The solar system has four solid, Earthlike planets. Earth, Mars and Venus are all reasonably familiar, but the fourth terrestrial planet—the innermost planet Mercury—is still largely unknown. If we want to understand these planets and how they formed, we need more data from Mercury.
There isn't much information on hand, just enough to raise interesting questions. Telescopic observations tell us that Mercury is small but heavy. It has no atmosphere thanks to its low gravity and the fierce solar radiation, almost seven times as intense as on Earth. We deduce that it has a relatively large iron core, three-fourths the diameter of the whole planet, and a thin rocky mantle only about 600 kilometers thick. Whereas Earth is basically a silicate planet with an iron core, Mercury amounts to an iron planet with a silicate rind. This fits the overall picture we have of the whole inner solar system: the farther we get from the Sun, the lighter the stuff of the planets. How this results from the solar system's early history is not certain; we have more than one hypothesis to choose from.
Beyond that, we have three more sources of information: pictures and geophysical data from the Mariner 10 mission, tantalizing images made with ground-based telescopes, and radar observations. And on the horizon are two spacecraft missions to Mercury.
Mariner 10
The tenth Mariner mission was a spacecraft lobbed past the inner planet on a converted missile in the 1970s. Its cameras captured only half of Mercury's surface during its three flybys, but what we saw looked mighty like the Moon—heavily cratered—but without the large dark lava plains and ringed basins. The pictures document a simple history of hot planetary formation, solidification of a crust, mantle and core, and a late flurry of crater-forming impacts. One curious feature not found on any other planet is a number of pushed-up ridges, which appear to result from a slight shrinkage of the whole planet.
The Mariner 10 instruments detected a planetary magnetic field, like Earth's geomagnetic field but much weaker. If that field is maintained by the same kind of dynamo as Earth's is, then Mercury's core probably has a little sulfur in it to keep it liquid (as Earth's core may also). Other evidence supports a liquid core, but the magnetic field needs to be known more precisely.
Telescopic Pictures
Because no spacecraft has gone to Mercury since the 1970s, ground-based telescopes have long been the only new source of images. The latest generation of electronic cameras have recently snapped useful pictures of surface features in visible light. The picture shown here is a composite of dozens of images selected from several hundred thousand snapshots taken at Mount Wilson observatory in August 1998. A Swedish experiment yielded more pictures covering the whole planet. Observations in the infrared give us hints of the composition of Mercury's surface, consistent with feldspar minerals low in iron. That is similar to the ancient highlands of the Moon and parts of the Earth's crust.
Radar Studies
In 1992 the Arecibo radar telescope began bouncing signals off Mercury. It yielded crude maps of areas not seen by Mariner, with suggestions of large features that might be volcanoes or lava basins like the lunar maria. It also saw unusually strong reflections from the north polar region, suggestive of ice hiding in places that are never exposed to sunlight. That's a funny thing to imagine on the closest planet to the Sun, where the temperature at noon reaches 450°C. But with no air to blow warmth around, ice can exist easily in shaded places. The same thing appears on the Moon.
In 2001 a technique was proposed for teasing out data about Mercury's core, based on advanced methods of radar interferometry. By taking radar snapshots at precisely the same points in Mercury's orbit, we could estimate the planet's libration, a tiny wobble caused by the gravity of other objects. In an analogy to the simple household test of telling a raw egg from a cooked one by spinning or shaking it, libration is a clue to the planet's interior. Earth's liquid core was first demonstrated by libration observations in the 1800s. Results announced in March 2007, after making about 20 of these exacting measurements, showed that Mercury's core is indeed likely to be liquid. New data from spacecraft could firmly settle this question.
Spacecraft Missions to Mercury
After 30 years, the planet Mercury is in the limelight again. The MESSENGER mission began in August 2004 with the launch of a spacecraft that will work its way into the solar system's hot zone by a series of flybys. With its first flyby early in 2008, its instruments have started answering a host of questions about Mercury and, by extension, all four of the rocky planets. Follow the latest results at the MESSENGER home page.
The European Space Agency and the Japanese space agency ISAS/JAXA are planning the BepiColombo mission to Mercury in 2013. Two spacecraft will perform flybys and settle into orbit in 2019 for a two-year nominal mission. Given the typical pattern of these things, the mission will surely be extended further if the spacecraft survive.
