New data from NASA's Messenger spacecraft has surprised scientists showing the planet Mercury has some of the most unusual internal dynamics ever seen.
The findings, which will appear in the journal Science, mean the planet closest to the Sun has evolved differently compared to the other terrestrial planets in the solar system.
After its first full year in orbit around Mercury, Messenger has returned a detailed picture of the planet's northern hemisphere suggesting a deep reservoir of high-density material exists below a thin crust surface.
Using the Messenger data, Dr Maria Zuber, of the Massachusetts Institute of Technology, and colleagues created a detailed elevation model showing the planet's northern hemisphere is far flatter than Mars or the Moon.
They also found extensive lowlands and a vast northern volcanic plain.
The researchers were also surprised to discover that the floor of several craters are tilted. For example, the 1500-kilometre wide Caloris impact crater is tilted to the extent that parts of its floor are now taller than the rim.
Unlike the Moon
"Prior to Messenger's observations, many scientists believed Mercury was much like the Moon," says Zuber.
"We thought it cooled off very early in solar system history, and has been a dead planet throughout most of its evolution."
Now Zuber and colleagues say there's compelling evidence that Mercury must have sustained intense geophysical activity for most of its history.
Messenger's also provided scientists with the first measurements of Mercury's gravity field showing the planet's crust is thicker at low latitudes and thinner toward the northern polar region.
They believe Mercury's outer shell is denser than previously thought, indicating a deep layer of iron sulfide below the surface.
The data also suggests Mercury has a huge iron rich liquid outer core and perhaps a solid inner core, together comprising about 85 per cent of the planet's radius.
By comparison Earth's core is about half our planet's radius.
The data means Mercury's mantle and crust occupy only the outer 15 percent or so of the planet's radius giving it a different internal structure compared to other terrestrial planets.
Strange and interesting world
Planetary scientist Dr Craig O'Neill from Sydney's Macquarie University says the new data shows Mercury is a far more interesting planet than previously thought.
"It seems the internal dynamics of Mercury is doing a lot more to the planet's crust than we gave it credit for," says O'Neill.
"The tilted crater floors are interesting and we don't really know what would have caused this."
According to O'Neill the discovery of what may be a liquid core, could also explain the magnetic field detected around Mercury.
"We used to think it was just residual traces of magnetism in the rocks," says O'Neill.
"But thanks to Messenger we now know there's something going on in Mercury's core."