EXCAVATION OF MARTIAN LOWER CRUST AND MANTLE BY THE ISIDIS IMPACT AND IMPLICATIONS FOR THE MARS 2020 MISSION

Introduction: Isidis Planitia (Figure 1) is a ~1500 km diameter basin centered at 13°N 87.0°E and is thought to be the last major impact basin that formed on Mars, approximately 3.9 billion years ago [1]. The Mars 2020 rover landing site has recently been selected to be Jezero crater, which lies between the inner and outer rings of Isidis (Figure 1). Thus, the circum-Isidis region represents a unique area for in situ investigations of basin-forming impact processes and their ejecta on Mars [2]. However, it is currently unknown how deeply the Isidis impact excavated, and thus it also unknown what types of materials the Mars 2020 rover will encounter and collect for possible future sample return. A recent study [3] modeled the formation of the similarly-sized South Pole-Aitken basin on the Moon, showing that SPA ejecta should contain a large fraction of upper mantle material, which is consistent with the observed enrichment in such mafic phases as orthopyroxene (OPX) in likely ejecta materials in the lunar highlands. An upper OPX-rich mantle has major implications for the petrology of the Moon and how it formed from an early magma ocean. Recent spectral observations identified orthopyroxene in the ejecta blocks of Argyre Planitia, another large multi-ring basin on Mars [4] and is the dominant mineral detected in hypothesized ejecta materials from Isidis itself [5]. By implication, we hypothesize that Isidis may also be capable of excavating the upper mantle on Mars. If Mars also has an upper mantle rich in low-Ca pyroxene, it would imply that the olivine rich upper mantle of Earth is unique in our solar system. If upper mantle materials are expected to be present in the circum-Isidis region, these could be important targets for Mars 2020 and Mars sample return. In this study, we use hydrocode simulations to model the Isidis impact and to determine the depth of origin and shock state for materials in the rim and beyond. Basin Size Constraints: Ref. [6] conducted a series of iSALE runs and admittance modeling of Isidis Planitia, varying the geothermal gradient, pre-impact crustal thickness, and projectile diameter. They concluded that the best-fit impact parameters for Isidis are a 120 km diameter dunite impactor striking a 60 km thick basalt crust with a 20 K/km geothermal gradient at 12 km/s. However, the diameter of the crustal collar formed from this size of impactor does not match the observed free air gravity anomaly of Isidis. Isidis, as a mascon basin, has a central free air gravity high surrounded by a free air gravity negative annulus. A mascon basin’s free air gravity low is associated with the location of its crustal collar, whose diameter does not change significantly with isostatic adjustment [7]. The diameter of the free air gravity annulus of Isidis is larger than the crustal collar produced by ref [6]’s best fit run. The diameter of the crustal collar is dependent on impact energy with a larger impact energy resulting in a larger diameter for the crustal collar. Therefore, in this work, we conducted a full parameter search of impactor parameters for impactors 120 km and larger.