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Haunted Hollows

by Perry Pezzolanella

One of the most exciting discoveries made by the MESSENGER spacecraft is the evidence that Mercury is rich in volatiles. These are substances that readily change from solid to vapor at low temperatures. Mercury is a hot planet where daytime temperatures reach 800ºF so it was thought that Mercury would lack volatiles. Instead of a dead planet, Mercury has proved to be geologically active.

The ferocious heat throughout Mercury’s history meant it could never have oceans or flowing water, but MESSENGER found higher amounts of sulfur and potassium than what is found on Earth and Mars, which have substantial volatiles themselves. It is possible that internal processes could concentrate the volatiles at the surface as they are capable of lowering the melting point of rock and enhancing volcanism. This could enable the movement of faults thereby creating explosive eruptions and fascinating landscapes, including haloed depressions known as hollows. Clusters of hallows occur all over Mercury and range in size from about a half mile across to several hundred miles across. They are found mainly in impact craters. Hollows look like the surface has been eaten away, leaving depressions up to 150 feet deep. They are bright and were first seen by Mariner 10 when it flew by Mercury in 1974. Hollows look freshly made with sharp rims and are not covered by ejecta from nearby craters. This means that hollows are quite young relative to other geology and may be forming this very day.

Hollows have flat bottoms, which mean that they are not volcanic vents. Volatiles near the surface are heated and, in the process of changing from solid directly to vapor, the ground collapses and appears eaten away. The vapor that is created escapes forever into the thin atmosphere. Hotter areas have more hollows than colder regions. It would seem that volatiles would have been depleted long ago given the relentless heat from the adjacent Sun, but most hollows are found in impact craters. The impact creates the crater and exposes volatile-rich rocks hidden deep underground that have been protected from the merciless Sun. Once it is exposed to the intense heat of the sun, the volatiles are lost to space and leave behind bright material.

Mercury turned out to be far more volcanic than originally thought. Lava plains cover about 27% of the planet. Vents surrounded by bright red deposits dot Mercury and are a result of explosive volcanism. Volcanism lasted a long time on Mercury from the ancient lava plains of about 3.5 billion years ago to the younger explosive vents of about a billion years ago. Mercury should have cooled faster than other terrestrial planets because it is smaller, so active volcanism should not have lasted as long, but volatiles kept the magma beneath the surface molten at a lower temperature and thus, prolonged the period of active volcanism.

Our knowledge of Mercury’s hollows and volcanism will improve in 2025 when BepiColombo, a dual orbiting spacecraft between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), arrives with equipment and cameras more sophisticated than MESSENGER. By understanding Mercury’s volcanic history there will be a better understanding of how the Solar System evolved. The hauntingly beautiful hollows of Mercury are an important part of that knowledge.