NASA probe offers new view of Mercury: an alien world right in our back yard

The overheated, underappreciated runt of the solar system is finally getting some attention.

Mostly ignored since a brief fly-by in the 1970s, Mercury, our solar system’s smallest, swiftest planet, received a longer house call last March: NASA’s $450 million Messenger probe, which achieved orbit, a tricky feat never before attempted.

(NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington) - A depiction of the MESSENGER spacecraft is shown flying over Mercury's surface displayed in enhanced color during the mission's second Mercury flyby in 2008.

Now, after poring over 100,000 images and reams of other Messenger data, space scientists have achieved consensus: Mercury is one weird world.

It is radically unlike the other rocky bodies of our solar system — Venus, Mars, Earth, the moon, and the moons of other planets. Its core is too big; its surface too scrunched. It looks shriveled, like a liposuction patient left with too much skin. It contains too much iron. Its internal structure — how the planet is built — is confounding. Its magnetic field is out of whack, asymmetrical. And its surface is strange, a jagged, ragged landscape of soaring escarpments, snaking faults, half-buried “ghost craters,” dead volcanos and mysterious pit-marked “hollows.”

“It’s been really spectacularly baffling,” said MIT’s Maria Zuber, of the Messenger data, which scientists reported on in two scientific articles and 57 presentations at the annual Lunar and Planetary Science Conference last week.

Mercury was long viewed as an inert lump, but Zuber and her colleagues now say it is still cooling and still shrinking, pushing up scarps — steep cliffs — that run for hundreds of miles. Not long ago (geologically speaking), volcanoes threw up showers of magma, which hardened into huge plains. There’s also evidence of mysterious explosions of interior gases that rocked the surface and left strange, pitted scars.

Massive interior forces have pushed and tilted huge stretches of the surface. Mercury’s biggest crater — the Caloris Basin, some 900 miles wide — has been so uplifted that much of its floor is taller than its rim. No other crater in the solar system looks like it.

“Everything is intriguing on the surface of Mercury,” said Nancy Chabot of the Applied Physics Laboratory at Johns Hopkins University, which built Messenger. “It has landforms that we’ve never seen on the rest of the terrestrial planets.”

Mercury might even experience Mercury-quakes. “I would bet some of those faults are still active,” said Messenger’s lead scientist, Sean Solomon of the Carnegie Institution for Science.

And despite being the closest planet to the sun, it apparently has buried water ice surviving beneath permanent shadows thrown by craters. “We’re almost certain of it,” Solomon said.

‘Back to the drawing board’

Just how Mercury was formed is another baffler. It is heavy with iron and sulfur — much more than Earth contains. But all of the rocky inner planets coalesced from the same disk of material. So why is Mercury so strange? “It’s like children in the same family,” Zuber said. “Same genes, same environment, yet they turn out so different.”

Some theorists say a giant space rock smashed into Mercury early on, ripping off a thick outer layer. But Messenger data throw that theory into chaos. Sulfur and other “volatiles” survive on the surface; a huge collision should have wiped them clear.