New research on the solar system's smallest and innermost planet could help scientists understand Earth's history. Six percent of Mercury — the equivalent of 60 percent of the continental U.S. — is covered with volcanic plains, providing evidence of how the planet and possibly others were formed.
"That's big news," said Jim Head, professor of geological sciences and lead author of the study, published today in the journal Science.
The team looked at images taken by the MESSENGER spacecraft, which has orbited Mercury since March. Jennifer Whitten GS compiled an image of the planet's north pole piece by piece using the images.
"Nobody has seen Mercury at these resolutions before," said David Hollibaugh Baker GS, who contributed to the research.
Since the first pictures of Mercury were taken in 1974, scientists have known that this area of the planet is covered in smooth plains, but the origins of the plains remained uncertain. Then from 2008 to 2009, the first MESSENGER flybys confirmed that the plains were produced by volcanic activity.
Still, the extent and source of the volcanism was in question. Two ideas were proposed. The first suggested the plains were produced like the islands of Hawaii — by scattered bursts of volcanic activity. The other proposed a massive eruption, producing huge flows of lava like those created by the 1980 eruption of Mount St. Helens in Washington.
The MESSENGER images revealed that the concentration of craters formed by debris impact is much lower on the plains compared to the rest of the planet. This observation pointed to the second hypothesis — great lava flows filled in the craters, leaving just a rocky outline — a "ghost crater," according to the study.
Some of these craters were more than a kilometer deep, meaning there was an enormous volume of lava. Head calculated that the same amount would cover Rhode Island in a layer 11,000 kilometers deep.
The volume of lava was so great that it also covered the fissures from which it poured, which is why there are few volcanic features visible on the plains. It's "just like when a bathtub is full," Head said. "In principle, you can't see the drain."
"This tells us that there was some very dramatic interior forces that were melting huge parts of the subsurface of Mercury and were possibly even expanding the planet slightly," Head said. This tells scientists about the formative years of the planets, he said. Scientists need to know the Earth's history to understand its current state.
But because of erosion and plate tectonics, the first half of the Earth's geological history has melted away. "It's like having an encyclopedia or a set of history books, and on the Earth the first 10 out of 20 are missing," Head said. But on Mercury, this history is still present. "That's why Mercury is so important," he said.
But there are still gaps in Mercury's history that must be filled before Earth's history can be understood. The researchers are now investigating the composition and mineralogy of the plains and trying to determine what could have caused such intense volcanic activity.
