Pictures from NASA's MESSENGER spacecraft orbiting the planet Mercury are showing something scientists say they didn't expect: Evidence of explosive volcanic activity in the planet's past.
Researchers from Brown University in Rhode Island were surprised because the planet was long believed to lack the volatile compounds that are the source of explosive volcanism.
Earth is rich in many volatiles such as carbon dioxide, water and some other compounds with low boiling temperatures that can create explosive eruptions, like the one that blew the top off Mount St. Helens in Washington in 1980.
Volatiles in rising lava change from liquid form to gas as the magma approaches the surface, causing great pressure that can create dramatic explosive eruptions.
When it comes to volatiles on Mercury, however, scientists had long believed the planet was bone dry.
That changed when the MESSENGER spacecraft began making its first approaches to the planet in 2008, capturing images of surface accumulations of volcanic pyroclastic ash, a telltale sign of explosive eruptions.
Scientists immediately wondered if all the volatiles on Mercury were exhausted in eruptions early in its history or if the planet may have retained them for a longer time.
The Brown researchers analyzed scored of pyroclastic sites across the planet's surface, using MESSENGER photographs that gave detailed views of the ash deposits with the vents that had spewed them out.
Analysis of the amount of subsequent erosion among the studied vents showed considerable variation. Evidence of explosions had not occurred around the same period.
"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," Brown geology graduate student Tim Goudge says. "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."
Some pyroclastic deposits are located inside impact craters on the surface of Mercury that are relatively young in geological terms, between 3.5 billion and 1 billion years old, Goudge says, proving the volcanic eruption had to have happened after the impacts, or the deposits would have been obliterated in the impacts.
This is a strong indication pyroclastic activity was not limited to the early period of Mercury's formation about 4.5 billion years ago, he says.
"These ages tell us that Mercury didn't degas all of its volatiles very early. It kept some of its volatiles around to more recent geological times."
The length of time it held on to those volatiles could be clues to how Mercury formed, geological sciences Professor Jim Head says.
"Together with other results that suggest the Moon may have had more volatiles than previously thought, this research is revolutionizing our thinking about the early history of the planets and satellites," says Head, who is also a co-investigator on the MESSENGER mission. "These results define specific targets for future exploration of Mercury by orbiting and landed spacecraft."
The Brown researchers have published their study in the Journal of Geophysical Research.
