Icarus http://doi.org/qb6 (2013)

Credit: © NASA/JOHNS HOPKINS UNIV./CARNEGIE INST. WASH'TON

The surface of Mercury is pockmarked by fresh, shallow, kilometre-wide depressions dubbed 'hollows'. A survey of recent images obtained by the MESSENGER spacecraft shows that the distribution of the hollows on Mercury's surface is consistent with an origin by sublimation of volatiles from a reservoir beneath the surface.

Rebecca Thomas and colleagues at The Open University, United Kingdom, mapped the occurrences of the depressions using available MESSENGER images. They find that hollows cover 0.08% of Mercury's surface and usually — but not exclusively — occur in clusters within impact craters. Most of the features occur in areas where heating of the surface by the Sun is high or where the subsurface has been heated by volcanism. The hollows could thus have been formed by sublimation from a volatile-bearing unit after it was exposed by an impact or volcanic eruption.

The hollows are present in many regions of Mercury, indicating that volatile-rich material is widespread across Mercury's subsurface.