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.

"We do not fully understand this yet," says researcher Larry Nittler, also with the Carnegie Institution. But, he adds, it almost certainly means that "Mercury was formed from building blocks fundamentally chemically different from those that formed the Earth and moon."

A volcanic past

The high concentrations of sulfur also would have led to explosive eruptions of volcanoes on Mercury. Evidence for this volcanism shows up in the new images of the surface that Messenger is beaming back.

"The presence of sulfur should tell us some new things about volcanism on Mercury," Dr. Nittler says.

It also is narrowing the range of possible explanations for one of Mercury's odd traits: Its core is unusually large for an object of Mercury's size, taking up a higher proportion of the interior than Earth's core occupies.

One explanation offered for this, Nittler says, is that Mercury started out with the right proportions. But it is so close to the sun that during the star's youth, when it was hotter, a lot of surface material evaporated.

But if that is the case, he continues, the abundance of sulfur, as well as of potassium and sodium, would be far lower than the abundances Messenger is measuring.

Scratch that idea, he suggests.

On the other hand, some of the Messenger measurements are broadly consistent with a scenario in which Mercury formed much like Earth did, but got whacked by another planet-size object, knocking off much of its original crust.

"This is a model that's still in the running," Nittler says.

Water ice in northern craters?

One of the major quests for the mission is to see whether permanently shadowed craters on the toastiest planet in the solar system cradle deposits of water ice.

This has been a source of speculation for 20 years, ever since radar beamed from Earth probed polar craters and detected signatures suggesting water ice is present, Dr. Solomon says.

Messenger's first test to see if water ice is present has come from the craft's laser altimeter.

The team targeted one crater showing the intriguing radar returns – a crustal dimple about 15 miles across in the planet's north polar region. After gathering data from several passes over the crater, the team built a contour map of the feature. Then they calculated the amount of sunlight the floor of a crater that deep and at that latitude would receive.

"This crater passes the test," Solomon says, noting that the portion of the floor in permanent shadow coincides with a portion of the crater floor showing a bright signature on radar that some have interpreted as water ice.

This result has not produced the smoking gun, he cautions, but it does make a strong argument for aiming Messenger's spectrometers at these areas to see if they can sniff out chemical signatures indicating water.

The approach is similar to the techniques used to discover water ice on the floors of high-latitude craters on the moon.

Indeed, the past several years have revealed that a moon once thought to be "dead" is far more interesting and dynamic than anyone though a decade ago.

The same is proving to be true for Mercury.

"We're finding out that Mercury really is a world in and of its own," says Ralph McNutt, a researcher at the Johns Hopkins University's Applied Physics Laboratory and the mission's project scientist. "Just like the Earth, it has its own personality. As we look at Mercury close up ... we're managing to explore a new world for the first time."