Craters' Age Suggest Mercury Saw Extended Period of Volcanic Activity

New analysis of the surface of Mercury suggests that the tiny planet saw volcanic activity for an extended period of its history.

Astronomers consider the revelation surprising because Mercury was long thought to lack the the volatile compounds that cause explosive volcanism in the first place. The new information has the potential to cause scientists to reevaluate their theories on how Mercury was formed.

Evidence that Mercury at one point supported a system of volatile compounds that could trigger volcanic explosions has been growing ever since 2008 after NASA's Messenger mission made its first observations of the planet closest to the Sun.

The Messenger data revealed evidence of pyroclastic ash - the telltale sign of a volcanic explosion - all over the surface of Mercury. However, it has not been clear whether Mercury's history contains a brief period where it had a wealth of volatile compounds, or whether the compounds lingered on the planet for a longer amount of time.

The latest evidence suggests that the latter is true. In a study led by Brown University researchers, 51 of Mercury's pyroclastic sites were evaluated using Messenger data.

Tim Goudge, a graduate student in Brown's Department of Geological Sciences, and his colleagues determined that some of the pyroclastic vents that expelled volatile compounds are more eroded than others, a revelation that suggests the volcanic explosions did not happen all at the same time.

"If [the explosions] happened over a brief period and then stopped, you'd expect all the vents to be degraded by approximately the same amount," Goudge said. "We don't see that; we see different degradation states. So the eruptions appear to have been taking place over an appreciable period of Mercury's history."

Using an established method of dating craters on Mercury, Goudge and his team found that some of these pyroclastic deposits are located in relatively young craters. Mercury formed about 4.5 billion years ago, and some of the volatile material has been located in craters that are only about 1 million years old, suggesting they formed much later in Mercury's existence. Similar material in other craters was determined to be up to 3.5 billion years old.

"These ages tell us that Mercury didn't degas all of its volatiles very early," Goudge said in a statement. "It kept some of its volatiles around to more recent geological times."

Goudge and his collaborators' work appears in the Journal of Geophysical Research: Planets.