One of the surprises revealed by the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission to Mercury was the discovery of thousands of “hollows”—shallow depressions up to roughly 1.6 kilometers wide—scattered across the planet’s surface. The hollows’ fresh appearance and relative youth compared to the craters with which they’re often associated suggest that contrary to prevailing theories, Mercury’s surface is still evolving as volatile materials excavated by impacts vaporize at the surface, creating these distinctive voids. Which materials these are, however, has been the subject of much debate.
Now Vilas et al. have, for the first time, identified several absorption features in MESSENGER’s spectral reflectance data associated with hollows in both the Dominici and Hopper craters. The absorption features are consistent with the presence of magnesium sulfide and possibly calcium sulfide. The finding is compatible with surface composition measurements that show a relatively low abundance of iron and a high abundance of sulfur on Mercury compared to the other rocky planets, as well as the general presence of magnesium on Mercury’s surface.
The researchers also observed variations in the spectral properties of two areas of hollows within the same crater. The material around hollows located on Dominici’s southern wall and rim has brighter halos and more well defined edges than the material around hollows in the center, suggesting that the southern material may have been more recently exposed. Likewise, the decrease in brightness observed in the central hollows may be due to the gradual addition of darkening material, such as graphite or submicroscopic iron, or the formation of a dark coating like those observed on some asteroids from processes associated with space weathering.
Collectively, the results of this study suggest that even within a single crater, these landforms can be of different ages or at various stages in their evolution. This finding is bolstered by the team’s observation that the hollows’ spectral signature is weaker in the Hopper crater; the lack of rays and fresh ejecta there indicate that the hollows are older, which means that absorption features can be correlated to age. The study is an important contribution toward better understanding Mercury’s geologic history, one of the mission’s primary science objectives. (Geophysical Research Letters, doi:10.1002/2015GL067515, 2016)
—Terri Cook, Freelance Writer
Citation: Cook, T. (2016), Demystifying Mercury “hollows”, Eos, 97, doi:10.1029/2016EO047675. Published on 11 March 2016.