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Second look at first rock from the sun

New data from NASA’S MESSENGER spacecraft suggests Mercury was shaped by volcanism.

By Peter N. SpottsStaff Writer for The Christian Science Monitor / October 29, 2008

Close up: A snapshot of Mercury taken Oct. 6 when NASA’s MESSENGER spacecraft flew by the planet for the second time this year. The new images are the most detailed yet.



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.

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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.