New research from NASA's Mercury orbiting Messenger spacecraft tells another story. Scientists find that volatile materials, such as potassium and sulphur, which would have been lost as temperatures spiked under either scenario, are present on the planet today.
"The chemistry is not what we expected," Messenger lead scientist Sean Solomon, with the Carnegie Institution of Washington, told Discovery News.
Perhaps Mercury formed from a metal-rich swath of planetary building materials that existed closest to the young sun, suggests scientist Denton Ebel, curator at the American Museum of Natural History in New York.
"The big conundrum at Mercury is the size of its core relative to its mantle. Between 60 to 70 percent of the mass of Mercury is in its core, whereas for the Earth it's only about 32 percent. That's a huge difference. The question is how did it get that way?" Ebel told Discovery News.
There's more for scientists to ponder than how Mercury formed. Messenger, the first probe to orbit the innermost planet of the solar system, revealed that Mercury has an asymmetrical -- and fluctuating -- magnetic field that is stronger in the north than in the south.