Smallest planet yields big surprises

Mercury has a complicated inside and an active geologic past

THE WOODLANDS, Texas — Mercury is even weirder than expected, NASA’s MESSENGER probe is showing.

Purple marks low elevation and white high elevation in this rendering of ancient volcanic plains in Mercury’s northern hemisphere. Images like these suggest the planet had an active geologic past. NASA, JHUAPL, CIW-DTM, GSFC, MIT, Brown University. Rendering by James Dickson and Jim Head.

For starters, the planet’s interior is built differently than anything else scientists have blueprints for. Unlike Earth’s, Mercury’s core — which gobbles up 85 percent of the planet’s radius — consists of three layers instead of two. At the planet’s heart lies a probable solid layer, surrounded by a swirling liquid iron layer, all encapsulated by a third, solid iron-sulfur layer.

The new MESSENGER results were presented on March 21 at the Lunar and Planetary Science Conference, as well as in two papers appearing online in Science. One paper discusses the gravity measurements leading to the new model of the planet’s interior, and the other describes surface features in the northern hemisphere.

Reconciling Mercury’s surface composition with its density has confounded scientists until now. Surface rocks don’t contain enough heavy elements, like iron or titanium, to account for the observed density when a standard, two-layered core is considered.

“So we had to ask ourselves, ‘How could this be formed?’” said Steven Hauck of Case Western Reserve University in Cleveland. The iron-sulfur shell surrounding the core solves that problem by providing the missing bulk. “It sits at the base of the rock layer, and we have high-density metal that sits right beneath it as a part of the total solid,” he says.

Researchers aren’t positive that the innermost solid core exists and how big it might be. “There is room for alternative models,” Hauck says. “But based on the observational data, the more probable models leads to this idea.”

Mercury scientists aren’t just focusing on subterranean mysteries. Researchers have identified wrinkly surface features that belie a shrinking planet.

Called fold-and-thrust belts, these features arise when Mercury’s core cools and contracts. As the core shrinks, so must the planet’s outer crust, which it accomplishes by sliding bits of crust beneath other bits of crust, Paul Byrne of the Carnegie Institution for Science in Washington, D.C., reported during a conference session devoted to MESSENGER results. New topographical data helped Byrne and his colleagues identify and measure the length of the sinuous features, some of which cover more than 1,000 kilometers.

Studying the surface crinkles will help scientists determine how much Mercury has shrunk and when the contraction occurred. “Mercury is exceptional in having such widespread evidence of that contraction,” said Sean Solomon of Carnegie.

In addition, new topographic maps reveal areas in the north that probably come from volcanic activity, and suggest that the planet had a much livelier geologic history than expected. Crater floors are tilted in some areas, meaning that volcanic or tectonic movement disturbed the basins after they were carved into the crust. And the floor of the Caloris impact basin — a massive crater spanning 1,500 kilometers — rises above the rims in some places, a feature not seen anywhere else in the solar system, said Maria Zuber of MIT. Some sort of very active process inside the planet, she said, must have boosted material above the crater’s rim.

Indeed, though scientists used to think Mercury was similar to the moon, they now doubt the planet quickly cooled and became a dead chunk of rock. “Mercury had a very active middle age,” Zuber said.

The mission isn’t over yet. With a one-year extension just approved, expect many more messages from Mercury.

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