Mars is the fourth planet from the sun and orbits the sun at a distance of about 141 million mi. Mars is named for the Roman god of war because it appears fiery red in the earth’s night sky.
Mars is a small planet that has about half the diameter of Earth and about one-tenth Earth’s mass. The force of gravity on the surface of Mars is about one-third of that on Earth. Mars has twice the diameter and twice the surface gravity of Earth’s moon. The surface area of Mars is almost exactly the same as the surface area of the dry land on Earth.
The Martian day, or the time it takes Mars to rotate once on its axis, is about a half an hour longer than an Earth day. Its year, or the time it takes to revolve once around the sun, is about two Earth years long. Mars has two moons, Phobos and Deimos.
THE INTERIOR OF MARS
Scientists believe that Mars’s interior consists of a crust, mantle, and core like Earth’s interior, but they do not know the relative sizes of these components. Because no spacecraft has ever brought instruments that can study Mars’s interior to the planet, the only real data that scientists have about the planet’s structure are its mass, size, and the structure of the gravity field.
Compared to Earth, Mars probably has a relatively thick crust. Beneath the surface is an area of volcanic activity in the northern hemisphere, it may be as thick as 80 mi. Beneath the landing site of the United States spacecraft Viking 2, it may be as thin as 9 mi.
The core is probably consists of mostly iron, with a small amount of nickel. Other light elements, mainly sulfur, could exist in the core also. If so, the core may be quite large. Mars does not have a significant magnetic field, so scientists believe that Mars’s core is probably solid.
Mars does not, and probably did not ever, have active plate tectonics. Because Mars is so much smaller than Earth, it must cooled quickly after formation and the crust thickened, forming one solid piece and eliminating any possibility of plate tectonics as it was on and still is on Earth. Though the Martian crust is not broken into separate plates, Mars’s liquid mantle has sculpted the planet’s surface. The molten rock has broken through the crust to form volcanoes and its motion has cracked the crust to form large rifts.
THE SURFACE OF MARS
The surface of Mars would be a harsh place for humans, but it is more like the surface of Earth than any other planet. The temperature on Mars does not get much cooler than the temperature at Antarctica. At the surface it ranges from about -140? C to 15? C (about -225? F to 60? F). During most of the year wind speeds are normally low around 4.5 mph, but during dust storms they can approach 40 to 50 mph. These winds often originate in large basins in the southern hemisphere and carry large volumes of dust from the basins to other regions, sometimes covering the entire planet in the storm. The dust is not sandy, as in a sandstorm on the earth, but has the consistency of flour.
The northern and southern hemispheres of Mars have different characteristics. The southern hemisphere has many impact craters and has a generally much higher elevation than the northern hemisphere. The southern highlands are probably the oldest ground on Mars. The northern hemisphere of Mars contains a much wider variety of geologic features, including large volcanoes, a great rift valley, and a variety of channels. The northern hemisphere also contains large expanses of relatively featureless plains.
Mars has the largest volcano in the solar system, Olympus Mons. It is 16 mi high (almost twice as high as the earth’s Mount Everest) and covers an area comparable to the state of Arizona. Near it, three other volcanoes almost as large-Arsia Mons, Pavonis Mons, and Ascraeus Mons-form a line running from southwest to northeast. These four volcanoes are the most noticeable features of a large bulge in the surface of Mars, called Tharsis. Another volcano, Alba Patera, is also part of the Tharsis bulge, but is quite different in appearance. It is probably less than 4 mi high, but has a diameter of 1000 mi. None of Mars’s volcanoes appear to be active.
The Tharsis bulge has had a large effect on the appearance of the surface of Mars. The Tharsis bulge includes many smaller volcanoes and stress fractures, in addition to the large volcanoes. Its presence affects the weather on Mars and may have changed the climate by changing the rotation of the planet. Valles Marineris (named for the U.S. Mariner spacecraft that discovered it) is the most notable stress feature associated with the Tharsis bulge. It is a great rift valley extending from the Tharsis region away to the east-southeast. It is about the same length as the distance from New York to California. This canyon system reaches widths of 440 mi and depths of 4 mi.
Three types of channels on Mars were probably formed by the action of water. These channels are unrelated to the canals thought to be seen in early telescopic views of Mars. Channel networks are similar in appearance to streambeds on the earth and occur in the southern highlands. These channels may date from a time early in Mars’s history when the atmosphere was thicker and liquid water could flow on the surface. Outflow channels, which giant floods may have formed, occur on the boundary between the southern highlands and the northern plains regions. Ares Vallis, where the Mars Pathfinder spacecraft landed, is one of these outflow channels. Landslides and other erosion probably formed fretted channels by enlarging preexisting channels. The Mars Pathfinder spacecraft found minerals in Ares Vallis that are similar to minerals that form near water on Earth, supporting the theory that Mars had liquid water at some point in its history.
Mars has small, permanent ice caps at its north and south poles. The caps increase in size in the winter of each hemisphere. The caps in the north and south are quite different from one another. The northern permanent cap is composed of water ice and is about 620 miles across. A seasonal cap of frozen carbon dioxide adds to the northern ice cap in the northern winter. The southern permanent cap is one-third the diameter of the northern cap because summer in the southern hemisphere 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 earth’s atmosphere is mostly nitrogen and oxygen, with only 0.03 percent carbon dioxide. The pressure of Mars’s 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.
The atmosphere of Mars contains very little water vapor. The level of water vapor averages about 0.016 percent, compared to the earth’s 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 Mars’s 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.
Mir’s 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 Mir’s 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 body’s 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 Thagard’s 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 Mir’s 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 station’s condition more closely. Just as scientific equipment from Salyut 7 was transferred to Mir, equipment from Mir will be transferred to Mir’s planned follower ship, the International Space Station (ISS), at the end of Mir’s 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.