When we say that Mars is the most earth-like of the planets, it isn't saying much. Compared with Earth, Mars is best described as an airless, toxic, frozen desert. Nonetheless it is still the only place in the Solar System apart from the Moon, where human beings could conceivably survive on a semi-permanent basis, using current technology and help from the Home Planet.

This module is concerned with Martian weather. Mars has weather, like the Earth, despite the very different atmospheric conditions. Mars has a very thin atmosphere, <1% Earth pressure, similar to our own at around 30,000 m (100,000 feet), and is made of 95% carbon dioxide, 2.7% nitrogen, 0.13% oxygen, with the remainder being constituted of inert gases, ozone and carbon monoxide. The atmosphere is very dry, with only 0.03% water vapour. With such a thin atmosphere, it rapidly becomes saturated, and so clouds can form. The temperature ranges from approx +20 C (70 F) to -110 C(-165 F), but the atmosphere is too rarefied to hold on to the heat, and so the perceived temperature would be much lower. Such conditions mean that liquid water cannot exist at the surface, so seas, or any bodies of open water, are an impossibility on present-day Mars. This has profound implications for weather on Mars.

Despite having no mountain ranges, there is a wide range in elevation on Mars: from the bottom of the Valles Marineris to the top of Olympus Mons is 26 km (16 miles). As pressure varies with altitude, this gives a variation of a factor of 10 in the range from the bottom to the top of the atmosphere. Mars is also subject to an annual drop in pressure of ~25%, because during winter, the temperatures are so low that a sizeable proportion of the carbon dioxide atmosphere freezes out at one or the other of the two poles, producing carbon dioxide frost over residual water ice caps. This also produces seasonal winds which lift the dust into planetwide dust storms, and give rise to a darkening of areas of the planet which was once interpreted as vegetation undergoing seasonal growth.

On average, Mars receives approximately 43% of the sunlight of Earth. However, Mars' orbit is so elliptical that the amount of sunlight reaching the surface during the closest and furthest approaches to the Sun, varies by a factor of two. Without any large expanses of open water to act as a reservoir for solar heat, the surface heats and cools quickly. Some areas heat and cool at different rates, due to the varied thermal inertia of the materials of which they are made. On Earth, the oceans circulate the heat to create a moderate temperature; on Mars, some heat is transferred by movement of the dust, which has its own thermal properties, and can cause reduced temperatures by cutting down the sunlight. The lack of water also simplifies the atmospheric circulation patterns. Broadly speaking, warm air rises over whichever hemisphere is having summer at the time and descends over the winter one.

The seasons are of radically different lengths because of the planet's orbital eccentricity.


Northern Season
Southern Season
Spring Autumn/Fall 194
Summer Winter 177
Autumn/Fall Spring 142
Winter Summer 156

The condensing of the atmosphere produces pressure differences, causing strong circulation towards the winter pole. In winter at high latitudes, the prevailing winds are westerlies and give Earth-like weather, with cyclones and anticyclones. Northern summers are quiet, with gentle east-west air flow; but southern summers produce dust storms, which are often big enough to drape the whole planet. Dust devils are common, and sometimes of great height. These seem to be the principal method of tranporting dust around the planet. The Pathfinder lander recorded and photographed several of these dust devils, which have also been seen from orbiting probes.

Clouds and fogs are common in the Martian atmosphere, and can be seen from Earth with even modest astronomical telescopes, as may be possessed by a school. We now know that white clouds are water-ice or carbon dioxide, while yellowish clouds are dust storms. The most visible and extensive clouds are over the north polar cap in autumn, and in the south in early spring. Hazes and fogs also form in the bottom of low-lying areas, such as the Valles Marineris. Clouds often form in the lee of major topographical features such as Olympus Mons and the Tharsis Ridge, when moisture-laden air is forced to rise over these features, reaching their peak in the afternoon.

There are also wide temperature differences even over very small distances. The Pathfinder lander discovered that within about 2 metres (6 feet), there can be a temperature difference of 20 degC ( F).

The students can look at Mars and compare its weather and prevailing conditions with those of the Earth. They can also look at the topography, although this is dealt with specifically in another unit. They could be asked to consider which features have the greatest effect on the creation of weather. Identification of cloud types on Earth, and then comparison with those in photos of Mars could be instructive, particularly if their formation and meaning is discussed. The practical project suggested for this module is the creation of a weather station for inclusion on a Mars lander, but this is not compulsory.

There has been much debate about the true colour of the sky on Mars. The images from the Viking landers of the 1970's suggested that the sky was pink, varying to yellow brown, particularly in the presence of dust storms. The interpretation of more recent images from the Pathfinder mission, however, suggest a much wider variation. The colour of the daytime sky being slightly blue or butterscotch, depending upon the dust content of the atmosphere. At sunrise and sunset deep purples, pinks and yellows are probably visible too.

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