However Mercury, with its massive heart of iron, got to be the oddball of the inner solar system's family, it was not by trials of fire, new research shows.
Excessive heating due to its close proximity to the sun has long been the favored explanation for why tiny Mercury has proportionally so much more metal iron than its sister rocky worlds Venus, Earth and Mars. Lighter elements just got boiled away, according to this theory.
Another theory suggests a massive body smashed into Mercury when it was young, blasting away its rocky shell and leaving behind a diminutive world that is more than half iron core.
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.
There's also geologic evidence for huge outpourings of lava that inundated large areas of Mercury to a depth of several miles -- enough to bury the state of Texas more than four miles deep.
The rock was so hot, it melted into the ground and created new channels that flowed onto the surface.
Other images from Messenger show a new type of landform called "hollows" -- rimless depressions where scientists believe volatile materials on the planet’s surface have been -- and possibly are being -- blasted into gases by sunlight or solar wind particles.
"It's quite conceivable that mercury is still geologically active," Solomon said.
"From the perspective of the science team, it's been a delight to realize that many of the theories about Mercury are no longer viable," he added. "Mercury has just been a long list of surprises."
After a seven-year voyage that included three flybys of Mercury, Messenger became the first probe to go into orbit around Mercury in March. The primary mission is scheduled to last for another six months.
A series of papers from the Messenger team is published in this week’s Science.