One of the planets in the solar system (VRML),
it is the fourth planet from the sun and orbits the sun at a distance of about 228 million
km (about 141 million mi). Mars (Ares in Greek) is the God
of War. The planet probably got this name due to its red color; Mars is
sometimes referred to as the Red Planet. (An interesting side note: the Roman god
Mars was a god of agriculture before becoming associated with the Greek Ares; those in
favor of colonizing and terraforming Mars may prefer this symbolism.) The name of the
month March derives from Mars.
Mars has been known since prehistoric times. It is still a favorite of science fiction writers as the most favorable place in the Solar System for human habitation. But the famous "canals" "seen" by Lowell and others were, unfortunately, were just imaginary.
Mars is a relatively small planet, with about half the diameter of Earth and about one-tenth Earth’s mass. Though Mars is much smaller than Earth, its surface area is about the same as the land surface area of Earth. 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.
When it is in the nighttime sky, Mars (video) is easily visible with the naked eye. Its apparent brightness varies greatly according to its relative position to the 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, which are named after the dogs of the Roman god Mars. These tiny bodies are heavily cratered dark chunks of rock and may be asteroids captured by the gravitational pull of Mars. Phobos orbits Mars once in less than one Martian day, so it appears to rise in the west and set in the east, usually twice each day. Deimos has the more ordinary habit of rising in the east and setting in the west.
Mars
appears as a fairly bright, starlike object in the night sky of Earth. It moves
through Earth’s sky fairly rapidly, on a timescale 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 miles) at opposition to about 375 million km (about 233 million miles) 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.
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. From that data scientists can
learn some things about density in different parts of the planet.
Except for
Earth, Mars has the most highly varied and interesting terrain of any of the terrestrial planets, some of it
quite spectacular:
- Olympus Mons: the
largest mountain in the Solar System rising 24 km (78,000 ft.) above the surrounding
plain. Its base is more than 500 km in diameter and is rimmed by a cliff 6 km
(20,000 ft) high (right).
- Tharsis: a huge
bulge on the Martian surface that is about 4000 km across and 10 km high.
- Valles Marineris: a system of canyons 4000
km long and from 2 to 7 km deep.
- Hellas Planitia: an impact crater in the
southern hemisphere over 6 km deep and 2000 km in diameter.
Much of the Martian surface
(video) is very old and cratered, but there are also much younger rift valleys, ridges,
hills and plains.
Mars has an appreciable
atmosphere. Until the beginning of this century it was thought that this atmosphere
might even be comparable with that of Earth, in which case, Mars would have been able to
support life of an advanced type. However, spectroscopic
work carried out in the first part of the 20th century showed that the atmosphere is
tenuous. In 1956 an analysis published by the French astronomer G. de Vaucouleurs
gave a ground pressure of 85 millibars. It was believed that the main constituent
must be nitrogen, with smaller amounts of carbon dioxide and water vapor. Had this
been so, the Martian atmosphere would have been as dense ase the Earth's. However, Mars has a very thin atmosphere composed mostly of the tiny amount
of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of
oxygen (0.15%) and water (0.03%). The earth’s atmosphere is
mostly nitrogen and oxygen, with only 0.03 percent carbon dioxide. The
average pressure on the surface of Mars is only about 7 millibars (less than 1% of
Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest
basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to
support very strong winds and vast dust storms that on occasion engulf the entire planet
for months. Mars' thin atmosphere produces a greenhouse
effect but it is only enough to raise the surface temperature by 5 degrees (K); much
less than what we see on Venus and 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.
