Into orbit

Hidden by the sun’s glare and dismissed as a heavily cratered rock no different than Earth’s moon, Mercury has for decades been relegated to the back burner of planetary research. But studies of the planet closest to the sun have now ignited.

On March 17, NASA’s MESSENGER spacecraft became the first probe to enter orbit around the metal-rich body (SN Online: 3/17/11). MESSENGER — short for Mercury Surface, Space Environment, Geochemistry and Ranging — completes a lap every 12 hours, traveling an oval path that swoops close to the planet’s north polar region and keeps a greater distance from the southern pole.

A yearlong effort to understand the planet’s volcanism, core, magnetic field and other features got under way April 4, when the craft’s suite of seven instruments began regularly beaming data to Earth. With MESSENGER gleaning new information, researchers will attempt to solve a number of unanswered questions about Mercury, many brought to light when the craft flew by the planet in 2008 and 2009.

These earlier encounters with Mercury revealed that volcanic upheaval played a major and perhaps dominant role in sculpting the planet’s surface. That finding was a surprise to scientists who had thought that the solar system’s tiniest planet wouldn’t have retained enough heat to drive widespread volcanic activity.

The first hints of volcanism came in the 1970s when Mariner 10, the only other spacecraft to fly past Mercury, captured images of smooth plains between heavily cratered regions. But it took MESSENGER’s flybys to uncover convincing evidence of lava flows. These flows had all but erased small craters within a Texas-sized impact basin called Caloris. More curious, the craft spotted more than 36 deposits on crater floors and the rim of Caloris indicating that some volcanic eruptions were highly explosive, powered by the rapid expansion of gases bubbling out of magma like carbon dioxide rushing out of seltzer water.

Explosive volcanism on Mercury is a puzzle because the planet’s hot birth should have immediately driven away compounds that easily vaporize, says MESSENGER lead scientist Sean Solomon of the Carnegie Institution for Science in Washington, D.C. Any remaining volatile compounds ought to have exited the planet early in its history, when scientists believe a massive body collided with Mercury.

During MESSENGER’s yearlong mission, gamma-ray and X-ray detectors will examine the composition of surface rocks to look for signs that volatile gases could in fact have driven the volcanism.

“We should get relatively unambiguous measurements of the atomic composition of surface rocks and their variation from place to place,” says Bill McKinnon of Washington University in St. Louis. “You can’t claim to know a planet’s history if you don’t know what the planet’s rocks are made of.”

Depending on what the craft finds, theorists may have to invoke nontraditional volatile compounds as possible explosive power sources or consider models in which comets or asteroids delivered volatiles after Mercury had begun cooling.

Hidden within

Studying surface rocks may also offer clues about Mercury’s unusually high density, another of the planet’s strange properties. This density, first deduced in the early 1950s from the planet’s gravitational influence on other planets, suggests that Mercury has an iron core that accounts for some 60 percent of its mass, says Solomon. In contrast, the metallic cores of Earth, Mars and most likely Venus make up only about 30 percent of the mass of those planets.

Explanations vary, from a collision that stripped away lightweight silicates in the planet’s outer shell to an abundance of iron in the region of the planet-forming disk from which Mercury emerged. But each theory predicts a different composition for surface rocks, so analyzing those rocks could point to the early solar system processes most important for the planet’s formation and evolution.

Identifying those mechanisms may offer insight about the formation of planets beyond the solar system, particularly those born close to their parent stars. “Almost everything we learn will be relevant for the interaction of close-in extrasolar planets with their host stars,” Solomon says.

Magnetic measurements made during orbit will further illuminate Mercury’s interior, including how heat transfers between layers of material at the planet’s core. The craft’s flybys already have confirmed a small but persistent magnetic field, suggesting that Mercury’s core is divided into two parts — a hot outer portion, made of molten iron, circulating around a cooler inner core of solid iron. The churning of the planet’s liquid outer core could act as a “dynamo,” driving the magnetic field.

Like the magnetic field of Earth, the only planet known for sure to have a liquid-iron dynamo, Mercury’s field broadly resembles that of a dipole, a bar magnet with a north and south pole. But the strength of the dipole field near Mercury’s core is only about one-thousandth the intrinsic strength of Earth’s field, and Solomon and his colleagues would like to find out why.

MESSENGER’s magnetometer will look for deviations from the field that a perfect bar magnet would generate. By comparing the observed deviations with those predicted by different dynamo theories, scientists hope to zero in on the model that best explains how the planet’s weak field comes about.

“The hardest question to answer is probably the origin of the magnetic field,” says Francis Nimmo of the University of California, Santa Cruz.

Inside to out

Researchers do know that Mercury’s persistent magnetic field is part of a larger story involving a delicate balance between heating and cooling inside the planet.

On the one hand, an active dynamo requires a source of internal heat to keep the planet’s outer core molten. On the other hand, both the MESSENGER and Mariner 10 flybys suggested that the planet has cooled substantially over time. Huge cliffs, or scarps, mark the tops of faults that mar Mercury’s surface, meaning cooling has caused the planet to contract like a shriveling raisin.

MESSENGER flybys revealed that the planet has shrunk about one-third more than previously estimated, pointing to a higher rate of cooling. Accounting for both the internal heating required for the dynamo and the cooling revealed by the scarps “has been a conundrum that’s interested me since Mariner 10,” says Solomon. MESSENGER’s examination of the scarps from orbit “will provide us with a story not only on the amount of the shrinkage of the planet but when it happened relative to other geological processes — the whole history of cooling.”

The rate and timing of the cooling should also hold clues to how Mercury’s outer core retains enough heat to remain molten and thus keep the dynamo and magnetic field alive.

Though Mercury’s active magnetic field sets the pockmarked planet apart from Earth’s moon, the two bodies share some secrets: Both may possess pockets of frozen surface water and therefore hold clues to the abundance of water in the solar system.

Like some areas of the moon, some of Mercury’s craters lie in permanent shadow and may be cold enough to trap ice deposits delivered by asteroids and meteorites over millions to billions of years. Radar studies from Earth have already suggested the presence of frozen water, and MESSENGER will seek confirmation by looking for a signature of hydrogen in the polar regions.

Finding ice on the planet closest to the sun would further feed the blaze of renewed interest that MESSENGER has ignited in Mercury. After presenting scientists with a storehouse of new knowledge, the spacecraft will end its mission on March 18, 2012 — unless NASA decides to extend the craft’s tour for an additional year.

Encores aside, MESSENGER will run out of fuel in a few years and crash into Mercury’s surface, melding with the planet it will have so thoroughly explored.