MARS

Mars is the fourth planet from the sun. It orbits the sun at a span of about 140 million miles. Looking up at Mars from Earth the planet Mars appears fiery red. The surface is almost exactly the same as the dry land on Earth. The time it takes Mars to rotate once on its axis is about half on hour longer than an Earth day. Phobos and Deimos are the names of the two moons of Mars. The atmosphere is cooler and drier than it was in the mid 1970's. Mars appears as a fairly bright, star-like object in the night sky of Earth. It moves through Earth's sky fairly rapidly, on a time scale of months. Because of the relative movements of Earth and Mars around the Sun, Mars appears to move backward in the sky for a short time around opposition, when the two planets are closest. As Mars and Earth orbit the Sun, the distance between them varies from about 75 million km (about 47 million mi.) at opposition to about 375 million km (about 233 million mi.) when the planets are on opposite sides of the Sun. This change in distance causes the apparent size of Mars to vary by a factor of 5 and its brightness to vary by a factor of 25. On February 25, 1995, Mars was at a distance of approximately 103 million kilometers (65 million miles) from Earth. The atmosphere is 95 percent carbon dioxide, 3 percent nitrogen and nearly 2 percent argon. Atmosphere pressure varies with the season and ranges from 6 to 10 millibars (1 millibar is approximately one one-thousandth of the air pressure on the surface of Earth.) The variation in pressure is caused by carbon dioxide freezing out at the poles of the planet in fall and winter. The level of water vapor averages 0.016 percent. There are six major types of clouds, which form in Mars's atmosphere. The Viking 2 lander recorded images of water and ice frost during the winter.