A varnish-like high-manganese rock coating in Jezero crater, Mars

Manganese-rich phases have been detected in situ on Mars by the NASA Opportunity and Curiosity rovers, and in the martian meteorite NWA 7034 (and its pairs). Notably, instruments on Curiosity in Gale crater have detected Mn-rich materials in many geologic contexts, including fracture fills, coatings, nodules, and cements; this variety suggests a complex, long-term manganese cycle or cycles in the region. The origins of these materials is not well understood, but their existence points to strongly oxidizing aqueous environments in Mars’ distant past. On Earth today, manganese cycling is primarily mediated by microbes, making manganese minerals on Mars important targets for detailed study. On Earth, a significant geologic setting for Mn-rich materials is rock varnish, a dark, shiny coatings composed of Mn- and Fe-oxides and clays. Varnishes are ubiquitous in arid environments on Earth and have recently been shown to be produced and modified by microbial communities. Such varnishes have long been predicted for Mars (as an abiotic feature) but have not been observed until now. In Jezero crater, the SuperCam and Mastcam-Z instruments on the Perseverance rover have now documented a dark, shiny, Mn-rich coating on the rock Hogback Mountain, which is in the Hogwallow Flats region of Jezero Delta sediments. SuperCam laser-induced breakdown spectroscopy (LIBS) analyses of 30- and 150-shot depth profiles penetrated through a thin, Mn-rich layer with MnO as high as 30 wt% (avg 11 wt% MnO over all shots). Preliminary chemistry results suggest that Ni is positively correlated with Mn; this is consistent with a Mn-oxide mineral, which adsorb Ni, Co, and other metals when available. Acoustic data from the SuperCam microphone obtained concurrently with the LIBS depth profiles show that the high-Mn coating is relatively hard, and that material properties change beneath the coating at ~40 shots (~12 µm) depth, in good agreement with the LIBS chemistry data. SuperCam reflectance spectra (0.40-0.85 um, 1.3-2.6 µm) of the coating suggest contributions from phyllosilicates and likely Mn-bearing minerals, including but not limited to birnessite, [(Na,Ca)_0.5(Mn⁴⁺,Mn³⁺)₂O₄·1.5H₂O]), which is the most common Mn-oxide in terrestrial rock varnish. So far, Hogback Mountain is the only SuperCam target with such high Mn. However, Mastcam-Z multispectral observations suggest that similar Mn-rich coatings are present on rock surfaces throughout the area. On Earth, varnish formation (and Mn-mineral formation in general) is associated with organic materials. Notably, at the nearby Berry Hollow abrasion patch, high intensity fluorescence signals indicate that possible organics were found by the SHERLOC instrument. Further investigation of these signals and colocated Raman signals is ongoing. This observation of a varnish-like coating on Mars represents a new geologic context for Mn-bearing minerals on that planet that expands the range of environments known to produce these materials, and opens up new opportunities to answer questions about potential biosignatures on Mars.