Saturday, May 30, 2009

Mercury: The Moon’s Twin

In many ways, Mercury has more in common with the lifeless moon of our own planet than with the other terrestrial planets. Its face is scarred with ancient craters, the result of massive bombardment that occurred early in the solar system’s history. These craters remain untouched because Mercury has no water, erosion, or atmosphere to erase them. The closest planet to the sun—with an average distance of 960,000 miles (1,546,000 km)—
Mercury is difficult to observe from the earth, and can only be viewed near sunrise or sunset.
Its surface, revealed in detail for the first time in images transmitted by such unmanned probes as Mariner 10 (in the 1970s), is pocked with moonlike craters.
Mariner 10 also discovered a weak but detectable magnetic field around Mercury. As a result, astronomers concluded that the planet must have a core rich in molten iron. This contention is consistent with the planet’s position closest to the center of the solar system, where most of the preplanetary matter—the seeds that formed the planets—would have been metallic in composition

The Terrestrial Roster

The terrestrial planets are Mercury, Venus, Earth, and Mars. Except for Earth, all are named after Roman gods. Mercury, the winged-foot messenger of the gods, is an apt name for the planet closest to the sun; its sidereal period is a mere 88 Earth days, and its average orbital speed (30 miles per second or 48 km/s) is the fastest of all the planets. Mercury orbits the sun in less than a college semester, or about four times for each Earth orbit.
Venus, named for the Roman goddess of love and fertility, is (to observers on Earth) the brightest of the planets, and, even to the naked eye, quite beautiful to behold. Its atmosphere, we shall see, is not so loving. The planet is completely enveloped by carbon dioxide and thick clouds that consist mostly of sulfuric acid. The name of the bloody Roman war god, Mars, suits the orange-red face of our nearest planetary neighbor—the planet that has most intrigued observers and that seems, at first glance, the least alien of all our fellow travelers around the sun. Here are some more numbers, specifically for the terrestrial planets. Notice that the presence of an atmosphere (on Venus and Earth) causes there to be much less variation in surface temperature.
If you recall, when we discussed the formation of the solar system, we mentioned a few observational facts that “constrained” our models of formation. A few rules of planetary motions are immediately apparent. All four terrestrial planets orbit the sun in the same direction. All except Venus rotate on their axes in the same direction as they orbit the sun. The orbital paths of the inner four planets are nearly circular. And the planets all orbit the sun in roughly the same plane. But the solar system is a dynamic and real system, not a theoretical construct, and there are interesting exceptions to these rules. The exceptions can give us insight into the formation of the solar system.