Mercury, a hard-baked pebble of a planet patrolling the inner solar system, has long been a bit of an inscrutable runt. But now that scientists are finally getting a close look, Mercury is proving to have just as much personality as its bigger siblings. 

The smallest planet in the solar system had not received much research attention until 2008, when NASA's Messenger spacecraft made its first flyby en route to entering orbit around Mercury earlier this year. [Read more about Messenger's journey here.]

Once dismissed as a "burned-out cinder," as Carnegie Institution of Washington planetary scientist and Messenger principal investigator Sean Solomon put it, Mercury is turning out to be anything but. The first large volley of findings since Messenger entered Mercury orbit in March paints a picture of a planet that does not fit expectations—nor does it fit in neatly with the rest of the objects in the solar system. Seven studies from early Messenger data, published in the September 30 issue of Science, reveal a world with an extensive volcanic footprint, an exotic surface composition and a puzzling geologic feature dotting its surface.

"I think it's a much more interesting place than people expected," says cosmochemist Larry Nittler, also of the Carnegie Institution and lead author of one of the new studies. Mercury had been neglected by robotic explorers partly because it is difficult to reach. The world had seen only fleeting visits from NASA's Mariner 10 spacecraft in the 1970s, and half the planet had never even been photographed before Messenger came along. It was not until 1985 that a researcher named Chen-wan Yen devised a practical way to send an orbiter there. Motivation for further missions was not aided by the fact that Mercury appeared a dead, desiccated world, a sort of super-size version of Earth's moon. "When Mariner took pictures of this cratered terrain and it looked like the moon, it was sort of disappointing," says James Head, a planetary geoscientist at Brown University.

Even though Mariner 10 saw only a portion of Mercury on its flybys, it did show that the planet had smooth plains where impact craters had been erased or filled in. But whether Mercury's plains had been plastered over by ancient lava or by material ejected from meteor impacts was uncertain. "For 35 years we didn't really know whether the plains on Mercury were volcanic in origin or not," Head says. "Now, I'm a volcanologist, and that's pretty darn frustrating."

Messenger's three flybys of Mercury settled the question—the plains were filled in by an episode of flood volcanism billions of years ago. Now that Messenger has more fully explored the planet, the scale of that volcanism is becoming clear. Head and his colleagues report in one of the new studies that volcanic flood plains in the northern hemisphere cover nearly five million square kilometers, more than 6 percent of Mercury's surface. (For comparison, the northern lava plains fill an area seven times the size of Texas.) Based on the width of the craters that the lava fills, the researchers estimate that the volcanic plains are one to two kilometers thick in some areas.

Some 3.8 billion years ago, Head and his colleagues conclude, volcanic activity played a large role in shaping Mercury's surface. "What we've discovered, we think here, is evidence that Mercury was very hot, and large amounts of lava came out over very short periods of time, maybe like we saw on the early Earth," Head says.

Mercury's surface today is quite different than Earth's—and just about any celestial object with which one might compare it. An x-ray spectrometer on Messenger shows that the innermost planet's surface is highly enriched in the element sulfur, with more than 10 times the abundance on Earth. Sulfur is a volatile element, meaning it could easily vaporize and escape from the surface. And the fact that it stuck around says something about the conditions under which Mercury formed. "It turns out that how sulfur behaves when rocks melt depends critically on the oxidation state," says Nittler, the lead author of the surface-chemistry study. The sulfur abundance suggests that Mercury formed from materials that were less oxidized than the materials that formed the rest of the terrestrial planets, Nittler says.

That may have something to do with Mercury's position in the solar system; the nearby sun could have separated the protoplanetary grains that helped form the planet from the oxygen-rich ice that accompanied them. Some meteorites known as enstatite chondrites could also have contributed to the formation of the planet as low-oxidation building blocks.

Another study using a different spectrometer on Messenger also indicated chondritic meteorites as a likely building material for Mercury. That study found that Mercury has too much potassium, another volatile element, to jibe with formation theories involving extreme heating early on.

"Mercury is now close to the sun, it probably formed close to the sun, and it had processes that gave it a very high fraction of metal" in its interior, Solomon says. Some had floated the idea that Mercury had once had a rocky outer shell, only to see it blasted away by a giant impact. The expectation, Solomon says, was that Mercury would be enriched in refractory elements and compounds that could withstand such extreme conditions. "One of the most surprising things is clearly that Mercury shows abundant evidence for having retained volatile components," he says.

Nittler and his colleagues also found that Mercury is depleted in aluminum and calcium relative to lunar rocks, indicating that the planet's superficial resemblance to the moon conceals a very different crust.

Messenger has only begun to reveal the surface composition of Mercury—it cannot yet resolve fine-grained regional differences—but even its preliminary data are unprecedented. "It's telling us what the surface of Mercury is like chemically, which we just didn't know before," Nittler says. "We had a lot of ideas and very, very little data."

The same goes for how Mercury's surface looks up close. High-resolution images from Messenger revealed a peculiar kind of landform, which its discoverers call a "hollow," on the floor of a number of impact craters. Hollows are shallow depressions, some of them a few kilometers long, many of which have bright interiors. They resemble some kind of extraterrestrial sinkhole. The researchers who identified the hollows from Messenger imagery say they do not yet know what produced the strange depressions. "It's a real surprise," says David Blewett, a planetary scientist at Johns Hopkins University Applied Physics Laboratory, who led the study. "There's nothing like this in any lunar craters."

The hollows could be collapse pits related to Mercury's large-scale volcanic activity, or it could be another example of volatiles in the crust. Condensed volcanic gases, for instance, could be buried by lava flows and hence insulated from escaping into Mercury's tenuous atmosphere. "Later on, when it's exposed by impacts or something, it sublimes away," Blewett explains. "Most of the hollows that we found are associated with craters, so you get the idea that the material that's susceptible to hollow formation is material that's been exposed from the subsurface."

Just six months after humankind got its first close-up look at the innermost world, one thing is already clear: Mercury, despite its small stature, stands out in the planetary crowd. "Messenger is really revealing a planet that is completely an oddball in every characteristic we can measure," Blewett says. "We don't know what kinds of rocks are on the surface, we don't know what is responsible for its color, its gravity is funny, its magnetic field is funny. It's just an odd planet."