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 earthís night sky. Mars is a small planet that has about half the
diameter of Earth and about one-tenth Earthís 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 Earthís 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 Marsís
interior consists of a crust, mantle, and core like Earthís interior, but they
do not know the relative sizes of these components. Because no spacecraft has
ever brought instruments that can study Marsís interior to the planet, the
only real data that scientists have about the planetís 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 Marsís 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, Marsís liquid mantle has sculpted the planetís
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 -140į C to 15į C (about -225į F to 60į 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 earthís

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 Marsís 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 Marsís
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 earthís atmosphere is mostly nitrogen and oxygen,
with only 0.03 percent carbon dioxide. The pressure of Marsís 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 earthís 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

Marsís 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. Mirís 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 Mirís 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 bodyís 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 Thagardís 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 Mirís 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 stationís condition
more closely. Just as scientific equipment from Salyut 7 was transferred to Mir,
equipment from Mir will be transferred to Mirís planned follower ship, the

International Space Station (ISS), at the end of Mirís 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.