Planetary physics

(astrophysics) The study of the structure, composition, and physical and chemical properties of the planets of the solar system, including their atmospheres and immediate cosmic environment.

The study of the structure, composition, and physical and chemical properties of the planets of the solar system, including their atmospheres and their immediate cosmic environment.

Planetary scientists attempt to synthesize their information about the structure and properties of each of the planets by constructing models of them. The most obvious gross properties of the planet are its mass and its radius. In constructing a model, it is required that the model be in hydrostatic equilibrium. This means that at any interior point in the model, the pressure must be great enough to sustain the weight of the overlying mass of material. Thus, given the mass and the radius, estimating the interior pressures through the principles of hydrostatic equilibrium, and knowing something about the compressibilities of materials, it is generally possible to place constraints upon the interior composition of the planets. Knowledge of the equatorial bulge and the rate of spin provides additional information about the distribution of mass in the interior of the planet.

Classes of chemical composition

The planets in the solar system have an extremely wide range of properties. This distribution of characteristics can be understood in part from a knowledge of the more abundant elements in nature and their volatility properties. Approximately 98% of matter in the Sun, and therefore also presumably in the matter from which the Sun and the solar system were formed, consists of the gases hydrogen and helium. Most of the remaining material consists of carbon, nitrogen, and oxygen, which in the presence of very large amounts of hydrogen tends to form methane, ammonia, and water. These substances are collectively called ices, and they evaporate at relatively low temperatures. Both the light gases hydrogen and helium and the ices are of quite low abundance on the Earth and the other inner planets in the solar system. What constitutes the bulk of the material in these planets is the rocky material, constituting only about 3 parts in 1000 of the solar mix of elements.

The differences in the volatilities of these materials, which correlate with the properties of the planetary bodies in the solar system, give information about the properties of the environment in which the planets formed in the solar system. The inner planets, composed predominantly of rocks, evidently formed in a rather hot environment, so that the volatile gases and ices were not condensed and did not collect along with the rocky material, which presumably was condensed. The comets, residing at very large distances from the Sun in the solar system, appear to be mixtures of rocky materials and of the ices. The outer giant planets, Uranus and Neptune, appear to be primarily composed of materials heavier than hydrogen and helium, probably mixtures of rocky and icy materials. The two largest planets in the solar system, Jupiter and Saturn, are much closer in composition to that of the Sun itself, although studies of their interior structures tend to indicate that there is some degree of enrichment in the heavier elements. These differences in composition thus indicate that the tendency to collect hydrogen and helium depends upon the size of the body which has formed, the larger bodies being more successful in gravitationally capturing the elusive hydrogen and helium. See also Comet.

These compositional classes provide a natural means for dividing the planetary objects within the solar system into separate groups, as follow.

Giant planets

The giant planets are Jupiter, Saturn, Uranus, and Neptune. Nowhere in the interiors of the giant planets can anything resembling a solid surface be expected. The temperatures in the interiors tend to be thousands to tens of thousands of degrees Celsius. The pressures range up to the order of 10⁷ atmospheres (10¹² pascals) and higher. Under these circumstances all materials behave like fluids. There may be a certain amount of compositional stratification, with denser fluids underlying lighter ones.

Terrestrial planets

The terrestrial planets include Mercury, Venus, Earth, and Mars. The Earth's Moon may also be considered a terrestrial planet. The Earth consists of a thin upper crust composed of rocks of relatively low density and low melting points, overlying a much thicker mantle composed predominantly of metallic silicates and oxides, which in turn overlies a substantial core, which is composed of much denser materials, believed predominantly to be iron with other elements, either alloyed or in solution.

It is conjectured that the interior of Venus is probably much like that of the Earth, with a core, a mantle, and a crust. Major structural features on the surface of the planet suggest extensive tectonic activity. The extent to which the crust of Venus is subject to extensive continental drift motions is quite unknown. See also Venus.

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. 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. See also Mars.

Mercury has only about half the mass of Mars. 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 that the crust and upper mantle are relatively rigid, and nothing resembling continental drift has probably taken place. See also Mercury (planet).

The Moon has a history which includes extensive episodes of melting and differentiation. The upper layers of the Moon, which is only 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. See also Moon.

Major satellites

The four Galilean satellites of Jupiter—lo, Europa, Ganymede, and Callisto—have masses which are all roughly comparable to the mass of the Earth's Moon. 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. See also Jupiter.

The Saturnian satellite system contains only one satellite comparable in mass to the Galilean satellites, Titan. Titan has a significantly 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 Galilean satellites. The atmosphere is completely opaque, and hence it is not known whether Titan has surface relief.

Atmospheres

The temperature at the surface of a planetary body 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.

The only terrestrial planets with atmospheres are Venus, Earth, and Mars. Both Mars and Venus have atmospheres composed predominantly of carbon dioxide. The element of next greatest abundance in the atmospheres of Mars and Venus is nitrogen, which is also the predominant element in the atmosphere of the Earth. The next most abundant element in the terrestrial atmosphere is oxygen, which is maintained there predominantly as the result of the operation of life.

Magnetospheres

Some of the planets contain substantial magnetic fields; others do not. Within the inner solar system, the Earth possesses a relatively strong field, Mercury a relatively weak one, and if Venus and Mars contain significant intrinsic fields, they are sufficiently weak that they have not been confirmed. On the other hand, Jupiter and Saturn have very strong magnetic fields. The generation of planetary magnetic fields appears to depend upon a combination of planetary rotation with an inner convecting layer having significant electrical conductivity. See also Magnetosphere; Planet; Van Allen radiation.