MARS - The Fourth Planet
Mars has such an eccentric orbit that the closest approach at opposition between earth and Mars comes every 15 to 17 years, when Mars is near perihelion in its orbit. At the last favorable opposition in August, 1971, Mars came within 60 million kilometers of the earth. At a time like that even the most casual observers of the heavens are struck by the planet's brilliant ruddy color, far outshining the brightest stars. The next favorable opposition will be in August, 1988.
Mars is a little more than half the earth's size, has about 11 percent of the earth's mass, and therefore has a mean density 75 percent of that of the earth. The Martian day lasts 24 hours and 37.4 minutes. Its axis of rotation tilts by 24° from the perpendicu lar to its orbital plane, giving the red planet seasons like those of the earth; but they last twice as long because the Martian orbital period is nearly 2 years.
Visual observers have made countless detailed maps of the numerous and varied surface features of Mars. Two observers are especially notable: the Italian astronomer Giovanni Schiaparelli (1835-1910) and the American astronomer Percival Lowell who mapped and named many Martian features. Through a telescope the red planet appears to have earthlike characteristics, such as white polar caps and large dark areas, which vary with the Martian seasons.
The recognition that Mars has polar caps dates back to the early 1800s. When Mars is closest to the sun, the south pole is inclined toward it. As a result the large southern polar cap recedes during the Martian summer, leaving behind a small residual cap or on some occasions none at all that can be seen from the earth. On the other side of the orbit the north pole is inclined toward the sun when the planet is farthest from the sun. The residual polar cap never quite disappears during the Martian summer in the northern hemisphere.
Mars has also for some time been known to have an atmosphere where vast yellow dust storms occur. Once or twice a Martian year (about 2 earth years) a storm grows to global proportions, enveloping almost the entire planet in a dense shroud of dust. White clouds in the Martian atmosphere have also been observed from the earth.
Although these various characteristics observed from here suggested great similarities to our planet, spacecraft revealed a surface topography that is as much like the moon as it is the earth. This quickly put to rest the many long-running romantic notions that seasonal changes were due to vegetation and the inference that Mars might thus be the home of intelligent life.
