Giant planets, terrestrial planets and major satellites
Dr HH Mate
Contd from prev issue
Mars : The mass of Mars is approximately one-tenth that of the Earth, and hence significant differences in the internal structure are to be expected. There appears to be less of a density contrast between the core of Mars and that of its mantle, suggesting that the amount of lighter material allied with the core-say, possibly sulphur-is increased relative to the Earth. Because the planet is smaller, the temperature increases less rapidly with depth than in the case of the Earth, and hence Mars should have a somewhat more rigid outer mantle and crust than the Earth.
There is no indication that large amounts of continental drift have taken place on Mars. On the other hand, tectonic activity has clearly played a large role in the history of Mars, since the planet can be roughly divided into a hemisphere which is of pre-dominantly ancient and heavily cratered terrain, and another hemisphere which is of much younger and less heavily cratered material. The density of craters on the surface of a planet such as Mars, with so little atmosphere that incoming massive bodies are not significantly impeded in striking the planet, is a measure of the relative age of the surface which has been exposed to space. Since the cratering rate apparently fell off rapidly throughout the first few hundred million years of the history of the inner solar system, differences in crater density frequently represent age differences of some few hundred million years back in the heavy cratering epoch.
Mercury : Mercury has only about half the mass of Mars, but has several distinct planetary characteristics. The mean density of Mercury is very high, indicating that Mercury probably has an abnormally large core predominantly composed of metallic iron. There is much evidence of extensive tectonic activity, although, like Mars, the increase of temperature below the surface of Mercury probably occurs sufficiently slowly so that the crust and upper mantle are relatively rigid, and nothing resembling continental drift has probably taken place.
Mercury is a very heavily cratered planet, with the craters of a given size apparently having been produced by smaller projectiles than in the case of Mars. The reason is that the distance of Mercury from the sun such incoming projectiles tend to have higher velocities than they do near the orbit of Mars, so that the resulting impacts are more energetic.
Moon : Although the Earth’s moon is technically a satellite, it makes sense to describe it as a planetary body, and planetary scientists consider the twin bodies of the Earth and the Moon as interesting examples of the extremes of planetary physics ranging from relatively large bodies to relatively small but still chemically differentiated objects.
The Moon has a history which includes extensive episodes of melting and differentiation, much of which can be reconstructed on the basis of the returned lunar samples. The upper layers of the Moon, which is just over 1% of the mass of the Earth, are quite rigid, and there is no evidence for extensive horizontal motions of the structural units.
The moon is unique in the solar system in having a relatively low density among the inner planets, and at best a very small core, indicating that the planet is practically devoid of metallic iron. Relative to the Earth it is also highly depleted in the more volatile elements. This unsual compositional pattern presumably requires an explanation in the mechanisms which resulted in the formation of the moon, about which there has been much controversy.
Major Satellites : The four Galilean satellites of Jupiter-Io, Europe, Ganymede, and Gallisto have masses which are all roughly comparable to the mass of the Earth’s Moon. It is, therefore, quite clear that they should be considered as planetary bodies in their own right by planetary scientists. The detailed images of these satellites returned by the Voyager space craft which passed through the Jupiter system revealed them to be very interesting places with many rich, complex, and exotic properties.
The most spectacular of these planetary bodies is undoubtedly Io. This satellite has a surface characterized by large deposits of sulphur and sulphur dioxide, which is in a state of continual change. It appears to have at any time several active volcanoes, each of which is likely to be spewing a stream of gas and entrained rocky particles about 60 miles or 100 kilometres or so above the surface.
Such volcanic plumes spread the gases and rocky materials from the volcano over a considerable portion of the surrounding terrain. This vigorous tectonic activity is understood to arise from a combination of orbital perturbations of Io by the other Galilean satellites and tidal damping by Jupiter, which results in the dissipation in the interior of Io of very large amount of heat.