Analysis of Purple Coatings by the SuperCam Instrument on the Perseverance Rover in Jezero Crater, Mars

Abstract

The NASA Perseverance rover has encountered numerous instances of purplish colored surficial material on rocks and pebbles throughout its traverse across the Jezero crater floor and the delta front. These enigmatic materials are visible on many different rock types and can vary in apparent thickness, from being very thin (microns) to several mm thick, and potentially forming in more than one layer. On Earth, such thin layers may form from a variety of processes, e.g., as coatings deposited on rock surfaces, exposed fracture fills, and/or alteration rinds/case hardening. The purple materials observed at Jezero typically unconformably overlie eroded natural rock surfaces, suggesting these features are possibly surface coatings of externally derived material. On Earth, coatings arise due to interactions between rock surfaces and the atmosphere, liquid water, and life. As such, they represent important targets for study on Mars.Using Laser-Induced Breakdown Spectroscopy (LIBS) and a microphone, SuperCam is able to analyze these coatings for chemical composition (LIBS) and material properties (recording the LIBS acoustic signal). By interrogating the same location with the LIBS laser multiple times, changes in composition and material properties with shot (depth) may be observed if the layer is thin enough. SuperCam has made 125-150 laser shot depth profiles on several of these coated rocks, at 4-5 locations on each. For each raster, we attempt to have at least one point on an uncoated area to compare with the coated surface profile. Whenever possible, SuperCam analyses of coatings were made at locations adjacent to a rover-made abrasion patch, where the upper ~mm is abraded off to expose the underlying rock. Here we focus on comparing compositions of depth profiles on purple coatings that are directly adjacent to abrasion patches; these targets are Cordoeil (sol 268), near the abrasion patch Dourbes, Chokecherry (sol 378) which is near the Alfalfa abrasion patch, and Pile_Bay (sol 582) located by the Novarupta patch. Coating compositions from these targets roughly matches that of the fine martian dust (e.g., Lasue et al., 2018, doi.org/10.1029/2018GL079210), potentially indicating a link between the two. Airfall dust is an important contributor to rock coating formation on Earth and may likewise play a role for coating formation on Mars.