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."