NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, now nearing the end of its fourth and final year of orbital operations at Mercury, is well into a low-altitude campaign that is returning images and measurements of the planet's surface and interior that are unprecedented in their resolution.
Early in its primary orbital mission, MESSENGER discovered thousands of peculiar depressions at a variety of longitudes and latitudes, ranging in size from tens of meters to several kilometers across and tens of meters deep.
MESSENGER Participating Scientist David Blewett at The Johns Hopkins University Applied Physics Laboratory (APL) said that these features, given the name 'hollows,' were a major surprise, because while they had been thinking of Mercury as a relic, a planet that wasn't really changing anymore, hollows appear to be younger than the planet's freshest impact craters, suggesting that Mercury is a planet whose surface is still evolving.
The team has since deduced that the hollows form through loss of a component in the rocks that is susceptible to sublimation (or a similar process) when exposed to the harsh environment of the planet's surface.
High-resolution images obtained by the spacecraft at low altitudes are revealing striking details about these hollows, including their young ages, their depths, and the diversity of locations in which they are found.