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Strange features on Mercury upend thinking about 'first rock from sun'

New results from NASA's MESSENGER spacecraft show Mercury to have features unlike anything scientists have seen elsewhere in the solar system. Here's one: a huge core for a planet this size.

By Staff writer / March 21, 2012

Mercury forms a crescent shape in this image, acquired by NASA in May 2011 as the MESSENGER spacecraft was high above Mercury's southern hemisphere.

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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From its bizarre multilayer core to its thin crinkly crust, Mercury is revealing itself as a fun house among planets.

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New results from NASA's MESSENGER spacecraft reveal craters with tilted floors. In one large basin, half of the basin floor sits higher than the basin's rim. A vast volcanic plain sports a tall, broad lump, with no obvious source for the rise. The planet's interior is dominated by a core structured vaguely like a peanut M&M – a solid-iron center, a molten iron layer, covered with a thin, dense shell. And Mercury's equator traces a slight ellipse, rather than a circle.

All in all, the first spacecraft to orbit Mercury is revealing the planet with many features unlike anything researchers have seen anywhere else in the solar system.

After looking at MESSENGER's data so far, “I understand far less than I used to” about Mercury, says Maria Zuber, a planetary scientist at the Massachusetts Institute of Technology in Cambridge, who leads a team analyzing the planet's surface features. “But I know more.”

One key message: Mercury has been geologically active through far more of its 4.6-billion-year evolution than previously thought.

Mercury is the solar system's munchkin planet – slightly larger than the moon. Given its size, “there was good reason to believe that Mercury cooled off early, and there hasn't been much of anything going on for most of its history,” Dr. Zuber says. But the evidence the team has gathered suggests that the broad, oddly deformed features they see in the planet's northern hemisphere were formed when the planet was well into middle age, she says.

The MESSENGER team unveiled its latest results Wednesday during a four-day planetary-science meeting in Woodland, Texas. The formal research papers are being published this week on Science Express, the online outlet for the journal Science.

Until MESSENGER arrived, only 40 percent of the planet had been mapped, focused on its eastern and western hemispheres. The maps were built from data gathered by NASA's Mariner 10 mission, which conducted flybys of Venus and Mercury. Messenger orbits Mercury's poles, giving full coverage of the planet.

An inside-outside tour the team provides begins with a core once thought to take up some 75 percent of the planet's radius – far larger than the core of any other planet compared with the planet's size. Speculation on the reason for such a large core centers on a collision early in Mercury's history that would have knocked much of its initial crust and mantle off the planet – with enough power to blast the material well out of Mercury's orbit.

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