Tiny planet offers up big surprises

A spacecraft sent to the least explored rocky planet in the solar system is providing surprising new 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, much more sulfur than expected and strange “hollows” across its surface that may hint at present-day geologic activity, according to data gleaned by the Messenger 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 formed.

When Mariner 10 flew by in 1974 and 1975, scientists got a tantalizing glimpse at Mercury’s moon-like features -- craters, flat plains of ancient lava -- and found it had a magnetic field. 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 of this year, when it sent back the first close-up images. It will continue to send data as it circles the planet for about a year.


Mercury, planetary scientists knew, is uncommonly dense -- most likely because its inner core of iron is very large relative to the rest of the planet. 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 such as iron tend to sink toward the center while lighter elements such as sulfur or phosphorus, which are more likely to evaporate, drift in the opposite direction. This, scientists had reasoned, would make those volatile lighter 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 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 the 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 and flooded the surface.

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 about 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 the planet’s craters that the researchers have named “hollows.” Though they’re not sure what’s causing the hollows, they think the features may have been created after an impact by space debris carved out a crater, exposing volatile elements such as sulfur.

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

It’s a process that may still be going on today, said study lead author David Blewett, a planetary scientist also at the Johns Hopkins laboratory.

The collection 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 and Planetology who was not involved in the research. Further, he said, it suggests that the early solar system 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.