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 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

September 2000

News Notes
 Planetary Geology

Mission to Mercury
Twenty-five years ago, Mariner 10 flew past Mercury for the second and final time. Since then the planet closest to the sun has orbited center stage with no visits from its adoring fans on Earth. “I think NASA had sort of forgotten about Mercury, although the scientific community hadn’t,” says Mark Robinson of Northwestern University.

After Mariner 10, which saw only half of the planet, scientists were left with knowledge about Mercury “similar to what we knew about the Moon at the dawn of the space age: no compositional data and only 50 percent of its view,” Robinson says. Mercury researchers had to interpret data using “Earth-based, geologic common sense to read between the lines.”

The planet has since been rediscovered. Radar imaging techniques have improved to provide scientists with intriguing information about the unseen side of the Moon-sized planet. At the American Geophysical Union meeting in June, American and European scientists presented their plans for the next mission to Mercury: the Messenger spacecraft, scheduled to launch in 2004. Messenger will use two flybys past Venus and Mercury to slow down before entering an elliptical orbit in 2009.

   Mariner 10 image mosaic 
   of Mercury.
“If we’re going to go into orbit around Mercury then we’re going to have to worry about the thermal environment of the planet,” says Ralph McNutt Jr. of The Johns Hopkins University Applied Physics Laboratory. Design plans aim to keep the body of the spacecraft cool under a thermal shield as Messenger spends two solar days or four Mercury years (about one Earth year) in orbit around the planet.

The spacecraft’s vantage point will allow it to map the entire planet and also provide close scrutiny of the stream of accelerated particles coming from the sun. Understanding the origin and nature of the planet’s magnetic field and how its thin atmosphere reacts to the sun’s blasting solar wind will be vital for the mission, said Lynn Burlaga of NASA’s Goddard Space Flight Center in Maryland. “Solar wind will interact with Mercury more strongly than Earth, by a factor of two to 10 at least.”

With a suite of spectrometers and a visible infrared spectrograph, Messenger will also investigate the composition of the surface to test hypotheses about Mercury’s high density and the origin of its core — considered rich in iron and extremely large for its volume as a terrestrial planet. It is unknown if the scorching heat of an early sun vaporized the planet’s outer mantle, leaving very few volatile elements like potassium in the crust, or if Mercury formed more like a dirty snowball rolling through the solar nebula, picking up grains rich in heavy metals along the way.

Another idea suggests a giant impact event stripped the planet, soon after it formed, of normal crustal elements such as aluminum. But this third alternative seems less likely considering that Earth’s early Moon-forming impact event kicked out a massive portion of material but did not appreciably affect the size of the mantle in relationship to its core, says William Boynton of the University of Arizona. Also, Mercury has no moon to offer in support of such an event.

Messenger will also investigate the volcanic nature of the planet, indications of tectonic deformation, the source of the planet’s magnetic field, the possible existence of a  liquid core and what the mysterious radar reflecting material is at the poles. Radar images of Mercury’s north pole in 1991 and south pole in 1994 appeared like fireworks in a dark sky, indicating to scientists at NASA’s Jet Propulsion Laboratory and Arecibo Observatory that Mercury sheltered frozen water permanently in shadow along high rims of its polar craters. Messenger will check for enhanced sulfur or hydrogen to resolve the debate between scientists who believe the material is indeed frozen water and others who suggest it is a result of extreme surface temperature differences or perhaps an indication of elemental sulfur.

European scientists plan to complement the Messenger mission with their own possible launch of a probe in 2007. The BepiColombo spacecraft concepts include a lander with a microrover and drilling device, says Rejean Grard of the European Space Agency in the Netherlands. Exploring Mercury, he says, may provide key insights for understanding the formation of terrestrial planets.

Christina Reed

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