Giant planets, terrestrial planets and major satellites

Dr HH Mate
Contd from prev issue
The Galilean satellites appear to represent a composition class which is slightly more volatile-rich than the pure rocky materials, characteristic of the inner solar system. In particular, the sulphur content is likely to be considerably higher. The water and carbonaceous contents may also be much higher. If Io ever had much water, it appears to have been lost from the body quite early in its history. It has been suggested that Io has a sufficiently large reservoir of sulphur that this may form an effective fluid layer, or ocean, underlying the solid surface crust.   
Europe, Ganymede, and gallisto all appear to have outer crusts composed of water ice. There are a variety of surface markings which indicate a history of cracking, cratering, and in some instances renewal of the ice surfaces. The mean density of Europa is sufficiently high that the planetary body is probably primarily composed of rock.
On the other hand, both Ganymede and gallisto have a significantly reduced density, suggesting that quite thick layers of an icy mantle are likely to be part of those planetary bodies. It would be surprising if the amount of tidal and radioactive heating in the interiors of these bodies was sufficient to maintain a substantial portion of this icy mantle in the form of a liquid brine. Nothing is known about the character of the underlying rocky core.
    The Saturnian satellite system contains only one satellite comparable in mass to the Gallilean satellites. Titan has a significant higher volatile content than the Galilean satellites. It has an extensive atmosphere, virtually unique in the solar system, largely composed of methane. It is not known what lies at the bottom of this atmosphere, but it has been reasonably speculated that there is a transition layer of heavier hydrocarbons. The satellite has a relatively low density, characteristic of an extensive content of ices, quite likely more than just water ice as in the gallilean satellites. The atmosphere is completely opaque, and hence it is not known whether Titan has surface relief.
    Oceans and atmospheres : With the possible exception of the Gallilean satellites, the Earth is the only planet in the solar system having oceans. Mars is too cold for substantial bodies of liquid water to exist upon its surface, although there is evidence in its surface features that water once ran through a number of channels for atleast short periods of time.
 Mars also has a substantial amount of ice in its polar caps. The atmosphere of venus is sufficiently hot that if any liquid water were to be placed upon its surface, it would quickly be evaporated into steam.
Venus contains very little water in its atmosphere, giving rise to the question of whether Venus has ever had substantial amount of water at any time in its past, or whether it has found some mechanisms for getting rid of the bulk of it. Both the Moon and Mercury are very dry.
    There are certain general principles which govern the structure and dynamics of planetary atmosphere. In most cases these atmospheres receive their primary heat input from above, resulting from  heating due to the sun. Most atmospheres contain some form of haze or condensed layer in the form of clouds, which results in a reflection of a portion of the incident sunlight back into space where it has contributed to the deposition of heat within the atmosphere. The remainder of sunlight is either absorbed within the atmosphere or transmitted or scattered downward to the ground where absorption takes place. The heat thus received by the ground must be reradiated into the atmosphere, which will transmit some of it and absorb some of it. The absorbed radiation from the ground will in part be re-radiated by the atmosphere toward the ground, adding to the heating effect that has taken place as the result of the original receipt of the corresponding energy. This enhancement of the heating effect is commonly called the green house effect, despite the fact that the mechanism by which a greenhouse keeps warm is somewhat different.
    The temperature at the surface of the planetary body therefore depends in a complex manner on the properties of the overlying atmosphere, as well as upon the distance of the planet from the sun. The atmosphere of Venus is very much hotter relative to the Earth than would be expected purely on the basis of the relative distances from the sun. The difference appears to arise from the extensive operation of the greenhouse effect within the very thick atmosphere of Venus.
 Russian spacecraft landed upon the surface of Venus had found quite large amounts of illumination by sunlight there, indicating that significant amounts of solar energy do manage to penetrate to the ground of that planet. The only terrestrial planets with atmospheres are Venus, Earth and Mars. Both Mars and Venus have atmospheres composed predominantly of carbondioxide.
The writer of this paper is a researcher, educationist, sociologist and author of books. He can be reached at [email protected]