Impact Delivery of Water at the Moon and Mercury

Cometary and asteroidal impacts at Mercury and the Moon likely supply some fraction of these bodies' near-surface reservoirs of water ice. At Mercury, MESSENGER spacecraft observations of permanently shadowed regions revealed bright and dark deposits, interpreted as comet- or asteroid-derived water and organics [1]. At the Moon, excavation of a permanently shadowed crater by the LCROSS mission confirmed the presence of substantial (~5 wt%) near-surface water [2]. Geophysical models of water delivery set limits on the relative roles of larger episodic events and ongoing micrometeorite impacts, assisting in both the interpretation of available data and the refinement of future geophysical strategies to characterize near-surface volatiles.

Determining to what extent comets and asteroids have contributed to the near-surface water detected at each body depends upon both the impact flux and the efficiency of impact delivery. Here we present new insights on delivery efficiency from simulations of impacts using the CTH shock physics code. Integrating these results with current estimates for the impact fluxes at the Moon and Mercury provides a more complete understanding of exogenous water sources. While prior analytical and numerical work has treated aspects of this problem, the effects of parameters such as impact angle and porosity have not yet been characterized in detail. We find that, while the inclusion of target porosity increases water retention, impactor porosity reduces delivery efficiency.

Fully 3-D simulations document decreased delivery efficiency with increasing impact angle obliquity. In addition, post-impact evolution of projectile material differs fundamentally between icy, cometary impactors and silicate, asteroidal impactors. Our calculations implement a variety of material types, probing the dependence of impact delivery on the projectile’s equation of state. The combined effects of impact angle, velocity, material models, and impactor flux demonstrate key differences in volatile delivery mechanisms at Mercury and the Moon.

[1] Neumann et al., 2013. Science 339, 296-300.

[2] Colaprete et al., 2010. Science 330, 463-468.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.