Mineralogy of the Nili Fossae region with OMEGA/Mars Express data: 1. Ancient impact melt in the Isidis Basin and implications for the transition from the Noachian to Hesperian

The Nili Fossae region located on the northwestern quadrant of the Isidis Basin, Mars, displays superb exposures of bedrock outcrops that reveal mineralogy and composition of the crust. Previous work has shown that this region exhibits the largest exposures of olivine‐dominated rock units on Mars. Visible‐infrared imaging spectrometer data acquired by OMEGA were calibrated to surface reflectance and analyzed to determine surface mineralogy. The dominant minerals identified are iron‐bearing mafic silicates (low‐ and high‐calcium pyroxene, olivine) and water‐bearing phyllosilicate (iron‐rich smectite clay). The strength and position of mineral absorption features were used to produce mineral indicator maps for the dominant species, and the maps were integrated with high‐resolution imaging from the THEMIS, MOC, and HRSC instruments, and MOLA topography. We show that olivine and phyllosilicate occur in spatially distinct outcrops; the olivine‐bearing rock unit is a meters‐ to tens of meters‐thick cap unit resting on phyllosilicate‐bearing bedrock, and the phyllosilicate units predate the Isidis basin‐forming event. On the basis of superposition, crosscutting, and geomorphic relationships, we interpret the emplacement of the olivine‐bearing units as having been contemporaneous with the Isidis impact event. By analogy with the Orientale basin on the Moon, we propose that the olivine‐bearing unit represents the surface exposure of the impact melt from the Isidis impact event. These results demonstrate that large regions of crust had been altered in the presence of water prior to the date of the Isidis basin‐forming event in the Late Noachian (≈3.96 Ga).