The water content of Phobos and Deimos is very important to assess, for both science and human exploration planning. The expected water content in Phobos and Deimos depends on their origin and evolution through time. The origin of Phobos and Deimos is still unknown. One intriguing hypothesis is that they are reaccreted impact ejecta from one or more giant impacts on Mars. Such an origin is commonly thought to imply that Phobos and Deimos would be depleted in volatiles, in particular in water, as expected in the hypothesis of the origin of the Earth's Moon by Giant Impact. But do giant impacts produce orbital debris that are necessarily devoid of water?
Insights into the survival of target water in impact events may be gained from examining the water content in tektites. Tektites are natural glasses derived from the hypervelocity impact-melting of terrestrial upper crustal rocks, that were then ejected from the impact site, lofted through, and in some cases, above, the Earth’s atmosphere, quenched, and distributed over an extended area called a strewn field. Extreme heating during impact melting, ejection, and atmospheric transit, result in massive water loss in tektites.
In this study, we review and produce a synthesis of what we know (and still don't) about the water content in tektites. We focus in particular on the Australasian tektites, as these have the most extensive strewn field and would have involved the largest impact among all tektite-forming impacts known. While most tektites are poor in water (~ 0.002 wt%), some do contain more significant amounts water (~ 0.02 wt%), even if substantially less than in volcanic glass such as obsidian (~ 0.1 – 1 wt%). Compared to other tektites, the Australasian tektites are the least poor in water, suggesting that the impact target material from which these tektites originated was rich in water, including possibly submerged. There is also a gradient in water content within the Australasian tektite strewn field, with water content decreasing away from the presumed source region, which is likely in or near Vietnam, toward Australia. Splash form Australasian tektites recovered in Australia have almost certainly been ejected out of the Earth’s atmosphere and traveled along a suborbital arc from Vietnam (or vicinity) to Australia. In spite of their high-energy history, these tektites still contain water.
The above considerations open the possibility that in the Giant impact hypothesis for the origin of Phobos and Deimos, not all water from the target (Mars), if substantial amounts were present in the planet’s crustal materials in the first place, would necessarily have been lost in the ejecta. Our study suggests that Phobos and Deimos should not be assumed to not contain water in the giant impact hypothesis for their origin.