Correction: A previous version of this story misspelled the name of Mercury’s largest crater. It is Caloris Basin. This version has been updated.

A depiction of the Messenger mission shows the spacecraft, equipped with a sun shield that protects its scientific instruments, near Mercury. Messenger needed seven years to reach the planet at a speed slow enough to go into orbit around it. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

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.

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.

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. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

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.

“It’s back to the drawing board” on theories of Mercury’s formation, said Solomon.

Also baffling: The planet’s internal structure. Its core is huge, making up almost 80 percent of the planet’s volume. (The Earth’s core makes up less than half of the planet.) And the core is strangely unlike Earth’s, with an extra solid layer floating on a deeper liquid layer. Zuber and colleagues imputed the core’s structure by calculating Mercury’s gravity field; it was so surprising that they did not believe their math, thinking their calculations had been muddied by the minute pressure of sunlight pushing on Messenger.

“It slowed us down, but it’s a fantastic find,” said Zuber.

Before Messenger, some of Mercury’s weirdness was already known: A day creeps by in six months, but a year zooms by in less than three months. A tin can would melt at noon, but deathly cold descends at night, the wispy atmosphere too thin to trap heat.

Asking the right questions

Mercury remained unexplored for so long due to its promixity to the sun, which makes it hard to inspect and harder to reach. Its mere existence baffled the ancient Greeks, who thought it was two planets. Seen just before sunrise, it was Apollo; just after sunset, it was called Hermes (the winged messenger of the gods, whom the Romans called Mercury).

The Hubble Space Telescope has never inspected Mercury, as it might be blinded in the attempt. Sending spacecraft to Mercury also tempts fate, as the sun’s enormous gravity threatens to suck interlopers toward a fiery death.

So when NASA’s Mariner 10 craft flew by in 1974 and 1975, it couldn’t linger.

In the intervening decades, two advances made the Messenger mission possible. First, space scientist Chen-wan Yen figured out that a long, spiraling trip from Earth could slow a probe enough to park it in Mercury’s orbit. That’s why Messenger’s voyage took almost seven years and covered twice the distance to Pluto (formerly the smallest planet but no longer considered a planet). Messenger flew by Earth once, Venus twice and Mercury three times, shedding speed at each encounter.

The second advance was a sun shield made from high-tech ceramic fabric; the six-foot by eight-foot rectangle protects the sensitive scientific instruments.

But to project scientist Ralph McNutt of the Applied Physics Laboratory, the decades of waiting were well worth it. NASA recently extended Messenger’s mission, which was supposed to last just a year, so more data will be pouring in.

“Mercury was seen as sort of a backwater,” McNutt said. “It looked like the moon; it looked like this dead planet. Why would we want to go there? It turns out we didn’t even know the right questions to ask.”