Multispectral Observations of Float Rocks Used to Investigate the Origin of Boulders on the Western Jezero Fan Front, Mars

Abstract

In Jezero crater, Mars, the Perseverance rover has explored the western fan and encountered loose pieces of rock separated from outcrops or “float” rocks. Comparing float rocks to in-place outcrops can provide key insights into the crater's erosional history and the diversity of units in the Jezero watershed that Perseverance cannot visit in situ. Here, we used multispectral observations from Perseverance's Mastcam-Z instrument to investigate the lithology and origin of float rocks found on the western Jezero fan front (sols 415–707). We identified four textural classes of float rocks (conglomerates, layered, massive, and light-toned) and investigated their physical characteristics, spectral properties, and distribution to interpret their source and mode of transport. Likely derived from local sedimentary fan outcrops, conglomerate and layered float rocks are highly spectrally variable and altered with differing ferric and ferrous signatures. Massive float rocks are the least altered with ferrous signatures and likely derived from local outcrop sources or more distal sources (∼50–250 km) in the Jezero watershed. Massive float rocks separate into two subclasses: massive olivine and massive pyroxene, which are likely derived from the regional olivine-carbonate-bearing watershed unit and the crustal Noachian basement unit, respectively. The unique light-toned float rocks have variable hydration and low Fe-abundance, but there is no local outcrop equivalent of these rocks on the crater floor or fan front, suggesting transport into the basin from a source region outside Jezero. Perseverance found no meteorites at the western fan, implying that fan sediments may be in the youngest ages estimated from crater counts (Hesperian).