EXPLORATION OF CARBONATE-RICH ROCKS IN THE MARGIN UNIT OF JEZERO CRATER ON MARS BY THE PERSEVERANCE ROVER

The Perseverance rover has recently completed exploration of the Margin unit (MU) in Jezero crater, a region along the western rim that exhibits strong orbital carbonate signatures. Based on orbital data, hypotheses for the origin of the MU included igneous processes, but the concentration of carbonate within a narrow range of elevations is also consistent with a shorezone deposit. Here we provide an overview of the Margin Campaign and hypotheses for the origin of the MU based on Perseverance data.

In situ results confirm that the MU is carbonate-rich, where carbonate occurs both as pore-filling cements and discrete grains, most likely due to both carbonation of olivine/serpentine and aqueous carbonate precipitation. Most abraded rock surfaces in the MU show petrographic textures consistent with clastic rocks (e.g., a well-sorted sandstone), and the eastern 0.5 km shows truncated packages of layers with both rimward and basinward dips, potentially consistent with beach bars. In this hypothesis, the MU formed as a sedimentary deposit in a shorezone setting, wherein some of the carbonate grains and cements formed due to contemporaneous low energy reworking and cementation of subaqueous or pore-filling precipitates.

However, the western MU does not show clearly exposed traction-transport structures in outcrop (e.g., cross-bedding), and is instead dominated by thick and basinward dipping layers of rock with outcrop textures ranging from structureless to faint internal layers, all characterized by extensive silica/carbonate mineralization. These rocks could reflect a continuation of eastern MU sedimentary processes, but we are also considering an igneous origin. In this hypothesis, the MU was deposited as a layered pyroclastic deposit (e.g., similar to proposed regional ignimbrites), or as lava flows (e.g., similar to Seitah olivine cumulates on the crater floor). These igneous rocks must have then experienced extensive alteration by surface or subsurface fluids to produce carbonate and silica, and could have been reworked in the lake to produce sedimentary rocks in the eastern MU.

Perseverance collected three rock cores from the MU for potential return to Earth via Mars Sample Return. If authigenic and deposited at the fluid/air interface, MU carbonates and silica would be a compelling target for sample return due to their recognized biosignature preservation potential.