Around 4.6 billion years ago, the universe was a chaos of collapsing gas and spinning debris. Small particles of gas and dust clumped together into larger and more massive meteoroids that in turn smashed together to form planets. Scientists believe that shortly after their formation, these planets -- and particularly Mercury -- were fiery spheres of molten material, which cooled over millions of years. Now, geologists at MIT have traced part of Mercury's cooling history and found that between 4.2 and 3.7 billion years ago, soon after the planet formed, its interior temperatures plummeted by 240 degrees Celsius, or 464 degrees Fahrenheit. 

They also determined, based on this rapid cooling rate and the composition of lava deposits on Mercury's surface, that the planet likely has the composition of an enstatite chondrite -- a type of meteorite that is extremely rare here on Earth. 

Timothy Grove, the Cecil and Ida Green Professor of Geology in MIT's Department of Earth, Atmospheric, and Planetary Sciences, says new information on Mercury's past is of interest for tracing 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."

Grove and his colleagues, including researchers from the University of Hanover, in Germany; the University of Liége, in Belgium; and the University of Bayreuth, in Germany, have published their results in Earth and Planetary Science Letters.