Mercury may once have had a large ocean of magma, Messenger data shows, shortly after its formation about 4.5 billion years ago.
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging probe has been orbiting the planet since March 2011. Later that year, a group of scientists analyzed X-ray fluorescence data from the probe - and identified two distinct, and unexplained, compositions of rocks on the surface.
An MIT team used the data to recreate the two rock types in the lab, and subjected each to high temperatures and pressures to simulate various geological processes. And, they say, there's only one explanation: a vast magma ocean that created two different layers of crystals, solidified, then eventually remelted into magma that then erupted onto Mercury's surface.
"The thing that's really amazing on Mercury is, this didn't happen yesterday," says Timothy Grove, a professor of geology at MIT. "The crust is probably more than four billion years old, so this magma ocean is a really ancient feature."
The scientists initially looked for scenarios in which both original rock compositions might be related. For example, both rock types could have come from one region, with one rock crystallizing more than the other.
But, they found, the two compositions were too different to have originated from the same region. And the easiest explanation, Grove says, is a large magma ocean, which over time formed different compositions of crystals as it solidified. Eventually it would have remelted, spewing lava onto the surface of the planet in massive volcanic eruptions.
Grove estimates that this magma ocean likely existed very early in Mercury's existence - possibly within the first one million to 10 million years - and may have been created from the violent processes that formed the planet.
As the solar nebula condensed, pieces collided into larger and largerchunks, eventually forming planets. That process of collision and accretiion could have produced enough energy to completely melt the planet.
"We're gradually filling in more blanks, and the story may well change, but this work sets up a framework for thinking about new data," says Larry Nittler of the Carnegie Institution of Washington, who was not involved in the study. "It's a very important first step toward going from exciting data to real understanding."