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The Mysterious Side of Mercury

Natural History, May, 2001 by Richard Panek

The MESSENGER launch in 2004 promises to lead us out of the dark.

The dark side of the Moon? Been there, done that. Another dark side in our solar system, however, is beckoning to some astronomers today, much as the Moon's did to an earlier generation. In this case the dark side is Mercury's, and as always in such discussions, its very existence depends on the definition of "dark."

The dark side of the Moon, for one, isn't--dark that is, at least in any day-and-night sense of the word. Every crater on the Moon's surface gets some sun. In fact, during Earth's new Moon phase, it's the Moon's "dark" side that is fully in sunlight.

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Still, that side is indeed dark to us, in the sense that we can't see it. As with many major satellites in the solar system, the Moon is in synchronous rotation with its host planet, meaning that it always shows the same face to Earth, never the other. That unseen frontier, perhaps more appropriately termed the far side, proved to be a most tempting target at the dawn of the space age.

Mercury's dark side is now starting to arouse a similar curiosity among astronomers. No, this dark side doesn't actually dwell in darkness either, although until the 1960s most astronomers believed that Mercury was in synchronous rotation with the Sun and that one side of the planet did indeed experience perpetual night. But in terms of what we know about the planet, just about half its surface remains, for all practical purposes, hidden.

Mariner 10 flew past Mercury three times: on March 29 and September 21, 1974, and on March 16, 1975. That's it. The photographs that the spacecraft sent back comprise just about the entire database of close or even reliable observations of the surface features of the innermost planet of our solar system--and they cover slightly less than half of Mercury's surface. The other half, though technically visible from Earth or from satellite observatories such as the Hubble Space Telescope, is still pretty much off-limits. The problem is the planet's proximity to the Sun--the same obstacle observers have always had to overcome when studying Mercury. Partly for that reason, observing the planet has historically served more as a means of gathering general astronomical information than as an end in itself. Transits of Mercury (when, from the point of view of an observer on Earth, the planet crosses the surface of the Sun) have been useful, for example, in determining the sizes of the planets relative to the Sun and the overall scale of the solar system (see "Celestial Events," November 1999). And a seeming anomaly in the planet's orbit around the Sun helped substantiate Einstein's general theory of relativity.

But Mercury itself?. Consider: Recently when I logged on to Yahoo! and followed the Science topic trail from Astronomy to Solar System to Planets, I found that Mercury had the fewest number of entries among the planets--a mere 4. Even Pluto had double that number, while Mars weighed in with a whopping 143.

Now, however, Mercury's relative anonymity just may be nearing an end. What most surprised astronomers during the Mariner 10 flybys some twenty-five years ago was the presence of a magnetic field, about 1/100 the strength of Earth's, possibly indicating an active interior. Now researchers revisiting that Mariner 10 data have found evidence of volcanic activity on the surface of the planet. New radar observations of Mercury's north pole even indicate the possible presence of water in the form of ice.

For these reasons, a NASA mission that was already in the works--the MESSENGER (an acronym for Mercury: Surface, Space Environment, Geochemistry, and Ranging) spacecraft, due to launch in 2004 and rendezvous with the planet in 2009--has recently assumed new significance. Unlike Mariner 10, MESSENGER would map the entire surface of Mercury and, in the process, possibly resolve some questions about planet formation.

In the meantime, observers can continue to content themselves with those infrequent occasions when Mercury ventures far enough from the Sun to provide us with a clear, if fleeting, glimpse. This month, Mercury makes its brightest appearance of the year. Look for it at twilight, trailing the Sun over the horizon in the west-northwestern sky--but look quickly, because the planet will be setting one to two hours after the Sun. By the end of the first week in June, the wash of light from the Sun will render it unobservable, and Mercury will once more be there but not there, dark but not dark, gone but--especially among astronomers--nowhere near forgotten.

The Mysterious Side of Mercury

Natural History, May, 2001 by Richard Panek

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