The planet Mercury may have had a large, rolling ocean of magma very early in its history after its formation about 4.5 billion years ago, US astronomers say. Scientists at the Massachusetts Institute of Technology have analyzed Mercury's current rocky surface using data from NASA's MESSENGER probe orbiting the planet to partially reconstruct its history over billions of years. The chemical composition of rock features on the planet's surface suggests a large ocean of liquid magma may have once been present, an MIT release reported Thursday. MESSENGER identified two distinct compositions of rocks on the planet's surface, scientists said, and experiments suggested only one phenomenon could explain the two compositions: a vast magma ocean that created two different layers of crystals, solidified, then eventually re-melted into magma that then erupted onto Mercury's surface. "The thing that's really amazing on Mercury is, this didn't happen yesterday," MIT geology Professor Timothy Grove said. "The crust is probably more than 4 billion years old, so this magma ocean is a really ancient feature." The study has been published in the journal Earth and Planetary Science Letters.