Examining the Potential Contribution of the Hokusai Impact to Water Ice on Mercury

Abstract

AbstractThe polar cold traps of Mercury host an estimated 1016–1018 g of water ice. The relative purity of the water‐ice deposits could indicate they were emplaced over a short time interval, and sharp albedo boundaries suggest that the water ice could have been emplaced relatively recently. Together, these lines of evidence are consistent with potential delivery by a single, large, young impact event. Hokusai is the most prominent large young impact crater on Mercury—if the bulk of Mercury's water‐ice inventory was indeed delivered by a single recent impact event, Hokusai is the best candidate source crater. This study constrains the impact conditions that created Hokusai from morphological and color observations of the crater and its ejecta. Results show that the Hokusai impact was recent and oblique. These parameters, coupled with retention and migration factors largely derived from existing lunar studies, are used to estimate the possible contribution of the Hokusai impact to water ice on Mercury. Assuming water‐rich asteroidal or cometary impactors, the Hokusai impact event could account for the inventory of water ice on Mercury for impact velocities less than ~30 km/s, a velocity that is achieved by 24–32% of impacts into Mercury, depending on the size distribution model employed. The single impact delivery scenario therefore remains viable for Mercury. Robust modeling of a single large impact event on Mercury would supply critical insight into the feasibility of this scenario.