HYDROUS IMPACT MELTS: CANTEENS AND CRUCIBLES OF THE INNER SOLAR SYSTEM

We previously have demonstrated that volatiles do not necessarily ionize and escape hypervelocity impacts. Natural and experimental melts generated by collisions with saturated sediments can trap significant volumes of target water. Typical impact melts (excluding tektites) can contain up to a few weight percent water, largely as dissolved hydroxyls. But much larger concentrations of OH^- and H₂O, ~ 5 to 20 wt%, can exist in discrete silicate melts enclosed by anhydrous matrices. Results from experimental impacts of hydrous projectiles into dry targets show in principle that melts might trap water from volatile-rich asteroids and comets.

We have argued that the gradual release, by diffusion and micrometeorite fragmentation, of water entombed in impact melt ejecta could contribute to the Moons’ water budget. Hydrous impact melts might also be important in other aspects of planetary evolution.

Hydrous impact melts that come to rest in thick melt sheets or consolidated ejecta would migrate and coalesce, or otherwise exsolve vapors, that could reach the surface either as froth or explosively. Such a process might explain the so-called “hollows” observed by Messenger on Mercury and their close association with impact craters. Similar post-impact migration of trapped water also could contribute to hydrothermal alteration of crustal materials on other small, dry bodies.

Fisk et al. (Science, 1998) showed that volcanic glasses on Earth are important crucibles of bacterial colonization in environments otherwise hostile to organisms. We have observed evidence that impact melt breccias, likewise, are sites of aggressive bacterial activity as the glass provides necessary substrates, nutrients, and energy. Some of these melts contain zoned carbonate globules similar to those, famously or infamously, observed in melt veins of ALH80041. While other similar globules have been identified in a number of terrestrial environments, ours is the first noted occurrence of such constructs in impact melts; and as such, they may provide an important point of comparison for the role of primitive organisms in their development on both planets.

Our findings continue to emphasize the importance of identifying and studying impact melt ejecta on Mars for understanding both the history of volatiles and potential life on its surface.