Compared TO the Earth. Mars has a thin atmosphere and Venus & thick one whereas Mercury and the Moon do not have one at all. Table 10.3 lists some of the gases which we might expect to find in planetary atmospheres because they are volatile compounds of the cosmically common elements. If a planet ever gets an atmosphere from anywhere it will gradually lose it because some of the molecules are moving sufficiently fast to escape from the planet’s gravity. Molecules move faster at higher temperatures and at any particular temperature light molecules move faster than heavier molecules. It seems likely that Mercury and the Moon have lost any atmospheres they may once have had because of their low surface gravities and low escape velocities. In the case of Mercury there is also the high surface temperature which reaches a maximum of 700K on the equator at noon. Mars has an escape velocity only a little greater than Mercury’s but on account of its greater distance from the Sun and consequent lower temperature it has been able to retain an atmosphere. Hydrogen and helium are cosmically the most abundant elements but, because they also have the lightest molecules, none of the inner planets has been able to retain these elements in their atmospheres.

Accurate studies of how planets lose their atmospheres are made difficult because the atmospheric temperature, and hence the rate of loss, depends on the composition. For example, if the Moon once had an atmosphere we do not know what its temperature was and hence cannot predict how rapidly it would have been lost. We can however say that the present planetary atmospheres are being lost at rates that are so slow that they are generally negligible, even over a period as long as the age of the Solar System.

The presence of the various constituents has been established spectroscopically. The only detected major constituent for both Mars and Venus is carbon dioxide (CO2). In the case of Venus, Venera spacecraft have entered the atmosphere and shown that it contains 97 per cent carbon dioxide. This proportion is similar on Mars; nitrogen and argon, which have been detected by the Viking space¬craft, constitute about two per cent of the atmosphere each. A number of minor constituents have also been detected in the two atmospheres. One notable molecule not detected on Venus is oxygen, which must be at least 50 times less abundant than carbon monoxide. On Mars there are twice as many carbon monoxide molecules as oxygen molecules. Carbon monoxide is thought to be formed from carbon dioxide in upper atmospheres by the action of ultraviolet light from the Sun. The two gases are formed in the relative amounts seen on Mars. Oxygen is presumably formed in the same way on Venus but quickly carried lower in the atmosphere where it combines with some other substance, possibly sulphur which may be found in the clouds.

The clouds on Venus are so thick that they always keep the planet’s surface hidden from view. In visible light, the cloud tops are a featureless pale yellow but in ultraviolet light a complex pattern of bright and dark swirls is visible. Ultraviolet photo graphs of venus are customarily processed by computer to en¬hance the contrast since the light areas are only a few per cent brighter than the dark ones. Observations of the Doppler shift of the light from the clouds and of the motion of cloud features across the disc show that the clouds and upper atmosphere of Venus rotates in the same retrograde direction as the planet itself, but with the much shorter period of only four days. This period corresponds to wind speeds of about 100ms-1 (i.e. 360km per hour) at the equator. Measurements by softlanding craft have shown wind speeds at the surface of only one or two meters per second. Astronomers have not yet succeeded in satisfactorily explaining the high speeds observed in the upper atmosphere and indeed some have suggested that the observations have been misinterpreted and that the winds are much slower.

The variation of temperature and pressure with height in the atmosphere of Venus. The visible cloud features are at an altitude of about 60km but the cloud layer extends from about 80km down to 35km. This layer reflects or absorbs much of the incident solar radiation. Only about one per cent of the radiation reaches the surface and here it is mainly absorbed and re-emitted as long-wavelength infrared radiation. This is absorbed by the atmosphere and the clouds; it cannot directly escape back into space. Consequently the atmosphere has been heated up to give a surface temperature of about 750 K; this whole process is called the GREENHOUSE EFFECT. In the atmosphere below the visible clouds, heat is transported mainly by convection; this limits the temperature gradient to about 10 degrees per kilometer.

