During the NASA Perseverance rover’s exploration of the Jezero crater floor, purple-hued coatings were commonly observed on rocks. These features likely record past water-rock-atmosphere interactions on the crater floor, and understanding their origin is important for constraining timing of water activity and habitability at Jezero. Here we characterize the morphologic, chemical, and spectral properties of the crater floor rock coatings using color images, visible/near-infrared reflectance spectra, and chemical data from the Mastcam-Z and SuperCam instruments. We show that coatings are common and compositionally similar across the crater floor, and consistent with a mixture of dust, fine regolith, sulfates, and ferric oxides indurated as a result of one or more episodes of widespread surface alteration. All coatings exhibit a similar smooth homogenous surface with variable thickness, color, and spatial extent on rocks, likely reflecting variable oxidation and erosional expressions related to formation and/or exposure age. Coatings unconformably overlie eroded natural rock surfaces, suggesting relatively late deposition that may represent one of the last aqueous episodes on the Jezero crater floor. While more common at Jezero, these coatings may be consistent with rock coatings previously observed in-situ at other landing sites and may be related to duricrust formation, suggesting a global alteration process on Mars that is not unique to Jezero. The Perseverance rover likely sampled these rock coatings on the crater floor and results from this study could provide important context for future investigations by the Mars Sample Return mission aimed at constraining the geologic and aqueous history of Jezero crater.

The univariate statistics of potassium (K) and thorium (Th) concentrations in the oceanic and continental crust of the Earth has been recently investigated from geochemical databases and airborne radiometric surveys [1]. This study demonstrates that the frequency distributions of these elements are scale-dependent. There are right-skewed for small-scale samples (typical volume of rocks analyzed for an individual sample in a dataset) but tend to be more symmetric for large-scale samples. The right-skewed behavior of K and Th is attributed to their incompatible behavior during partial melting or fractional crystallization. The scale-dependence and evolution toward normal distributions are a direct consequence of the central limit theorem applied to K and Th concentrations. The results of the results of this study may be applied to Mars, using the Mars Odyssey global maps of K and Th concentrations [2]. In light of available K, Th concentrations at the rock-scale (in-situ samples and martian meteorites), we infer that each “pixel” of these maps reflect a right-skewed distribution of K and Th concentrations at smaller-scales, where K and Th-poor rocks, such as basalts, are spatially dominant. In turn, K, and Th-rich rocks, such as those found by Curiosity at Gale crater [3], may occur globally, though their spatial extension must be limited to account for the values reported by Mars Odyssey. The global, but sparse occurrence of K, Th-rich rocks at the surface is consistent with a buried felsic crust, outcropping at the favor of tectonic or impact events. These conclusions will be discussed in the context of the inferred constraints about the structure of the martian crust from Insight data [4]. [1] Baratoux, et al., Earth and Space Science, in press. [2] Boynton et al. JRGP, doi:10.1029/2007JE002887. [3] Sautter et al. doi:10.1038/NGEO2474. [4] Knapmeyer-Endrun et al., Science, doi: 10.1126/science.abf8966

Through rover missions and martian meteorites received on Earth, the surface of Mars has showed unexpectedly elevated concentrations of transition metals usually measured in minor and trace concentrations in silicate rocks compared to the average crust. Gale crater presents one of the most diverse geological records in terms of its complex fluid and magmatic history described through the sedimentary and igneous records, respectively. Transition metals, such as Mn, Co, Ni, Cu, and Zn, are highly concentrated within various sedimentary rocks and diagenetic features, suggesting their mobilization through fluid circulation. This paper presents the first compilation of elevated concentrations of transition metals measured by the Curiosity rover and reviews the origin of such metals in Gale crater, highlighting the existence of a hydrothermal or magmatic-hydrothermal deposit in its vicinity. The discovery of felsic magmatism on Mars opens up to novel perspectives in terms of the type of metal deposits that current and future exploration could evidence at the surface of Mars and raise questions about the global abundance of such metals. Constraining the abundance of transition metals is also a central question for exobiology purposes. Because on Earth living organisms use transition metals for their survival and functioning, should live have arisen on Mars, the availability of such chemical elements at the surface could have been essential for its development. An accurate assessment of in situ metal resources and potential risks for health will be key for the preparation of human exploration of Mars as recently announced by NASA.

The univariate statistics of Potassium (K), Thorium (Th) and Uranium (U) concentrations, in the Earth’s oceanic and continental crust are examined by different techniques. The frequency distributions of the concentrations of these elements in the oceanic crust are derived from a global catalog of mid-ocean ridge basalts (MORB). Their frequency distributions of concentrations in the continental crust are illustrated by the North Pilbara Craton, and the West Africa Craton. For these two cratons, the distributions of K, Th and U derived from geochemical analyses of several thousand whole rock samples differ significantly from those derived from airborne radiometric surveys. The distributions from airborne surveys tends to be more symmetric with smaller standard deviations than the right-skewed distributions inferred from whole rock geochemical analyses. Hypothetic causes of these differences include (i) bias in rock sampling or in airborne surveys, (ii) the differences in the chemistry of superficial material, and (iii) the differences in scales of measurements. The scale factor, viewed as consequence of the central limit theorem applied to K, Th and U concentrations, appears to account for most of the observed differences in the distributions of K, Th and U. It suggests that the three scales of auto-correlation of K, Th and U concentrations are of the same order of magnitude as the resolution of the airborne radiometric surveys (50 – 200 m) and concentrations of K, Th, U are therefore generally heterogenous at smaller scales.