NASA MESSENGER: Ancient volcanic explosions shed light on Mercury's origins

Two pyroclastic vents on the floor of Mercury’s Kipling crater, top, would likely not have survived the impact; they are more recent. 

The false colour image of the same spot, bottom, marks pyroclastic material as brownish red.

The surface of Mercury crackled with volcanic explosions for extended periods of the planet's history, according to a new analysis led by researchers at Brown University.

The findings are surprising considering Mercury wasn't supposed to have explosive volcanism in the first place, and they could have implications for understanding how Mercury formed.

On Earth, volcanic explosions like the one that tore the lid off Mount St. Helens happen because our planet's interior is rich in volatiles - water, carbon dioxide and other compounds with relatively low boiling points.

As lava rises from the depths toward the surface, volatiles dissolved within it change phase from liquid to gas, expanding in the process.

The pressure of that expansion can cause the crust above to burst like an over-inflated balloon.

Mercury, however, was long thought to be bone dry when it comes to volatiles, and without volatiles there can't be explosive volcanism.

But that view started to change in 2008, after NASA's MESSENGER spacecraft made its first flybys of Mercury.

Those glimpses of the surface revealed deposits of pyroclastic ash - the telltale signs of volcanic explosions - peppering the planet's surface.

It was a clue that at some point in its history Mercury's interior wasn't as bereft of volatiles as had been assumed.

What wasn't clear from those initial flybys was the timeframe over which those explosions occurred.

Did Mercury's volatiles escape in a flurry of explosions early in the planet's history or has Mercury held on to its volatiles over a much longer period?

This latest work, available in online early view at the Journal of Geophysical Research: Planets, suggests the latter.