NASA experts find that Mercury's oldest plains were probably made near its core. Its surface features originated from heating as well as the crushing of its rocks almost 400 kilometres deep in the region where its mantle touches its iron core.
It is clear that volcanic activity spewed up the rocks found as the oldest ones on the surface of the planet. Experts recreated the material that was like the volcanic deposits on the planet.
Last year, NASA's MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) spacecraft's mission to the planet found the surface to be heterogeneous, with a low iron content as well as the alkaline surface. There was a high concentration of sulphur on a terrestrial planet in the solar system in its mantle.
"We have a hard job explaining where in the solar system Mercury formed, and how it happened," says David Rothery of the UK's Open University. He was not involved in the latest research.
Two types of regions of Mercury were discovered. While the older inter-crater plains, as well as heavily cratered terrains, were discovered to be 4.2 billion years old, the young northern volcanic plains were 3.7 to 3.8 billion years old.
It is not completely clear, though, how different regions developed here.
Mercury's cooling history right after it was formed between 4.2 and 3.7 billion years ago was explored and the origins were discovered to be a rare meteorite, a kind of enstatite chondrite, which is now believed to represent the planet's building blocks.
"The key finding is that by varying pressure and temperature on only one type of composition, we 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," the first author of the study, Dr. Asmaa Boujibar, said at Goldschmidt conference in Yokohama, Japan.
"The pressures were high, up to 50,000 times the Earth's atmospheric pressure, and the sort of pressure where you can form diamonds," said Dr. Boujibaar. "This is the pressure of Mercury's core-mantle boundary."
Timothy Grove, the Cecil and Ida Green Professor of Geology in MIT's Department of Earth, Atmospheric, and Planetary Sciences point out how the rare materials trace the earth's early formation.
"Here we are today, with 4.5 billion years of planetary evolution, and because the Earth has such a dynamic interior, because of the water we've preserved on the planet, [volcanism] just wipes out its past," Grove says. "On planets like Mercury, early volcanism is much more dramatic, and [once] they cooled down there were no later volcanic processes to wipe out the early history. This is the first place where we actually have an estimate of how fast the interior cooled during an early part of a planet's history."