The Circum-Isidis Capping Unit: An Extensive Regional Ashfall Deposit Exposed in Jezero Crater

Several mineralogically diverse regions across Mars are capped by dark-toned rock formations emplaced during the Noachian-Hesperian transition, an era encompassing a shift in volcanism from dominantly explosive to effusive. However, these caprocks' origins are uncertain, limiting insight into the nature of this shift. We explore the potential volcanic ash origin of a widespread (similar to 50,000 km(2)) mafic caprock in the Circum-Isidis region via an integrated photogeologic and remote-compositional analysis. We also investigate whether this unit is genetically equivalent to a mafic rock formation exposed in the floor of Jezero Crater. We find: (a) the Jezero Floor and Capping Units are morphologically, stratigraphically, and compositionally similar, suggesting a shared formation mechanism, and (b) the tonal layering and draping characteristics of the Capping Unit are most consistent with a volcanic ash origin atop the ultramafic Olivine-Rich Unit, also an ash. Our hypotheses can be tested by the Perseverance rover and studies of returned samples.