Diameter: 4,221 miles (6,794km).

Mass: 0.11 Earth masses

Rotation period: 1.03 days.

Inclination of equator to orbit: 25.2 degrees.

Mean orbital speed: 15 Miles per second (24.1 km/s)

Mean distance from the sun: 142 million miles (228 million km)

THE STRUCTURE OF MARS

Mars is often cited as being the most Earth like planet in the solar system – it has basins, plains and highland regions that are recognized as continents. It is tilted on its axis to a similar degree to earth, which endows the planet with four seasons. Its day is 24 hours and 37 minutes long. Like Earth, Mars has polar ice caps and it retains some semblance of an atmosphere...but that is where the similarities end.

The Martian atmosphere is thin – equivalent to an altitude of 140,000 feet on Earth, and consists of 95% carbon dioxide. Although carbon dioxide is a greenhouse gas which helps keep a planet warm, Mars' atmosphere is so tenuous it retains little solar heat. As well the planets highly elliptical orbit accentuates seasonal differences, and as a consequence the climate is extremely harsh, with temperatures ranging from –193F in the polar winter to 62F in the southern summer. At some locations on Mars it is cold enough for Carbon Dioxide to freeze out of the atmosphere as dry ice. Furthermore its surface is dry and desolate, with no eco system or oceans.

Four and a half billion years ago Mars was a very different planet..it had a thicker atmosphere and enough heat left over from its formation to melt water ice at the planets water. Judging from the main rivulets, channels and canyons that have been seen by space probes, and by analysis of the composition of Martian meteorites found on earth, abundant water must once have flowed across this planets surface. Where is the water now? Well water still exists on the planets polar caps and atmosphere. In 2000 planetary scientists analyzing images from NASA's Mars Global Surveyor space craft reported gullies in crater walls that suggest ground water may lay at depths of 330–1,300 feet. Other space craft findings pointed to thick deposits of sedimentary rocks, possibly laid down in now vanished lakes and oceans.

Mars consists mainly of silicate rock and it retains a modest metal core. The surface is a desert of rusty rocks and dust racked by intense surface winds that on occasion whip up immense dust  storms. At other times, particularly as the planet approaches perihelion when it is closest to the sun, the storms take on global proportions, engulfing the entire planet for months at a time. Such a storm occurred in 1971 at the beginning of the Mariner 9 mission, and again in 1977, during the Viking mission. The Mars pathfinder space craft has also seen evidence of recent dust storms.

This planet has some spectacular features, one of the most prominent being Olympus Mon's, potentially the largest volcano in the solar system. It rises 13 miles above the surrounding plains on the planets western hemisphere, and covers an area as a large as Arizona. Three other large volcanoes lie at the south east in a region of ancient volcanic activity called the Tharsis bulge, or ridge. The Valles Marineris, just south of the equator, is also remarkable. This system of canyons up to four miles deep forms an immense gash stretching some 2500 miles across the planet. Scientists think that activity in the Tharsis bulge broke open the crust and widened the canyon as ice washed out of the canyon walls.

                                                 THE MOONS OF MARS

Mars' two moons are tiny and may in fact be asteroids captured by the planets gravity. The larger of the two, Phobos, is a potato shaped body measuring 8.4 miles across and orbits Mars every 7.7 hours. This moon is closer to its primary than any other moon in the solar system, and orbits Mars below the synchronous orbit radius, thus it rises in the west, moves very rapidly across the sky, and sets in the east, usually twice a day. In fact it is that close to the Martian surface that it cannot be seen above the horizon from all points on the surface of Mars. Further news on this moon is that quite simply its doomed....because its orbit is below synchronous altitude, tidal forces are lowering its orbit, and in about 50 million years it will either crash onto the surface of Mars or break up into a ring.  The soviet space craft Phobos 2 detected a faint but steady outgassing from the moon, unfortunately Phobos 2 went offline before it could determine the nature of the material, yet water may be the likeliest answer. The most prominent feature on Phobos is the large crater named Stickney. Like Mimas' crater Herschel the impact that created tickney must have almost shattered Phobos.                                                                                 

Even smaller is Deimos, lying two and a half times further out than Phobos. This chunk of rock orbits Mars in just 1.26 days. Both of these moons may be composed of carbon rich rock like c–type asteroids, but their densities are so low that they cannot be pure rock. They are more likely composed of a mixture of rock and ice. Both are heavily cratered, and images from the Mars Global Surveyor indicate that Phobos is covered with a layer around a meter thick.

SATELLITE

DISTANCE (KM)

RADIUS (KM)

MASS (KG)

Phobos

9000

11

1.08e16

Deimos

23,000

6

1.80e15