Summary of Chapter

The asteroids are small bodies, the great majority being confined to the space between Mars and Jupiter in the asteroid belt. Their orbits are prograde but on average somewhat more eccentric and more inclined than the orbits of the major planets. They have a total mass of the order of 1022kg, and there is the order of 109 bodies greater than 1 km across. Ceres, with a radius of 479 km, is by far the largest asteroid, containing the order of 10% of the total mass.

Beyond the asteroid belt there are near-Earth asteroids, the Trojan asteroids which are near the L4 and L5 points of Jupiter, and the Centaurs, with perihelia outwards from Jupiter's orbit and semimajor axes less than that of Neptune.

The asteroids nearer than Jupiter are thought to be the remnants of material in the space between Mars and Jupiter that the gravitational field of Jupiter prevented from forming into a major planet. Throughout Solar System history collisions in the asteroid belt have been common, and so the population has evolved considerably. There has also been a net loss of material, the present mass being only about 0.1% of the original mass.

An asteroid is categorised according to its reflectance spectrum and geometrical albedo. There are 14 Tholen classes (Figure 3.7), with about 80% of classified asteroids falling into the S class and about 15% into the C class. Class C dominates the outer asteroid belt and Class S the inner belt. A class C asteroid is thought to consist of an undifferentiated mixture of silicates, iron-nickel alloy, hydrated minerals, and carbonaceous materials. A class S asteroid has a surface that consists of mixtures of silicates with iron-nickel alloy. The population variations across the belt, and those seen in other classes of asteroid, could be the result of lower temperatures and weaker heating at greater heliocentric distances in the solar nebula.

Comets are small bodies that are distinct from asteroids in that they develop a coma, a hydrogen cloud, and tails when within 10 AU or so of the Sun. Studies of these huge structures indicate that the solid nucleus of a comet - typically only a few kilometres across - contains a significant proportion of icy materials, particularly water, in a loose aggregation with rocky and carbonaceous materials. The volatile materials are liberated by solar radiation to form the coma and hydrogen cloud, and then driven off by the solar wind and by solar radiation to form the tails.

The comets have a wide variety of orbits. Long-period comets have orbital periods greater than 200 years and enter the inner Solar System from all directions. About 1000 have been recorded. It is inferred that they are a very small sample of a cloud of 1012 - 1013 bodies greater than 1 km across, 103 - 105 AU from the Sun, called the Oort cloud. This cloud, with a present day mass of about 1025kg, is thought to consist of icy-rocky planetesimals ejected into large orbits from the giant planet region during the formation of the giants, and during any subsequent giant planet migration.

Short-period comets have orbital periods less than 200 years. Most of them have periods less than 20 years and orbital inclinations less than 35°. These are the Jupiter family comets. A few hundred are known. It is inferred that most of them (the Jupiter family comets) are a sample of the Edgeworth-Kuiper (E-K) belt, planetesimals left over because of ineffective accretion in the solar nebula beyond the giant planets, plus a proportion scattered out by the giant planets, particularly Uranus and Neptune. The remaining short-period comets, typically in higher inclination orbits, are the Halley family comets. These might in some cases be samples of the inner Oort cloud. The Centaurs could be the larger members of a population in transition from the E-K belt to the family of short-period comets.

Meteorites are small rocky bodies that survive passage through the Earth's atmosphere to reach the Earth's surface. The three classes are stones, stony-irons, and irons. Stones account for about 95% of the meteorites observed to fall to Earth. Most of them contain silicate chondrules that define the subclass called chondrites, the remainder being achondrites. About 6% of the chondrites are carbonaceous chondrites, primitive bodies that contain not only silicates, but also hydrated minerals and carbonaceous material. Radiometric dating shows that the oldest components of meteorites - the calcium-aluminium inclusions and the chon-drules - solidified 4570 Ma ago. These are the oldest ages obtained in the Solar System, and are taken to be the Solar System's age. A small number of meteorites have come from Mars and the Moon. Micrometeorites are largely derived from the rocky component of comets.

The composition of the particularly primitive C1 chondrites matches that of the observable part of the Sun, except for elements that are volatile, or reside predominantly in volatile compounds. These are greatly undersampled in meteorites. The C1s have enabled the relative abundances of many elements in the Solar System to be significantly refined.

Spectral reflectances of asteroids and meteorites show matches between

• the carbonaceous chondrites and class C asteroids;

• the iron meteorites and class M asteroids, which are the iron-nickel cores of (partially)

differentiated asteroids;

• most achondrites and the rare V class asteroids, which are from the silicate crust of fully differentiated asteroids, of which Vesta is the most prodigious parent;

• the ordinary chondrites and S class asteroids.

A least some stony-irons are thought to come from the core-silicate mantle interfaces of (partially) differentiated asteroids.

4 Interiors of Planets and Satellites: The Observational and Theoretical Basis

Our understanding of planetary and satellite interiors is considerable, but will always be limited by their inaccessibility - we have to rely on external observations. These are made by telescopes on the Earth or in Earth orbit, and by instruments on spacecraft in the vicinity of the planetary body. For Mars, Venus, the Moon, Titan, and of course the Earth, we also have observations made at the surface of the body. For the Earth and the Moon we have additionally sampled materials from below the surface, though for the Moon only the upper metre or so has been sampled. Even in the case of the Earth, the deepest samples are from only about 100 km -less than 2% of the distance to the centre, and brought to us by volcanoes. Nevertheless, good models of planetary and satellite interiors have been developed.

The basic features of a model are a specification of the composition, temperature, pressure, and density, at all points within the interior. Planets and large satellites are close to being spherically symmetrical, so, at least as a first step, this reduces to a specification of properties versus radius from the centre, or, equivalently, versus depth from the surface. Not all of the features of a model are independent. For example, the density of a substance depends on its pressure and temperature. Some of the relationships between various features are poorly known.

A model will embody certain physical principles. For example, if the material at some depth is neither rising nor falling then the net force on it in the radial direction must be zero. With such principles the model is then used, with initial depth profiles of the various features, to derive properties that are observed externally, and the depth profiles are varied until an acceptable level of agreement with the actual observations is obtained.

We shall now look at the main types of external observations that are available for modelling planetary and satellite interiors. Table 4.1 lists many of the spacecraft missions that have made particularly important contributions.

0 0

Post a comment