A team of researchers has unveiled the secret behind Mercury's curious surface.
NASA researchers have found that several volcanic deposits on Mercury's surface require mantle melting to have started close to the planet's core-mantle boundary, which lies only 400km below the planet's surface and making it unique in the solar system.
The recent MESSENGER mission to Mercury has shown that the surface of the planet is very heterogeneous, but it can be classified into two main types of regions. One is an area of relatively young Northern Volcanic Plains (NVP) - these are between 3.7 and 3.8 billion years old. The other area is older (4 to 4.2 Ga) and consists of intercrater plains and heavily-cratered terrains (IcP-HCT), which is between 4 and 4.2 billion years old.
The older regions contain several previously unexplained features, including a large magnesium-rich spot, which is around 10 000 000 km square - around the size of Canada although because Mercury is much smaller than the Earth this spot takes up around 15 percent of the planet's surface.
Until now, there has been no satisfactory explanation of how the formation and history of the planet would have allowed these heterogeneous areas to develop without invoking melting of a heterogeneous mantle.
The researchers looked for the answers by simulating early conditions on Mercury. Mercury is believed to have formed under highly reduced conditions. Enstatite chondrites are similarly reduced and may be a good proxy to the chemical building blocks. So the researchers took the same chemistry as found in enstatite chondrites, and began to subject them to the sort of pressures and temperatures found in the deep mantle of Mercury.
First author Asmaa Boujibar said that the key finding is that by varying pressure and temperature on only one type of composition, they could produce the variety of material found on the planet's surface. These findings indicate that the older terrains are formed by material melting at high pressures up to the core-mantle boundary, while the younger terrains are formed closer to the surface.
This study is reported at the Goldschmidt conference in Yokohama, Japan.
