiter’s (LRO) Narrow Angle Camera (NAC) images, Lunar Orbiter Laser Altimeter (LOLA), and Clementine UVVIS multispectral images. The map describes nine geologic units that collectively make up three formations: (1) basin materials, including the peak ring, rim and wall, and ejecta from recent, large impacts (>5 km diameter) onto the basin floor; (2) floor materials; and (3) volcanic materials. Crater counting was used to provide relative ages and age estimates of the different units, which placed most units near the Lower/Upper Imbrian boundary. The exceptions are the units encircled by the peak ring, which were emplaced in the Upper Imbrian, and the Eratosthenian pyroclastic deposit (Mest, 2011).

To develop those concepts further and to better characterize Schrödinger Basin for future lunar surface missions, we mapped the exposed mineralogy of the basin and surrounding region using spectral data from the Moon Mineralogy Mapper (M³). The results were assessed in a geomorphological context using new LRO images and LOLA data. We took a different approach than did Mest (2011) to mapping Schrödinger, and while there are similarities, there are important differences that are highlighted in this paper. The interpretation, which incorporates results from spectroscopic, morphologic, and tectonic features, reveals new insights about the crust of the lunar farside, the effects of South Pole-Aitken Basin on regional geology, and the formation of a peak ring basin.