Curious surface features known as hollows line the interior of the Raditladi impact basin on Mercury. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
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.