Scientists have discovered that the core of Mercury might have formed from a very rare type of meteoroid - one that's hardly ever found in meteorites on Earth.
The discovery was made thanks to data from NASA's MESSENGER spacecraft, which took readings of Mercury's surface between 2011 and 2015. With this data in hand, astronomers have managed to create synthetic 'Mercury' rocks of their own, and they're starting to reveal a few of the scorching planet's best-kept secrets.
But to understand those, you need to know what goes on when planets first begin to take shape.
The planets of our Solar System were originally formed when clumps of dust and gas - and later larger meteoroids - collided and grouped together. The surface of Mercury is now pocked with craters left over from this formation process billions of years ago.
When MESSENGER orbited Mercury, it used an X-ray spectrometer to determine the chemical composition of more than 5,800 lava deposits on the planet's surface. Unlike Earth, Mercury still clearly shows these tell-tale signs of its original formation.
Researchers from MIT correlated each of these data points with their position on the planet. Those that came from heavily cratered areas were assumed to be older, dating back to 4.2 billion years or so, while those from smoother regions are thought to be younger, from around 3.7 billion years ago.
"It’s true of all planets that different age terrains have different chemical compositions, because things are changing inside the planet," explains geologist Timothy Grove. "Why are they so different? That’s what we’re trying to figure out."
To solve the mystery, the team developed their own synthetic rocks in a lab furnace, with the rocks designed to match Mercury's insides when the planet was younger (lava deposits not yet solidified as rock) and older (solidified rock deposits).
When the researchers analysed the synthetic rocks, they found tiny crystals with a chemical composition matching that of enstatite chondrite meteorites - very rare rocks that only make up 2 percent of the meteorites that fall to Earth.
The discovery suggests that a rare meteoroid was the main starting material for the formation of Mercury.
The behaviour of the rock also revealed some secrets about the way Mercury might have cooled. Around 4.2 billion years ago, there appears to have been a temperature drop of 240ºC (432ºF) over a period of just 500 million years on Mercury - a significant level of climate change.
"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," said Grove. "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."
The team acknowledges that their conclusions are largely hypothetical, but in the absence of an actual chunk of Mercury's surface for chemical analysis, it's likely to remain our best insight into the planet's composition - for now, at least.
"The next thing that would really help us move our understanding of Mercury way forward is to actually have a meteorite from Mercury that we could study," said Grove. "That would be lovely."
The findings have been published in Earth and Planetary Science Letters.