The scarred Kipling crater on Mercury (Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Like its traveller god namesake, the planet Mercury is hard to pin down. It now seems that volcanic blasts have rocked it for aeons, and this doesn’t mesh with theories of its birth. It even raises the prospect that Mercury may have formed further out in the solar system and migrated in.

Such volcanic explosions happen on Earth when lava boils water and volatile compounds underground, and they smash through the surface. Mercury, though, is closer to the sun, so any volatiles within should have boiled off as the planet formed 4.5 billion years ago.

That view changed during fly-bys in 2008, when NASA’s Messenger probe spotted volcanic ash deposits and telltale vents in the ground. Still, the blasts could have happened just after Mercury’s birth.


Now, Timothy Goudge of Brown University in Providence, Rhode Island, and his colleagues have analysed higher resolution images taken when the probe began orbiting Mercury in 2011.

They looked at 51 deposits and their source vents and found that some were more eroded – by material flung up by impacts, say – than others, so they can’t all have formed at the same time. What’s more, most of the vents were inside impact craters, which can be dated, suggesting that the explosions occurred intermittently between 3.5 and 1 billion years ago, not just after Mercury formed.

Wandering planet?

The results should challenge planetary scientists to come up with new ideas about Mercury’s birth, says Goudge. “The formation mechanism for Mercury now has to be able to explain this observation of volatiles in the planet’s interior.”

David Rothery of the Open University in Milton Keynes, UK, who has also spotted relatively young vents in Messenger data but was not involved in this work, agrees: “We have a mystery about how Mercury formed.”

Rothery suggests the possibility that Mercury formed further out and migrated in. Astronomers think gas giants like Jupiter and Saturn may have migrated from their orbits in the early days of the solar system, so perhaps something similar happened to Mercury as well. “We’re at a bit of a loss,” he adds.

The idea that volcanic explosions rocked Mercury for most of its existence also clashes with another theory about its formation: that a large object might have hit the youthful Mercury and destroyed most of its crust. This is used to explain the mystery of why the modern Mercury has a large core and thin crust – but such an impact would also have removed any volatiles, apparently ruling out volcanic explosions.

Rothery is part of the team working on BepiColombo, a joint mission between Europe and Japan due to launch in 2016 and reach Mercury in 2024. It is equipped with more advanced sensors that could clear things up. “When we know Mercury’s composition and its geological evolution more clearly, we will be able to pose better questions about how it formed,” he says.

Journal reference: Journal of Geophysical Research: Planets, DOI: 10.1002/2013JE004480