Forgotten Planet

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.

Following the sun

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.

Rich in metal

Mercury’s composition also poses a puzzle. Mariner 10 data confirmed that the planet is extremely dense, leading scientists to conclude that it must be two-thirds iron. “It’s by far the most metal-rich planet,” notes Solomon, and researchers aren’t sure how it assembled.

Standard theory suggests that all the planets condensed from a swirling disk of gas and dust that swaddled the young sun. Small grains in the disk collided and stuck together to form bigger and bigger particles, which eventually grew into planets.

Interactions between the embryonic particles underlie a planet’s composition. Because the inner part of the solar system disk is denser than the outer regions, the drag force between particles there is larger. Drag slows lighter-weight materials, such as silicates, much more than heavier particles. Once they decelerate, the silicates spiral into the sun. This leaves a much higher concentration of metals in the inner part of the disk, where Mercury assembled, than in more distant reaches.

However, that theory may not entirely explain the planet’s high density, notes theorist Alan P. Boss of the Carnegie Institution. The embryonic material that coalesced to make Mercury and the other terrestrial planets was probably gathered over a wide range of distances in the inner disk, including regions where the metal density was lower than that of Mercury today.

In another scenario, ultraviolet light from the young sun shining on a fledgling Mercury might have burned off a crust of low-density rock, leaving the planet as an iron-rich cinder.

According to a third theory, collisions with an asteroid as large as Mercury could have drastically altered the planet’s composition, stripping away light-weight minerals from the crust and upper mantle. Both these alternative theories allow for the possibility that after Mercury formed, it migrated in from farther out in the disk.

Although scientists have yet to determine which model is correct, a recent analysis of Mariner 10 images by Robinson and G. Jeffrey Taylor of the University of Hawaii in Honolulu suggests that Mercury arose in its current location and never budged.

From the Mercury images, as well as previous studies of moon rocks, Robinson and Taylor estimated the average amount of iron oxide present in minerals on the planet’s surface. Their results, which show a relatively low abundance of iron oxide, support the idea that the planet assembled at its current location. Data from Earth, the moon, Venus, and Mars show lower amounts of iron oxide the closer a planet lies to the sun, Robinson and Taylor say.

Their analysis, in combination with work by Paul G. Lucey of the University of Hawaii, also supports the view that volcanic activity spawned the vast, smooth plains imaged by Mariner 10. Researchers had already suspected such an origin but couldn’t prove that the material forming the plains differs from that of its surroundings. The new work indicates that the plains have a distinctive coloration and thus a different composition from adjacent areas. The findings make it more plausible that mammoth upheavals of lava once were common on the planet.

Temperature extremes

When it comes to extremes in temperature, Mercury is the champ. It simultaneously ranks among the coldest and hottest planets in the solar system, notes Chapman. The planet rotates under the broiling sun as if it were a slab of meat in a slow rotisserie: One rotation lasts nearly half an Earth year. Because Mercury’s atmosphere is much too thin to transport the sun’s heat from the day side to the night side, the planet’s sunlit side reaches a temperature of 427ºC, while its dark side drops as low as -173ºC, Chapman says.

Given Mercury’s proximity to the sun, finding permanent deposits of ice on the planet would seem about as likely as hell freezing over. Yet radar studies a decade ago showed that scattered areas at the north and south poles were highly reflective at radio wavelengths, an indication that the regions might contain ice. Follow-up studies have confirmed the radar findings and indicate that patches of ice might lie as far as 750 kilometers from the poles.

Scientists suggest that the ice resides at the bottoms of craters where sunlight never reaches. Recent evidence of water ice in craters at the poles of the moon (SN: 3/14/98, p. 166) strengthens the argument that Mercury may also have such frozen reservoirs. To find out, MESSENGER and BepiColombo will employ the method that scientists used to search for lunar ice. The craft’s gamma-ray and neutron spectrometers will look in the polar regions for hydrogen.

An abundance of hydrogen is an indicator of water. Ultraviolet spectrometers on the craft can also search for the hydroxyl molecule, an even stronger signpost of water.

