.. phere is warmer than in the north. The southern seasonal cap is larger than the northern cap because more carbon dioxide is frozen out in the south than the north because Mars is farthest from the sun, and therefore coldest, in the southern winter. Carbon dioxide may also make up the southern permanent cap. Regions of striped-looking terrain, probably formed of layers of dust and ice, occur at the edges of both polar caps.

Climate cycles almost like the ice ages on the earth may have caused this layering. THE ATMOSPHERE OF MARS The atmosphere of Mars is 95 percent carbon dioxide, nearly 3 percent nitrogen, and nearly 2 percent argon with tiny amounts of oxygen, carbon monoxide, water vapor, and other gases. The earths atmosphere is mostly nitrogen and oxygen, with only 0.03 percent carbon dioxide. The pressure of Marss atmosphere varies with the season, ranging from 6 to 10 millibars (1 millibar is almost one one-thousandth of the air pressure at 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 pressure also varies with altitude and is about a factor of ten less on the top of Olympus Mons than on the floor of Hellas Planitia.

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The atmosphere of Mars contains very little water vapor. The level of water vapor averages about 0.016 percent, compared to the earths average level of about 2 percent. The water content of the atmosphere on Mars varies seasonally and by location and can form clouds and even frost. Six major types of clouds form in Marss atmosphere. The polar hood is a haze of water and perhaps carbon dioxide ice that forms over the polar regions in the fall and can cover much of the northern plains.

Wave clouds form on the sheltered side of large obstacles, such as craters, and have very distinct ridges. Convective clouds form in high areas at midday. Orographic clouds form when air lifts over large-scale objects like Olympus Mons, and are most common in spring and summer when the water vapor content of the air is highest. Ground hazes occur in low areas at dawn and dusk and probably consist of water ice. Wispy high-altitude clouds sometimes occur just at dawn and dusk. The Viking 2 lander recorded images of water-ice frost during the winter. Past Space Stations One past space station is Mir.

Mir was a Russian space station designed to provide long-term accomodations for crewmembers while they orbit the earth. Mir was launched on Febraury 19, 1986. Crewmwmbers reached Mir aboard Soyuz spacecraft and, more more recently thtrough an American space program aboard a spaceshuttle. Mir was the first space station designed for expansion and was originally only a single module. Now Mir consists of seven modules. Mir replaced the Salyut series of space stations as the most important part of the Russian manned space program.

The Salyut series of space stations were smaller and simpler stations that helped develop most of the technology needed to build Mir. The Mir space station is composed of seven modules that together weigh about 109,000 kg and are about 19 m long without any visiting spacecraft. The Mir core module is the control center and living quarters for the Mir station. The 20-ton module measures about 4.18 m in width and about 13 m in length. At each end of the main part is a hatch fitted to connect with other spacecraft called a docking port. The rear port leads through a tunnel into the living compartment, which contains a kitchen, exercise equipment, two sleeping compartments that are smaller than phone booths, and a toilet stall. Mirs first crew was Salyut 7 veterans Leonid Kizim and Vladimir Solovyov.

They flew to the Mir core module in the Soyuz-T 15 spacecraft in March 1986 to activate and check Mirs systems. They undocked and flew to the abandoned Salyut 7 station to salvage scientific equipment and dropped off the recovered equipment at Mir. They returned to earth in July 1986. Mir flew unmanned until February 1987. Except for two short periods from July 1986 to February 1987 and from March 1989 to September 1989, Mir has been staffed without interruption. Normally, teams of two or three cosmonauts work on board in six-month shifts.

There are, however, occasional exceptions. For example, medical doctor Valeri Polyakov set a new world space-endurance record by living on Mir for 438 days-long enough for a spacecraft to travel to Mars. During that time, Polyakov studied his bodys reactions to prolonged weightlessness. He returned to earth aboard Soyuz-TM 20 in March 1995. With him was Yelena Kondakova, the first woman to complete a long-duration stay in space.

She lived aboard Mir for 168 days. Also in March 1995, U.S. astronaut Norman Thagard began a 114-day Mir flight, breaking the U.S. 84-day space-endurance record set on Skylab in 1974. Thagard reached Mir on Soyuz-TM 21 with cosmonauts Vladimir Dezhurov and Gennadi Strekalov.

He returned to earth with his Russian crewmates on the space shuttle Atlantis, which docked with Mir for the first time on June 29, 1995. Since Thagards visit, six other U.S. astronauts have lived on Mir. German astronaut Thomas Reiter arrived at Mir aboard Soyuz-TM 22 in September 1995. He returned to earth in February 1996, after 179 days in space, having completed two space walks to install European instruments outside of the station. Mir was over a decade old when its career was nearing an end.

In 1997 the station experienced a small fire, failure of the oxygen generation system, a temperature-control failure that made the living quarters uncomfortably warm, failures of Mirs main computer and navigation system, and a collision with a supply ship. None of the onboard cosmonauts and astronauts were hurt, but the incidents caused crew members and engineers to monitor the stations condition more closely. Just as scientific equipment from Salyut 7 was transferred to Mir, equipment from Mir will be transferred to Mirs planned follower ship, the International Space Station (ISS), at the end of Mirs career. Space shuttle missions to Mir ended in mid-1998 and the first component of ISS was scheduled for launch in late 1998. ISS was assembled in orbit from U.S., Russian, European, Japanese, and Canadian parts.