Mars Facts

Mars - fourth planet from the Sun

Mars is about 6700km in diameter, intermediate in size between Earth (12,756km) and our Moon (3,476km).
Mars orbits the Sun in 687 Earth days. A Martian day (sol) is a little longer than ours at 24 hours 39.6 minutes. The Martian year contains 669 sols.
The planet's axis of rotation is tilted at 25.2 °, a little more than Earth's so Mars experiences seasons comparable with ours but almost twice as long.
The orbit of Mars about the Sun is noticeably more elliptical than ours resulting in some 40% less heat when Mars is furthest from the Sun. Because of this, southern summers are shorter and warmer than northern ones. The axial tilt precesses on a 51,000 year cycle and because of this, in some 30,000 years time, northern summers will be warmer and shorter.
The tilt of the axis is also thought to vary considerably over very long periods. It may have reached 60 ° at different times in Martian history. This will have had a big effect on the temperature of the different parts of the Martian surface and hence its climate and surface features.
The distance from Earth to Mars varies considerably. Every 2.14 years Earth converges with Mars as they orbit about the Sun. When the two planets are at their closest it is called an opposition. During the March 1997 opposition the distance from Earth to Mars was about 100 million kilometres. The closest approach since observations began will take place in 2003 when Mars will be just over 50 million kilometres away. Exploration spacecraft are launched to take advantage of the times when Earth and Mars are closest together because the cost of getting to Mars varies according to distance.
The atmosphere of Mars is much thinner than Earth's with a surface pressure less than one hundredth of ours. It is composed of 95% carbon dioxide, 3% nitrogen and nearly 2% argon with very small amounts of other gases including water vapour. The atmosphere is very dry but, because of the low pressure, it is nearly saturated in water vapour. The atmospheric pressure varies somewhat throughout the Martian year depending on how much carbon dioxide is condensed at the winter polar ice-cap.
The polar ice caps expand and contract throughout the Martian year. They are composed largely of carbon dioxide (dry ice). The northern ice-cap can reach from the pole to 65 ° latitude in the northern winter but the longer, cooler winter ensures that the southern ice-cap is larger. The polar ice all but disappears during the summer although a core of water ice remains.
The gound temperatures at the Martian equator vary between minus 113°C at night to around 7 ° C during daylight. The air temperature is around minus 58°C
Ice crystal clouds occur high in the atmosphere of Mars particularly in the early morning and evening. They commonly form near the summits of very high volcanoes. Early morning freezing fog is common in the topographically low areas.
Dust storms are common seasonal occurrences particularly at high latitudes or where temperature gradients are sharp. They can be local or, occasionally, as in 1971, become planet-wide after several weeks. The dust can take months to disperse after the storm has blown out.
Mars has two small natural satellites ('moons') known as PHOBOS and DEIMOS, named from the horses 'Fear' and 'Dread' that drew the chariot of the Roman God of War. Both are very dark, cratered, irregular in shape and covered with a layer of impact debris. The spectral reflectivity of both suggests that they are small asteroids of carbonaceous chondrite composition which were gravitationally captured by Mars very early in its history.
- Phobos (top of page), about 22 km across, is the closest to Mars and is slowly spiralling in towards it. Unless it breaks up through gravitational stress it will collide with Mars in about 100 million years time to create a crater basin some 300km in diameter.
- Deimos (bottom of page) is only 14km across. Both Phobos and Deimos keep the same face towards Mars as they orbit the planet.

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