Both MESSENGER and BepiColombo plan to explore the composition of Mercury’s tenuous atmosphere. The planet’s envelope of gas is so thin that the molecules don’t generally collide with each other but bounce repeatedly off the surface like rubber balls, forming what planetary scientists call an exosphere.

Several processes may contribute to the buildup of the exosphere: emission of gases from Mercury’s interior, sunlight or solar wind ions evaporating material from crystal rocks, or space debris smashing into rocks and vaporizing them. To distinguish among these processes, spectrometers on the craft will compare the composition of the exosphere with that of surface rocks.

Entering orbit

It won’t be easy for a spacecraft to enter orbit around Mercury. Resisting the pull of the sun’s gravity takes a lot of fuel and couldn’t be done with the propulsion systems available before the mid-1980s, notes Solomon. In 1985, Chen-wan Yen at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., discovered a clever way to get to Mercury. Two flybys of Venus followed by two of Mercury would give a spacecraft the gravitational kick required to orbit the innermost planet, she proposed.

That’s just the prescription MESSENGER will soon follow. Even with those assists, more than half the 1,097-kilogram weight at launch will be fuel, says project manager Ralph L. McNutt of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. An umbrellalike heat shade will shield the craft’s detectors from the relentless sun, and solar panels will help power the craft once it begins an elliptical orbit about Mercury in 2009. Then, MESSENGER will radio back to Earth the first close-up images of Mercury since 1975, mapping features as small as 125 meters across.

BepiColombo will rely on an innovative fuel to travel to Mercury. The craft will use atoms of xenon ionized by the sun’s energy. An electric field will accelerate the ions to high velocity as they shoot out of the rear of the craft, enabling the craft to slip into orbit about Mercury rather than spiral into the sun. A combination of solar ion propulsion and gravitational boosts from the moon and Venus would give a travel time of about 3.6 years, compared with 6 years, the shortest time using ordinary rocket fuel.

Solar ion propulsion has only been demonstrated once beyond Earth’s orbit, on a NASA mission called Deep Space 1. The European Space Agency (ESA) intends to conduct a second test flight before using the method for the first of its Mercury-bound missions in 2008, says Rüjean Grard of ESA’s European Space Research and Technology Center in Noordwijk, the Netherlands.

Touring Mercury, the planet gripped most tightly by the sun’s gravity, also presents an opportunity to test with unprecedented accuracy Einstein’s theory of general relativity. That theory describes exactly how the sun curves space-time and so affects the orbit of Mercury.

The first ESA launch will probably feature two missions—a spacecraft that will explore Mercury’s magnetosphere and a small lander that will plunge into the surface of the cratered planet. The Mercury Magnetospheric Orbiter will have a highly elliptical orbit, enabling the cylindrical craft to investigate the outer edge of the planet’s magnetic field, some 12,000 km from the surface.

With luck, the magnetosphere mission will begin taking measurements while MESSENGER is still touring Mercury. “Having two spacecraft taking comparable observations at different orbits at the same time gives you a three-dimensional [view of Mercury] that’s enormously valuable,” says Solomon.

The missile-shaped lander will penetrate 2 to 3 m into Mercury’s soil, leaving its top poking out just a few centimeters. A camera on an extendable mast will image the surroundings in exquisite detail. Other instruments will measure the flow of heat from the planet’s interior, record quakes, and assess the magnetic properties of surface rocks. Connected by an umbilical cord to the lander, a minirover will carry an alpha X-ray spectrometer to nearby rocks to record their composition.

A year later, in 2009, ESA plans to launch another craft, which will fly over Mercury’s poles, imaging features as small as 10 m across. Solar cells will cover three sides of the Mercury Planetary Orbiter, which will be shaped like a truncated pyramid.

The studies, adds Grard, have implications far beyond our planetary system. “As far as we know, the terrestrial planets are the cradle of life in our solar system,” he says. “Now, we have a big question: Is there life anywhere else in the universe?”

Understanding how every terrestrial planet formed and survived—even those that don’t contain life—may shed light on how some planets may have managed to become a haven for living things, Grard notes.

Mercury, the tiniest and in some ways the most bizarre member of the terrestrial family, may have a lot to answer for.