Forgotten Planet - Mercury

Science News, July 8, 2000 by Ron Cowen

Mercury: The solar system's inner frontier

Not far from the sun lies a tiny planet with a big image problem. A fleeting presence often hidden by the sun's glare, Mercury is the neglected child of the solar system.

Planetary scientists have preferred to gaze upon more exotic-looking orbs than pockmarked Mercury, whose appearance bears a superficial resemblance to the moon's familiar facade. Only one mission, Mariner 10, has flown past the planet, and it viewed less than half the surface.

Scientists know almost as little about Mercury, the solar system's innermost planet, as they do about distant Pluto. Simultaneously hot enough on one side to melt zinc and cold enough on the other to freeze methane, tiny Mercury represents a world of extremes that may push planet-formation theories to their limit.

Indeed, Mercury's very existence can teach scientists volumes about how the other terrestrial planets--Earth, Mars, and Venus--assembled, says Sean C. Solomon of the Carnegie Institution of Washington (D.C.). Information about Mercury may even help scientists calculate the likelihood that other stars harbor Earthlike planets, he speculates.

"To understand processes that affected all the planets, we have to understand the extreme outcomes," Solomon notes. "Mercury has the potential to give us much deeper insight into questions about how planets can assemble."

Last month, at the spring meeting of the American Geophysical Union in Washington, D.C., scientists discussed Mercury's many mysteries as well as plans by two space agencies to investigate those puzzles. NASA's $286 million MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) spacecraft would examine Mercury's atmosphere and entire surface for 1 Earth year with a suite of detectors including cameras, spectrometers, and a magnetometer.

The European Space Agency's more ambitious proposal, not yet funded, consists of a trio of spacecraft called BepiColombo. The craft include two satellites and a vehicle that would land on the surface, deploying a tiny, tethered rover.

"One of the things that's really neat and exciting about studying Mercury is the chance to see [the half] that's never been seen before," says Mark S. Robinson of Northwestern University in Evanston, Ill.

Faithfully following the sun over the horizon at sunset or appearing in the dawn's early light, Mercury--as seen from Earth--never strays far from the star of the solar system. When directly overhead, it's swamped by scattered sunlight. Although it's easiest to see when it hangs low in the sky, Mercury's light must then pass through 10 times the usual thickness of Earth's turbulent atmosphere. As a result, most ground-based telescopes produce blurry images of the planet.

Earth-orbiting telescopes don't have to contend with atmospheric blurring, but many of them, including the Hubble Space Telescope, can't peer anywhere near Mercury, lest stray sunlight damage their sensitive detectors.

Thus, almost everything astronomers know about Mercury comes from Mariner 10, which studied the planet during three flybys in 1974 and 1975. The spacecraft, however, always imaged the same face of Mercury, leaving half the planet unexplored.

But what the spacecraft did find stunned researchers. Most scientists had assumed that the planet, geologically dead on the outside, was just as inactive on the inside. At Mercury's equator, however, the craft detected a persistent magnetic field of a few hundred nanotesla equivalent to about one-hundredth that of Earth's.

The only convincing model to account for planetary magnetic fields relies on the concept of a dynamo. In this scenario, the continual rise and fall of molten, electrically conducting material deep within a rotating planet generates an electric current. The current in turn creates a magnetic field.

Could a planet as small as Mercury, which should have cooled substantially since the birth of the solar system, stay hot enough to keep its core partially liquid? The riddle is all the more puzzling because a planetwide system of huge cliffs attests that Mercury has indeed cooled and shrunk. The cliffs represent places where the planet has buckled as it vented its heat and contracted, explains Clark R. Chapman of the Southwest Research Institute in Boulder, Colo.

MESSENGER, scheduled for launch in 2004 and entry orbit around Mercury 5 years later, will be able to check whether the planet has a liquid outer core, says Solomon, principal investigator for the mission.

To do so, the craft will precisely track the planet's rate of rotation. Mercury's rotation varies because the sun's gravity creates a twisting force as the slightly eggshaped planet follows its elliptical path around the sun. The twisting force would cause the rotation rate to vary by twice as much if Mercury's surface floats on a partially molten core than if the entire planet is solid, scientists calculate.

Instruments aboard both MESSENGER and BepiColombo will measure the tiny oscillation. In combination with new gravity maps of Mercury, obtained by measuring how much the spacecraft speeds up or slows down as it passes over different parts of the planet, the MESSENGER experiment will determine the size of the core and how much of it is solid.