Second look at first rock from the sun

Stunning craters, vast volcanic plains, and a narrow stretch of landscape that the Mariner 10 flybys missed in the mid 1970s are among long-hidden facets of Mercury that are finally coming into view.

Yesterday, scientists with the MESSENGER mission to Mercury unveiled new results from the second of three flybys the spacecraft must make before it takes up its orbit around Mercury in 2011.

The Oct. 6 flyby nearly ended one basic task in exploring Mercury – getting a full planet’s worth of close-up pictures of the surface. Researchers have waited more than 30 years to fill in the missing 55 percent of the planet that Mariner 10 couldn’t capture. With this year’s January and October flybys, scientists have enough images to cover 90 to 95 percent of the planet’s surface – although the MESSENGER images do so in far greater detail than did Mariner’s.

Sailing with the solar wind
Measurements from the craft’s laser altimeter, combined with the detailed images, will allow researchers to begin piecing together a detailed geological history of the surface. The craft has also uncovered additional chemical elements in the planet’s magnetic “tail.” This provides insights into the composition of the planet’s surface, where the atoms originate.

New measurements of the planet’s magnetic field suggest that its orientation is more stable than Earth’s magnetic field. This opens a window on processes driving the internal dynamo that generates the field.

And the craft is making a bit of serendipitous aerospace history by using the pressure of the sun's light as a kind of "wind" to help make small course changes as it closes in out its quarry. Although the craft carries no solar sail, this pressure is significant enough in Mercury’s vicinity to allow the craft itself to serve as a substitute sail. *

For a mission whose motto for now could be, “I have not yet begun to orbit,” the results are exciting, notes Sean Solomon, the mission’s lead scientist and director of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington.

MESSENGER “is showing us new aspects of the planet well before the orbit phase,” says Mr. Solomon. The new information, he adds, will sharpen the questions scientists ask of the planet once the orbiter arrives for its one-Earth-year science mission.

For instance, since the days of Mariner 10, scientists have been puzzling over Mariner images that show craters with smooth internal plains or large swaths of smooth surface outside large craters.
Some have argued that the material is accumulated ejecta cast near and far as objects slammed into Mercury’s surface nearly 4 billion years ago. Others have suggested that the plains are caused by lava welling up beneath the crust and overspreading the surface.

During a press briefing in Washington yesterday, planetary scientist Maria Zuber showed how laser-altimeter data, combined with photos, has tipped the scale toward the idea that volcanism played a key role in the planet’s formation. She highlighted two enormous, side by side craters – one looked relatively fresh, the other was largely buried beneath a plain, with the ancient crater rim barely visible.

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.]