Strange Hollows Discovered on Mercury
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"These hollows were a major surprise," says David Blewett, science team member from the Johns Hopkins University Applied Physics Laboratory. "We've been thinking of Mercury as a relic – a place that's really not changing much anymore, except by impact cratering. But the hollows appear to be younger than the craters in which they are found, and that means Mercury's surface is still evolving in a surprising way."
Hollows inside the Raditladi impact basin. Credit: NASA/Johns
Hopkins University Applied Physics Laboratory/Carnegie Institution of
Washington [larger image]
"We've never seen anything quite like this on a rocky surface."
If you could stand in one of these "sleepy" hollows on Mercury's surface, you'd find yourself, like Ichabod Crane, in a quiet, still, haunting place, with a black sky above your head.
Another example of hollows in crater Tyagaraja. Courtesy Science/AAAS [larger image]
As the planet closest to the Sun, Mercury is exposed to fierce heat and extreme space weather. Blewett believes these factors play a role.
A key clue, he says, is that many of the hollows are associated with central mounds or mountains inside Mercury's impact craters. These so-called “peak rings” are thought to be made of material forced up from the depths by the impact that formed the crater. Excavated material could be unstable when it finds itself suddenly exposed at Mercury's surface.
"Certain minerals, for example those that contain sulfur and other volatiles, would be easily vaporized by the onslaught of heat, solar wind, and micrometeoroids that Mercury experiences on a daily basis," he says. "Perhaps sulfur is vaporizing, leaving just the other minerals, and therefore weakening the rock and making it spongier. Then the rock would crumble and erode more readily, forming these depressions."
A fresh impact crater. Hollows are present on a section of the
crater wall that has slid partway down toward the floor. Courtesy
Science/AAAS [larger image]
But they're still there.
"The old models just don't fit with the new data, so we'll have to look at other hypotheses."
To figure out how the planets and Solar System came to be, scientists must understand Mercury.
"It's the anchor at one end of the Solar System. Learning how Mercury formed will have major implications for the rest of the planets. And MESSENGER is showing that, up to now, we've been completely wrong about this little world in so many ways!"
What other surprises does Mercury hold? The sleepy hollows of the innermost planet may be just the beginning.
Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA
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