Spacecraft uncovers surprising data on Mercury's characteristics

September 29, 2011
Spacecraft uncovers surprising data on Mercury's characteristics
By Amina Khan | McClatchy-Tribune News Service

LOS ANGELES — A spacecraft sent to the least explored rocky planet in our Solar System is providing new and surprising information that may rewrite what scientists believe about the growth of planets.

Mercury, the tiny planet closest to the sun, has a lopsided magnetic field, far too much sulfur and sports strange hollows across its surface that may even hint at present-day geologic activity, according to data gleaned by the Messenger mission spacecraft.

The results, published in a package of seven papers in Friday's edition of the journal Science, may force scientists to throw out many ideas about how Mercury first formed.

Planetary scientists got a tantalizing glimpse at Mercury's moonlike features - craters, flat plains of ancient lava - and discovered it had a magnetic field when the Mariner 10 mission first flew by in 1974 and 1975. The Messenger mission, armed with a suite of instruments including cameras, element-sensing spectrometers and a magnetic field detector, was designed to answer questions dangled by that decades-old snapshot.

Launched in 2004, the spacecraft flew by the planet three times before entering orbit in March, when it sent the first close-up images back to Earth. It will continue to send data as it circles the planet for about a year.

Mercury, planetary scientists knew, is uncommonly dense - mostly likely because its inner core of iron is very large for its size. This led some scientists to theorize that Mercury had once been perhaps two to three times larger and its outer layers had been stripped away, either from the Sun's fierce glare or major impacts from asteroids.

As molten balls of rock coalesce to form a planet, the heavier elements, like iron, tend to sink to the bottom while lighter elements like sulfur or phosphorus, which are more likely to evaporate, float to the top. This, scientists had reasoned, would make those volatile elements the first to get stripped away, leaving the planet comparatively dense.

But when scientists used Messenger's gamma-ray and X-ray spectrometers to analyze elements on Mercury's surface, they found that the planet was rich in phosphorus and that sulfur was 10 times more abundant on the surface there than on the Earth or Moon.

"At this point, the origin of Mercury's large core is still a mystery," said Larry Nittler, a cosmochemist at Carnegie Institution of Washington who led the X-ray spectrometer study.

Another paper found that the vast flat plains on Mercury were not caused by eruptions from volcanoes but were fashioned from large amounts of lava that seeped up from cracks in the ground and flooded the surface. The scorching-hot lava also carved teardrop-shaped islands into the surface, which are visible near the edges of these plains.

Mercury is "essentially wallpapered by huge volumes of lava," said James Head, a planetary geoscientist at Brown University and lead author of that study.

"Volcanism is important because it represents the pulse of the planets," he added. "It's like the blood of the interior: Is it not doing much inside, or is it really active?"

Researchers also discovered that the planet's magnetic field is shifted 300 miles north from the equator. This could mean there are strange, lopsided internal dynamics occurring in the churning liquid metal in the core.

"There's something very intriguing going on with that that we don't understand yet," said study lead author Brian Anderson, a space physicist at Johns Hopkins University Applied Physics Laboratory.

Messenger's imaging instruments revealed another surprise: strange, bright, bluish depressions inside of the planet's craters that the researchers have named "hollows." Although they're not sure what's causing the hollows, they think the features may be created after an impact by pieces of space debris carves out a crater and exposes volatile elements such as sulfur.

Bombarded by heat and particles from the Sun, the sulfur would have evaporated, leaving the rock it was embedded in to crumble, creating the hollows.

It's a process that may still be going on today, said study lead author David Blewett, a planetary scientist at Johns Hopkins University Applied Physics Laboratory.

"Mercury's a small planet," Blewett said. "It was sort of considered to be like the moon - an old, burned out cinder ... but to find some landscape modification apparently taking place today, and producing these spectacular and unique features, is a first."

Altogether, the suite of new studies "is really going to revolutionize our understanding of this little planet," said Jeffrey Taylor, a planetary scientist at the Hawaii Institute of Geophysics & Planetology who was not involved in the research.

Further, he said, it suggests that the early solar system, full of the debris that coalesced to form the planets, may have looked very different than how we currently envision it.

"It may require a whole new look at the inner solar system," he said.

Amina Khan writes for the Los Angeles Times.

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