Spectroscopically, liquids and solids are more difficult to identify than gases and as a result the composition of the clouds is not known with certainty. The commonly held view is that the clouds are mainly droplets of concentrated sulphuric acid containing about one quarter water. Such droplets are white, not yellow, so it is likely that there are impurities as well. The ultra-violet markings can be explained by differences of a few per cent in the diameters of the droplets between the darker and lighter parts of the clouds. In the lower clouds the droplets may coagulate to form a rain which would be the most corrosive fluid in the Solar System!

Unlike the dense atmosphere of Venus, with its surface pressure of 90 atmospheres, that of Mars is very thin – only 0.006 atmospheres at the surface. The variation of temperature with height in the Martian atmosphere for three locations. Unlike Venus, Mars shows horizontal, as well as vertical variations in temperature. There is no greenhouse effect in the thin atmosphere so the average surface temperature is 230 K, below the freezing point of water. In general there is only a small amount of cloud, about five per cent of the surface being covered at any one time. At middle latitudes clouds of water-ice are seen; nearer the poles they are of solid carbon dioxide. The clouds are often associated with surface features. As the winds blow over a large crater, a pattern of LEE WAVES is set up. Where the air is rising it expands and cools and ice-clouds can form by condensation. Such clouds are also found downwind from terrestrial mountains. Nearer the equator localized condensation clouds are sometimes found. They form when air rises and cools as it passes over, highly elevated surface features.

Mars also has dust storms. Small storms are common but once every Martian year there is a great storm and a large part of the surface is obscured by dust in the atmosphere. The storm starts when Mars is close to perihelion. It normally begins at about the same place in the southern hemisphere and within a month almost all this hemisphere is obscured. Sometimes the storm spreads into the northern hemisphere as well and the entire planet can be shrouded by dust. The dust generally settles over a period of a few months. During a storm, the atmospheric temperature can rise by as much as 50K because of absorption of solar radiation by the dust. The storms appear to be produced by something similar to a terrestrial hurricane which pumps dust from the dry surface to an altitude of 20km whence it spreads out in a layer between 20 and 30km high.

The main external drive for the large-scale behaviour of the atmospheres of the inner planets is the larger amount of radiation received per unit surface area at the equator than at the poles. This causes a temperature contrast between the equator and poles although it is quite different for each planet. In terms of the mean
atmospheric temperature the contrast is less than 2 per cent for Venus. 16 per cent for the Earth and 40 per cent for Mars. Because of its large mass, the atmosphere of Venus responds very slowly to the influx of solar radiation and since the atmospheric motions can carry heat from the equator to the poles more rapidly than this there is very little horizontal temperature variation. The atmosphere of Mars has little mass so that it rapidly responds to the solar radiation and a larger temperature contrast is possible. For similar reasons there is little difference between day and night-time temperatures on Venus but a large difference on Mars. This large diurnal variation on Mars causes strong winds.

Volcanic activity on Venus, Mars and the Earth is likely to have released an atmosphere predominantly of carbon dioxide, water vapour and nitrogen. On the Earth the most abundant constituent, water, has condensed to form the oceans whereas carbon dioxide has reacted chemically with the oceans and surface rocks to leave nitrogen as the most abundant atmospheric species. The appearance of photosynthesising plants on the Earth about two billion years ago was responsible for the release of oxygen which now makes up about 20 per cent of the atmosphere. There is no evidence of similar life on any other planet, which explains the absence of major amounts of oxygen in the atmospheres of Venus and Mars. Judging by the amounts of carbon dioxide in their atmospheres we can see that Venus has had a release of gases by volcanic activity similar to the Earth, but that Mars has been much less active. There can have been little loss of carbon dioxide from either the Earth or Mars or loss of water from the Earth, whereas, although Mars has little water now, it has been estimated that 5000 kg mr2 have been lost to space. If this is correct, then the relative amounts of water and carbon dioxide emitted to the atmospheres were similar for Mars and the Earth. In contrast there is very little water on Venus compared to the amount of carbon dioxide. Why this is so is not certain but it seems quite likely that at the distance of Venus from the Sun the temperature was too high for water to condense from the primitive solar nebula so that Venus never had any crustal water to emit to the atmosphere. By contrast, carbon dioxide could condense chemically combined into carbonates which would release the gas when heated.

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