The quicksilver planet piques new curiosity among astronomers.

Mercury, the Rodney Dangerfield of planets, is finally getting respect.

For some 25 years, the tiny planet could light a fire under only a handful of solar-system scientists. After Mariner 10's three fly-bys in the mid-1970s, many astronomers dismissed the planet as too boring - too much like Earth's moon to be worth the price of an orbiter.

Now, however, Mercury's stock is rising. Last fall, the European Space Agency approved an ambitious 2009 mission - a pair of orbiters and a surface probe - to study the first rock from the sun. The United States is building a Mercury orbiter for launch in 2004. And Japan hopes to launch its own Mercury mission in 2005.

This mini-armada is being aimed at Mercury for several reasons, the planet's faithful say. Dramatic discoveries during the past decade, such as radar evidence for water ice in polar craters and features that return radar echoes unlike any others in the solar system, have piqued scientists' cur-iosity. Technological advances are putting more-capable sensors into smaller, lighter packages. And, perhaps most important, scientists discovered an approach to getting there that cuts mission costs.

Whatever the reasons, Mercury's die-hards say it's about time their planet commanded a place on FIRST ROCK FROM THE SUN:

the exploration itinerary.

"Mercury has been horribly neglected," complains Robert Strom, a planetary geologist with the University of Arizona's Lunar and Planetary Laboratory who played a key role on the imaging team for the Mariner 10 mission.

Indeed, researchers say that they know less about Mercury than about any other planet but Pluto. Yet, they add, unraveling Mercury's secrets is vital to understanding the formation of the rest of the inner planets, which also include Venus, Earth, and Mars.

Mercury's mysteries arise from its extremes. With a diameter that would allow it to fit comfortably between Boston and Anchorage, Alaska, it's a planetary munchkin. Its orbit traces an ellipse around the sun, while those of the other inner planets are more nearly circular. Sunrise to sunrise, its "day" is twice as long as its "year," which is 88 Earth days long. Temperatures on its sunny side reach a withering 800 degrees F. at the equator. In the deepest recesses of polar craters, temperatures never top minus 274 degrees F.

Dense, but not clueless

No planet is more dense than Mercury. And it is the only inner planet besides Earth that displays a global magnetic field. Yet unlike Earth, Mercury's enormous iron core is thought to be frozen, lacking the molten, flowing layers that set up Earth's planetary magnet.

Quite apart from its physical attributes, Mercury holds a soft spot in many a physicist's heart. The planet provided the first full test of Albert Einstein's theory of general relativity, which he successfully invoked to explain why Mercury's closest approach to the sun shifts from orbit to orbit.

For astronomers, however, Mercury tends to reveal its secrets only grudgingly.

Mountaintop telescopes have had a tough time viewing the planet because it orbits close to the sun and can be viewed only around sunset and sunrise. Since Mercury never rises far above the horizon, astronomers have to look at it through more of Earth's atmosphere, which blurs Mercury's features.

Last year, scientists from Boston University announced that they had successfully tested a prototype telescope camera to study Mercury's tenuous atmosphere. In the process, they supplied the first surface images from a region of Mercury that Mariner 10 missed. But the ability to study surface features in any detail from the ground remains elusive. The Hubble Space Telescope, which might be able to return spectacular images, is forbidden to view Mercury; the sun would fry the orbiting observatory's instruments.

This has left the field to two contestants - spacecraft and ground-based radar.

In November 1973, the National Aeronautics and Space Administration (NASA) launched Mariner 10, which was designed as a reconnaissance mission in preparation to send an orbiter, Dr. Strom says. The mission made the first accurate mass measurement of the planet, discovered its exceedingly thin atmosphere and its magnetic field, and produced photo mosaics of 45 percent of its surface.

Those photos have sustained Mercury's hard-core students ever since. The images were given a fresh look in 1997, when Mark Robinson, now a Northwestern University planetary geologist, and Paul Luci from the University of Hawaii subjected the images to new processing techniques. They built up a color photo that gave researchers fresh insights into the volcanic processes that helped shape Mercury's surface. Their images also yielded evidence that Mercury's chemical composition was different from that of the other inner planets.

