Second look at first rock from the sun

Using the altimeter, she and her team determined that the fresher-looking scar – some 62 miles across – was four times deeper than its neighbor. That neighbor, it turns out, “was filled in by solidified lavas,” filling a volume of some 15,000 cubic kilometers. Picture the combined Washington-Baltimore metropolitan areas covered with a blanket of volcanic rock some 12 times thicker than the height of the Washington Monument.

“That’s an awful lot of volcanic material in one place for such a little planet,” Dr. Zuber says.
“When we had information on Mercury only from Mariner 10, there was a fair amount of ambiguity about whether or not volcanism was even an important process on the planet,” she says. After MESSENGER’s first two flybys, “we now have a better understanding that volcanism is quite an important process, and we’ve even beginning to quantify that in an important way.”

Launched in August 2004, the $446 million effort is designed to round out the history of planet formation in the solar system, particularly among the inner planets. Mercury is the smallest of the lot, sitting at one end of the size and composition spectrum. Giant gas balls like Jupiter anchor the other end.

A history waiting to be told
Mercury sports the oldest surface of any inner planet, based on the number of craters dotting its surface.

It is also denser than the other inner planets, after adjusting for size and gravitational pull. The tiny planet’s iron core accounts for some 60 percent of the planet’s mass – twice the value for Earth’s core – taking up most of its interior.

And the planet is the only inner planet other than Earth to be cloaked in a magnetic field – though one much weaker than Earth’s. A molten outer core appears to be the dynamo generating the magnetism.

Mercury’s position in the planetary lineup and its intriguing traits suggest it has an important story to tell about the evolution of the solar system, researchers say.

During MESSENGER’s year-long science mission at Mercury, scientists say they hope to sort out which of three notions about Mercury’s formation is correct. One posits that the planet formed from heavier, iron-rich dust that would have dominated the mix of material at that distance from the sun when the solar system formed some 4.5 billion years ago.

Another holds that Mercury may have initially formed with a composition more like other rocky planets, but with the sun so close, heat removed much of the easily vaporized material making up the surface. A third notion suggests that the planet lost most of its outer layers after a large object smacked into it, relieving it of its original crust and much of its mantle.

Each pathway yields a different prediction for the mixture of chemical elements MESSENGER’s instruments should detect on the planet’s surface.

* [Editor's note: The original version of this story incorrectly attributed the push the spacecraft gets from the sun to solar wind rather than to radiation pressure from sunlight.]

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