JEZERO CRATER WESTERN FAN CHEMISTRY AND MINERALOGY OBSERVED BY PERSEVERANCE/SUPERCAM ON MARS

Perseverance landed on igneous terrain near the front face of a river delta/fan formation on the western rim of Jezero crater (18°N, 77°E) on the eastern terminus of Isidis Planitia. The Perseverance team has been exploring the delta/fan deposits since April 2022, first along the front (~30 m height), and since February 2023, on the top of the delta fan. Grain sizes along the front (“Shenandoah fm”) correspond mostly to medium to coarse sand, with a traceable interval of light-toned coarse siltstone at Hogwallow Flats and Yori Pass, well above the base of the front. SuperCam observations indicate sandstones dominated by olivine and pyroxene, with variable carbonate, serpentine, and other phyllosilicates indicated by infrared (IR, 1.3-2.6 micron) observations. LIBS and Raman spectra also confirm olivine as the primary igneous mineral. The light-toned siltstone interval contains a significant fraction of sulfates, which likely resulted from a period dominated by intense lake evaporation. The regolith-dominated region below the front was explored while depositing a contingency sample cache for the Mars Sample Return mission. This area contains some light-toned Al-dominated float rocks of unknown origin, interpreted to have undergone intense aqueous alteration. Perseverance climbed onto the delta fan top (“Tenby fm”) and has been exploring a so-called curvilinear unit apparently dominated by fluvial bar or delta plain deposits. The lower portion consists of medium to coarse sandstone, with coarser-grained upper portions up to pebble conglomerates. Clasts and boulders litter the surface in some areas, and some bouldery lenses appear in the upper delta front as well. IR spectra of the delta/fan top indicate stronger olivine signatures and lower Fe³⁺ than the delta front or crater floor, with carbonate and minor Si-OH or Al-OH phases. Many clasts and boulders are olivine-rich with higher Mg #s (up to 70), but some are low-Ca pyroxene-rich. Except for the sulfate-containing interval on the delta front, the sediments indicate neutral to slightly alkaline pH with variable oxidation conditions, including some relatively anoxic regions on the delta fan top. In terms of habitability, while coarser grain sizes of the delta front and top are not as conducive to organic preservation, the possible hydrated silica and potentially anoxic conditions strongly favor organic preservation.