"Mariner 10 gave us some very fruitful data," says Dr. Robinson, who is on the science team for the upcoming US mission to Mercury. If astronomers have been pleased with Mariner's data, they were flabbergasted with results from radar. In 1991, researchers "pinging" Mercury's surface with radar signals discovered what they interpreted to be evidence of water ice deep in north polar craters. The craters were visible to radar because the orbits of Mercury and Earth are slightly offset.

"Finding ice was a huge shock," recalls Paul Spudis, deputy director of the Lunar and Planetary Institute in Houston.

Since then, researchers have found evidence of ice at both poles, with further radar studies planned for later this year. While some planetary scientists caution that the signals could be showing sulfur deposits, rather than ice, others who lean toward the ice explanation hold that the evidence is more compelling than the evidence the recent Lunar Prospector mission mustered for water ice in lunar craters.

Strom points out that radar is finding other features that demand a closer look. In 1996, radar data showed what looked like an enormous shield volcano nearly 400 miles across. The feature never showed up in Mariner images because of Mariner's limited coverage. The radar signatures are strong enough to suggest that if the formation is a shield volcano, it may have formed fairly recently.

"It could be as big as Mons Olympus on Mars," Strom adds.

What he finds truly intriguing, however, are mysterious radar signatures that defy description - a collection of bright dots some 1,200 miles in diameter. "There is no other radar signature like that in the solar system," he says.

With such morsels on their plates, it's little wonder that planetary scientists are eager to return for seconds.

Messenger's 5-year journey

One opportunity comes in 2004, when NASA is scheduled to launch the Messenger mission, which would arrive at Mercury in 2009 and orbit the planet for 12 Earth months. The five-year journey includes two Mercury flybys, which will supply close-up images of parts of the planet no human has seen before. In addition, flybys of Earth and Venus will help Messenger catch up with its fleet quarry.

"If you don't do that, Mercury would zip by at 10 kilometers a second," or about 22,000 miles an hour, making for a short visit, says Sean Solomon, a Carnegie Institution scientist and Messenger's lead researcher.

The instrument package for the $286 million project is designed to help answer a range of questions, Dr. Solomon explains.

"Mercury is the densest planet. How did it wind up mostly metal?" he asks, explaining that its iron core is about the size of Earth's moon. It is covered by a relatively thin layer of rocky material about 370 miles thick.

One explanation holds that Mercury formed as a metal-rich object in roughly its current location. Another suggests that Mercury wandered among the other budding planets, taking hits from passing planetesimals (small rocks that formed planets) that stripped it of much of its outer cover of rock. Clues to which explanation is right will come from analyzing the chemical composition of the surface minerals from orbit.

Another puzzler is Mercury's magnetic field and the processes that generate it. To unravel that knot, researchers must figure out if Mercury, like Earth, has a liquid core.

"If Mercury has a liquid core, the planet's equator should do a slight twist in orbit" as the sun's gravity tugs on it, says Maria Zuber, a planetary scientist at the Massachusetts Institute of Technology, in Cambridge. Dr. Zuber notes that the twist is small - on the order of about 1,000 feet per Mercurian year. Using devices that can measure variations in topography to within about 20 inches, as well as making gravity measurements, she will look for the telltale twist.

Evidence of water on the moon and Mercury could provide clues about Mercury's origins, adds Strom.

If the evidence for water on the moon is truly from water, the source for both objects is likely to be passing comets, he says. But if the lunar readings aren't water at all, but an artifact of the sun's interaction with the moon's surface, "then what is that stuff on Mercury?" he asks. One possible source would be volcanoes, he says.

Models that try to explain Mercury's origins also predict characteristics like sulfur content, he notes. Additional information should come from the European Space Agency's Bepi Colombo mission.

The mission, scheduled for launch in 2009, consists of two orbiters and a surface probe. Instead of using standard chemical propellants, Bepi Colombo's planners hope to use an ion propulsion system similar to one being used on NASA's Deep Space 1 spacecraft, currently en route to Comet Borrelli this September. Planners hope it will cut the mission's travel time to half that of Messenger's.

One of Bepi Colombo's two orbiters will carry an instrument package similar to Messenger's. The other craft will travel through the teardrop tail of Mercury's magnetic field to study the planet's "space weather."

Looking at the range of missions heading toward the first rock from the sun during this decade, MIT's Zuber lays out what could become a bumper sticker for Mercury missions: It's time has come.

(c) Copyright 2001. The Christian Science Publishing Society

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