BROWN (US) — Vast plains around Mercury’s north pole were created more than 3.5 billion years ago by lava flows that covered 6 percent of the planet’s surface—an area that would cover nearly 60 percent of the continental U.S.
“For more than 35 years we have been uncertain about the role of volcanic activity on Mercury,” says James W, Head III, professor of geological sciences at Brown University.
In a paper published in Science, Head and more than two dozen colleagues from Brown and other institutions used data from the orbiting Messenger spacecraft to examine Mercury’s largely unseen northern high latitudes.
Lava flows on Mercury more than 3.5 billion years ago—a mile deep in places— would have covered nearly 60 percent of the continental United States. (Credit: Brown U.)
Early in the planet’s history—some 3.5 to 4 billion years ago—huge volumes of lava poured out from cracks in Mercury’s surface, flooding the surrounding, low-lying plains “like a bathtub,” Head says, with volume that would have been enough to bury the state of Texas more than four miles deep.
The lava flows “don’t build a volcano like we see on Hawaii; rather, they cover up the place where the lava is coming out, and they’re very difficult to understand in the context of the current Earth eruption conditions,” Head says.
“But one thing is clear,” adds Jennifer Whitten, a graduate student in Head’s research group and a contributing author on the paper. “So much lava comes out so rapidly, that it’s an entirely different type of volcanism than we see at Hawaii. This gives us big clues about what’s going on in the interior of Mercury.”
The lava flows give scientists insight into how planets are born, how they evolved, and whether they’re still active, Head says. “For example, on the Moon, we see very little evidence of activity of volcanic processes over the last 2 to 3 billion years.”
To confirm the plains were caused by very rapid volcanism, the researchers looked at a feature about 125 miles outside the volcanic zone. There, they saw evidence of a fissure vent, with wide river-like channels emanating from the crack.
“These features are similar to lava flows and very fluid flows that erode much of the surrounding terrain,” says Debra Hurwitz, also a graduate student with Head, and a contributing author on the paper.
Researchers believe this is similar to what happened in the smooth plains, but the clues have been largely erased by the sheer volume of lava that poured out.
“When trying to understand the origin of the smooth plains, it is helpful to look at the margin of the plains and its surroundings,” says Caleb Fassett, a postdoctoral researcher at Brown now teaching at Mount Holyoke College, and a contributing author on the paper. “Where the plains thin and meet surrounding terrain, the differences between the plains and pre-existing terrain can help give us insight into how the plains were formed.”
Messenger has been orbiting Mercury since March. The same spacecraft executed three flybys of Mercury in previous years, which gave the scientists their first look at the smooth plains. The team hopes that other instruments will allow them to better understand the minerals and chemical composition in the northern high latitudes, as well as to compare the volcanic activity there with other regions that experienced volcanism.
“This one deposit is so huge,” Head says, “volcanism has got to be important elsewhere.”
Researchers from the University of Arizona, Johns Hopkins University, Southwest Research Institute, the Smithsonian Institution, Carnegie Institution of Washington, University of Hawaii, Arizona State University, and the German Aerospace Center contributed to the research that was funded by the NASA Discovery Program.
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