Monday, June 29, 2009

“I Can’t Breathe in Mercury!”

Like the earth’s moon, Mercury possesses insufficient mass to hold—by gravitation—an atmosphere for very long. In the same way that mass attracting mass built up planetesimals, so the early planets built up atmospheres by hanging on to them with their gravitational pull. If an atmosphere was ever associated with Mercury, the heating of the sun and the planet’s small mass helped it to escape long ago. Without an atmosphere to speak of, the planet is vulnerable to bombardment by meteoroids, x-rays, and ultraviolet radiation, as well as extremes of heat and cold. In sunlight, the planet heats to 700 K. In darkness, with no atmosphere to retain heat, it cools to 100 K. Despite the absence of atmosphere, regions at the poles of Mercury may remain permanently in shadow, with temperatures as low as 125 K. These regions, and similar regions on the earth’s moon, may have retained some water ice.

Close Encounter with Mercury

If Mercury was difficult for a professional astronomer like Schiaparelli to observe, it is even more challenging for the amateur. It is never farther than 28 degrees from the sun (due to its small orbital radius) and always seen very low in the sky, either in the west just after sunset or in the east, just before sunrise. Because it is visible only close to the horizon, obstacles and atmospheric conditions (light pollution, smog, and turbulence) may often make it impossible to see. Like the moon (and, as we saw in Chapter 2, Venus), Mercury exhibits phases as different fractions of its face are seen to be illuminated by the sun. The best time to see Mercury is at its crescent phase, because it appears largest in the sky at this time. The reason for the variation in size with phase is that when the planet is on the near side of the sun (at a distance of approximately 0.6 A.U. from us), it is backlit and closer and thus appears large. When it is on the far side of the sun, it is fully illuminated (full), is 1.4 A.U. away, and appears smaller. To get a good look at Mercury, you need a telescope, preferably fitted with an eyepiece that offers about 150magnification. It is also possible to see Mercury in the daytime, but this can be dangerous. Because the planet is so close to the sun, there is a real danger that you might accidentally focus on the sun. Doing so for even a moment can permanently damage your eyesight! If you want to look for Mercury during the day, you should consult a good ephemerides guide (see Chapter 17 and Appendix E) and use a telescope fitted with setting circles (see your telescope’s instruction manual and Chapter 17) to locate the planet precisely. For added safety, always keep a solar filter on the telescope until you have precisely located the planet.
Better yet: Restrict your viewing of Mercury to just before sunrise or shortly after sunset.

Lashed to the Sun

In the days before space-based telescopes and probes, earthbound astronomers did the best they could to gauge the rotation of Mercury. The nineteenth-century astronomer Giovanni Schiaparelli observed the movement of what few, indistinct surface features he could discern and concluded that, unlike any other planet’s, Mercury’s rotation was synchronous with its orbit around the sun.
Synchronous orbit means that Mercury always keeps one face toward the sun, and the other away from it, much as the moon always presents the same face to the earth. Technology marches on. In 1965, by means of radar imaging, unavailable to Schiaparelli in the nineteenth century, astronomers discovered that Mercury’s rotation period was not 88 days, but only 59 days. This discovery implied that Mercury’s rotation was not precisely synchronous with its orbit, but that it rotated three times around its axis every two orbits of the sun.