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Diameter6794 km, 0.53 x Earth's
Mass0.11 ME
Volume0.15 VE
Average Density3.94 g/cm3
Average Distance from Sun228 x 106km = 1.523 AU
Eccentricity of Orbit0.093
Inclination of Equator to Orbit25°
Inclination of Orbit to Ecliptic
ColourOrangey red
TemperatureDay: 22°C
Night: -120°C
AtmosphereVery thin carbon dioxide
Length of Day1.02 days
Length of Year687 days
No. of MoonsTwo
Surface PressureNegligible
Gravity0.38 x Earth's
Escape Velocity5.02 km/s

The Sojourner Rover from the Pathfinder mission Mars is the fourth planet from the Sun. It is thought to be the most likely place in the solar system to support extra-terrestrial life. Although Mars is now very cold and has little water, it is believed to have been much warmer and wetter in the past. There is no flowing water on the surface - instead it is trapped in the permafrost and the ice caps. The NASA spacecraft Pathfinder has also photographed a thin icy dew in the mornings.

Marsí equator is tilted to the plane of orbit by about 25 degrees. This means it has seasons like Earth, but because the year is twice as long, the seasons also last twice as long.

The surface of Mars is rocky and cratered with huge canyons across it. It is called the Red Planet because of the colour of the dust. This is an orangey-red colour because it contains a lot of oxidised iron (rust).

Some of the canyons that stretch across the surface were created by water that flowed across the planet thousands of years ago. The largest of these, however, was not created by water but by the cracking of the planet's surface. It is called Vallis Marineris and stretches for about 4000 km, or about the same distance as the width of the United States. It is so deep that if you dropped Mount Everest inside it, the mountain would just peek above the top. There are also ice caps at both the North and South poles, made of water and carbon dioxide. These can be seen to grow in the winter and reduce in the summer.

Vallis Marineris

Mars has the highest volcano in the Solar system, Olympus Mons. It is three times the height of Everest. Olympus Mons can only exist on Mars due to the low gravity, and if it were put on Earth it would collapse under its own weight.

The largest impact crater on the Martian surface is called Hellas Crater. The impact that caused it was so great that it sent shockwaves around the planet and caused a huge volcanic region on the other side called the Tharsis Mons.

Phobos (right) and Deimos (left) compared with the asteroid, Gaspra (top) Mars has two moons, called Phobos and Deimos. Both are thought to be captured asteroids. Phobos is the larger of the two and orbits very close to the planet. It completes one orbit every 7 hours, so if you were stood on the surface looking up, Phobos would cross the sky about three times a day. There is a huge crater on the moon called Stickney crater. This stretches about half way along the surface. The collision that created it was thought to have been so severe that it shattered the inside of Phobos, which may break up into a ring around Mars some time in the next few million years.

Deimos is the smaller moon and orbits much further away. The surface of the moon is very smooth because it is covered in a fine dust called "regolith". This was thrown up by the many impacts that can be seen on its surface, before settling down to fill in the deepest craters. Deimos spins once in the same time it takes to orbit, so always keeps the same face towards Mars.

A new mission to Mars is planned for launch in 2003, called the Mars Express. This will carry the Beagle 2 lander, which will search for signs of life on the planet. Beagle 2 is an ESA mission, which is partly being built here in the UK.

The Beagle 2 lander as it would look on the surface of Mars
All Rights Reserved Beagle 2

Planets Introduction




The Moon









Extra-Solar Planets

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Authors: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001