New York, June 28 (IANS): Based on an analysis of cooling rate and the composition of lava deposits on Mercury's surface, a team of geologists has found that the planet likely has the composition of an enstatite chondrite -- a type of meteorite that is extremely rare on Earth.

The new information on Mercury's past is of interest for tracing the Earth's early formation, according to Timothy Grove from Massachusetts Institute of Technology.

"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 said.

"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," he added.

Grove's team utilised data collected by NASA's MESSENGER spacecraft. During its mission, MESSENGER produced images that revealed kilometre-thick lava deposits covering the entire planet's surface.

An X-ray spectrometer onboard the spacecraft measured the X-ray radiation from the planet's surface, produced by solar flares on the sun, to determine the chemical composition of more than 5,800 lava deposits on Mercury's surface.

In the study, published recently in the journal Earth and Planetary Science Letters, the team recalculated the surface compositions of all 5,800 locations and correlated each composition with the type of terrain in which it was found -- from heavily cratered regions to those that were less impacted.

The researchers determined the chemical compositions of the tiny crystals that formed in each sample in order to identify the original material that may have made up Mercury's interior before it melted and erupted onto the surface. 

They found the closest match to be an enstatite chondrite, an extremely rare form of meteorite that is thought to make up only about 2 per cent of the meteorites that fall on Earth.

"We now know something like an enstatite chondrite was the starting material for Mercury, which is surprising, because they are about 10 standard deviations away from all other chondrites," Grove said.