Mercury may have a lot in common with Earth, but close-up images and data captured by NASA'S MESSENGER probe this year show it's still a bit of a planetary weirdo.
"Mercury is not the planet described in the textbooks," James Head III of Brown University said in a telephone briefing on Thursday. "The innermost planet has had a long and much more exciting life than anyone expected or predicted."
There are dips in Mercury's surface, just as there are hills and valleys on Earth, and both are rocky planets. But those on Mercury have been dubbed "hollows" to differentiate them from impact craters and other depressions on the small, hot orb closest to the Sun.
New MESSENGER images those small, shallow, irregularly shaped hollows may still be forming because they show little evidence of scarring from incoming space rocks, said David Blewett of the Johns Hopkins University Applied Physics Laboratory, lead author of one of the reports.
Mercury has a magnetic field, just as Earth, Jupiter, Saturn, Uranus and Neptune do, but the planet's magnetosphere is so small – about 1 per cent the size of Earth's – that it offers little protection from the charged particles that make up the solar wind blasting off the Sun.
MESSENGER – which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft – looped around the inner solar system 15 times over six years before beginning its orbits around the planet on March 18.
The spacecraft's findings were presented in a special package of articles in the journal Science.
Astronomers have long wondered whether Mercury had volcanoes on its surface. MESSENGER's fly-bys confirmed that it does, but the six months of orbital observation show that the lava flows are different from those on Earth.
Just like Earth, Mercury has lava flows. But these are deep flows that smoothly cover the small planet's northern polar region, with no Earth-type volcanoes in sight.
Scientists believe that the source of the lava has been buried under the flow. The polar lava flow covers more than 6 per cent of Mercury's surface, an area equivalent to 60 per cent of the continental United States.
When the lava flowed out of cracks in the planet's surface some 3.5 billion to 4 billion years ago, it filled craters more than a 1.6 km deep, Mr. Head said. The volume would have been enough to bury the state of Texas under more than a 6.4 km deep blanket of lava.