Icarus 200 (2009) 446–462 Contents lists available at ScienceDirect Icarus www.elsevier.com/locate/icarus Geologic context of in situ rocky exposures in Mare Serpentis, Mars: Implications for crust and regolith evolution in the cratered highlands A.D. Rogers a,b,∗ , O. Aharonson a , J.L. Bandfield c a Division of Geological and Planetary Sciences, California Institute of Technology, MC 150-21, Pasadena, CA 91125, United States b Department of Geosciences, Stony Brook University, 255 Earth and Space Sciences, Stony Brook, NY 11794-2100, United States c Department of Earth and Space Sciences, University of Washington, Johnson Hall 070, Box 351310, 4000 15th Avenue NE, Seattle, WA 98195-1310, United States article info abstract Article history: Received 28 May 2008 Revised 15 October 2008 Accepted 20 November 2008 Available online 16 December 2008 Keywords: Mars, surface Infrared observations Geological processes Global acquisition of infrared spectra and high-resolution visible and infrared imagery has enabled the placement of compositional information within stratigraphic and geologic context. Mare Serpentis, a low albedo region located northwest of Hellas Basin, is rich in spectral and thermophysical diversity and host to numerous isolated exposures of in situ rocky material. Most martian surfaces are dominated by fine-grained particulate materials that bear an uncertain compositional and spatial relationship to their source. Thus location and characterization of in situ rock exposures is important for understanding the origin of highland materials and the processes which have modified those materials. Using spectral, thermophysical and morphologic information, we assess the local and regional stratigraphy of the Mare Serpentis surface in an effort to reconstruct the geologic history of the region. The martian highlands in Mare Serpentis are dominated by two interspersed surface units, which have distinct compositional and thermophysical properties: (1) rock-dominated surfaces relatively enriched in olivine and pyroxene, and depleted in high-silica phases, and (2) sediment or indurated material depleted in olivine and pyroxene, with relatively higher abundance of high-silica phases. This is a major, previously unrecognized trend which appears to be pervasive in the Mare Serpentis region and possibly in other highland areas. The detailed observations have led us to form two hypotheses for the relationship between these two units: either (1) they are related through a widespread mechanical and/or chemical alteration process, where less-mafic plains materials are derived from the mafic bedrock, but have been compositionally altered in the process of regolith formation, or (2) they are stratigraphically distinct units representing separate episodes of upper crust formation. Existing observations suggest that the second scenario is more likely. In this scenario, plains materials represent older, degraded, and possibly altered, “basement” rock, whereas the rocky exposures represent later additions to the crust and are probably volcanic in origin. These hypotheses should be further testable with decimeter-resolution imagery and meter- resolution short wavelength infrared spectra.  2008 Elsevier Inc. All rights reserved. 1. Introduction The nature and origin of intercrater surfaces within the mar- tian cratered highlands have been somewhat enigmatic since the Mariner era (Malin, 1976; Greeley and Spudis, 1981; Tanaka et al., 1988; Edgett and Malin, 2002). These surfaces may consist of impact debris, sediments deposited through fluvial and/or ae- olian action, and/or volcanic deposits. It is likely that all of these modification processes have played a role in intercrater plains for- mation to some extent, however the relative contribution of each is largely unknown (Tanaka et al., 1988). Determination of the rela- * Corresponding author at: Department of Geosciences, Stony Brook University, 255 Earth and Space Sciences, Stony Brook, NY 11794-2100, United States. Fax: +1 631 632 1509. E-mail address: adrogers@notes.cc.sunysb.edu (A.D. Rogers). tive contributions has been hampered by the presence of a veneer of mobile material, ambiguity of or lack of diagnostic morphologic features, and lack of high-resolution geomorphic, thermophysical and compositional data. The fine component of the martian regolith, here meaning the upper few centimeters of sand and dust deposits found glob- ally across the surface, has been well-characterized with orbit- ing and landed spectrometers. The composition of dust deposits in high-albedo regions likely includes framework silicate minerals (Bandfield and Smith, 2003; Ruff, 2004), as well as small amounts of carbonates (Bandfield et al., 2003) and nanophase oxides (Allen et al., 1981; Morris et al., 1990; Bell et al., 1993). Low resolu- tion visible/near-infrared (VIS/NIR) and thermal infrared (TIR) re- mote spectroscopic observations have shown that high- and low- Ca pyroxenes, plagioclase, olivine and high-silica amorphous, pri- mary and/or secondary phase(s) dominate the mineralogy of low- 0019-1035/$ – see front matter  2008 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2008.11.026