Insights into the Sedimentary Record and Processes of the Western Delta of Jezero crater (Mars) as observed by the Mars 2020 rover Perseverance. (Invited)
Résumé
Since its landing in Jezero crater in February 2021, the western delta of Jezero has been one of the main targets for the Perseverance rover to explore and sample sedimentary rocks that lead us to better understand the environmental evolution of the region, and could host traces of past biosignatures.
During the first year, the rover explored the floor of Jezero crater, focusing on aqueously altered igneous rocks. It also provided the opportunity to remotely observe the main delta front and its remnants (e.g., the Kodiak butte). This allowed us to distinguish several beds of sandstones (with local occurrences of boulders up to 30 cm) arranged into bottomsets, foresets and topsets morphologies. This tripartite geometry and steep slopes of foresets are characteristic of a Gilbert-type delta, formed by the deposition of fluvial sediments prograding into a standing body of water, here a paleolake whose level can be constrained by the transition from the foresets to topsets. Massive beds of boulder conglomerates (with boulders up to 1.5 m) have also been observed at or close to the top of many locations along the delta’s front, hinting at a transition to higher energy flows. Collectively, these elements argue for a polyphase complex depositional history of the delta through time.
The toe of the current delta front was reached by the rover on Sol 422 (April 2022) when Perseverance arrived at the Enchanted Lake outcrop, at the base of the southeastern end of the promontory informally named Cape Nukshak on the distal end of the delta. The first in-place sedimentary rocks that were observed were a succession of thinly-laminated medium/coarse sandstones and mudstones. Then, Perseverance pursued its route towards the delta and started its ascension at Hawksbill Gap to assess the first half of the lower delta succession. Strata at the base of Hawksbill Gap are mostly composed of fine to coarse-grained rocks ranging from mudstones to granule conglomerates, displaying planar to low-angle cross-stratifications.
These fine-grained detrital rocks are likely to have been deposited by fluvial to deltaic processes. There, the rover collected the first sets of paired sedimentary rock samples (coarse sandstone to micro-conglomerate) that will represent the fine- and coarse-grained lower delta succession once returned to Earth.