How Mercury is like Saturn (and other surprises from NASA's orbiter)

The planet Mercury, long a backwater on the solar-system exploration itinerary, is turning out to be one hot destination.

New observations of the first rock from the sun – gathered by NASA's Messenger orbiter and described Thursday – reveal some surprises.

For example, the planet's mineral makeup is far different from the composition researchers were expecting. And its magnetic field in some ways is more like Saturn's than that of Earth, the only other terrestrial planet with an active internal dynamo generating such fields.

The report from the orbiter also gave some support to a hypothesis based on previous radar observations that northern craters whose floors are in perpetual shadow may in fact be loaded with water ice.

Until now, no spacecraft has orbited Mercury. All that planetary scientists have had to go on from previous spacecraft were results from three Mariner 10 flybys in the 1970s and three Messenger flybys as the craft made its way from Earth to Mercury.

But what a difference an orbiter makes. Just as orbiters sent to the moon, including NASA's Lunar Reconnaissance Orbiter, have revealed an object far more dynamic than previously thought even a decade ago, so Messenger is showing that Mercury is more than "the burnt-out cinder of the solar system," as some in the planetary-science community had characterized it.

"We had many ideas about Mercury that were incomplete or ill-formed" as a result of the lack of information gathered from an orbiter, says Sean Solomon, a researcher at the Carnegie Institution for Science in Washington and the Messenger mission's lead scientist.

Debunking some theories

"We are confirming a few of the theories that preceded us," he says. "But many of those theories are being cast into the dustbin of science."

Messenger was launched in 2004 and arrived at Mercury to begin its Earth-year-long mission last March. It has now spent 88 days – one Mercurian year – mapping the planet's surface in unprecedented detail, taking the measure of its surface composition, and doing the same for its magnetic field and its interaction with local "space weather."

The craft passed a significant milestone on Sunday as it made its closest approach to the sun and lived to tell the tale. The craft was designed to withstand the high heat and radiation environment at Mercury's distance from the sun, but there's nothing like seeing it survive the real thing, as opposed to Earthbound tests, to instill confidence that the craft has the right stuff.

One surprise involves the composition of Mercury's surface. Messenger has two types of spectrometers on board to unravel this puzzle, which holds the key to understanding both the processes under which Mercury formed as well as the geological processes that continue to sculpt it.

For instance, the team has found that Mercury has a concentration of sulfur in its crust that is some 10 times the amount found in Earth's or the moon's crusts.

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