Seeing a world in a grain of sand: learning about Mars from sand in river and dune deposits

Two current Mars rovers, Curiosity and Perseverance, landed in sedimentary basins in their quests to seek habitable environments and signs of life on Mars. Sedimentary basins can geologically concentrate signals of Mars’ igneous, aqueous, and climatic diversity. Here, we briefly synthesize rover observations of mineral and lithic grains in the coarser size fractions, medium sand to pebbles, which carry information about igneous and transport processes on Mars.

Mars is a dominantly basaltic world with igneous complexity due to fractional crystallization and crustal assimilation, which means sedimentary systems on Mars begin with a wide range of primary minerals with differing grain sizes. This enhances the impact and importance of size, shape, and density sorting during transport, creating downstream systems with distinct compositional trends and alteration pathways.

In Gale crater, the provenance for fluvio-deltaic sedimentary rocks is dominated by plagioclase-phyric basalts. The fluvial system concentrated cm-scale plagioclase phenocryst lathes in coarser-grained deposits, whereas finer mafic phases concentrated in silt. Multiple cobbles showing concentrated plagioclase lathes in basaltic groundmass support this interpretation and, in comparison to analogous systems in Iceland, indicate that sorting may be enhanced by magmatic heterogeneities that also concentrate phenocrysts. The large scale of the plagioclase phenocrysts allowed multiple approximations of their crystal chemistry and modeling of magmatic formation. An unconformably overlying lithified eolian sandstone shows an unusual abundance of coarse sand grains whose chemistry appears to be similar to the underlying preserved fluvial deposits, consistent with sedimentary recycling of coarse clasts via creep. Similarly, coarse sand was not observed in modern dunes.

Perseverance explored the fluvio-deltaic deposits of the Western Fan in Jezero crater and found that, in contrast, the preserved coarse phenocrysts in Jezero are dominantly mm-scale olivine grains. There is less groundmass, suggesting olivine cumulate sources, but easily-weathered olivine generates more Fe–Mg carbonate and sulfate alteration products.