asteroids and comets Asteroids
The asteroids are irregularly shaped rocky or iron-rich bodies, most (but not all) of whose orbits lie between those of Mars and Jupiter. The largest is about 900 km across, and they range downwards in size with apparently no lower limit (Fig. 1). About 5000 are known that exceed a kilometre or so in size. Most are too small to have undergone differentiation, but the few largest ones may have experienced partial melting to the extent of having surface flows of basalt lava very early in the history of the Solar System. The supposed heat source was either the decay of short-lived radioactive isotopes, such as aluminium-26, or electromagnetic induction.
Once thought to be the remnants of a planet that broke apart, the asteroids are now regarded as
planetesimals that were never able to come together to form a planet because the gravitational disturbance caused by Jupiter made mutual collisions so energetic that fragmentation was just likely as accretion.
A few asteroids have orbits that cross that of the Earth, and collisions with such Earth-crossing asteroids are responsible for many of the impact craters found on the Earth.
Meteorites
Meteorites are chunks of material that are collected after they fall to Earth. On entry into the Earth's atmosphere, typically at speeds of tens of kilometres per second, the surface of a meteorite is heated to incandescence by friction. Freshly fallen meteorites are thus often recognizable by the black, glassy surface that results, but some incoming bodies break into several fragments before reaching the ground. Only objects more than a few metres across hit the ground fast enough to produce an impact crater. Towards the other end of the scale, most
meteors are sand-sized objects that are completely vaporized by the heat of their passage into the atmosphere. Their demise is marked by a visible ‘shooting star’.
Almost all meteorites are collisional fragments of asteroids, and can be dated radiometrically to about 4.6 billion years (4.6 × 10
9 years, 4.6 Ga), which represents the birth of the Solar System. Most are stony, and of these the majority consist of
chondrules, which are globules of silicate minerals, embedded in a finer-grained matrix. These meteorites are therefore referred to as
chondrites. The abundances of the non-gaseous elements in some chondrites match the elemental abundances in the Sun, and these objects are reckoned to be among the most primitive (least altered) material in the Solar System. Because of their relatively high carbon contents they are known as
carbonaceous chondrites. Many chondrites contain in their matrices tiny grains whose isotopic composition attests to their pre-solar origin; for example, silicon carbide and diamond believed to have been carried from the atmosphere of giant stars by stellar winds. Other
stony meteorites are basaltic, having clearly crystallized from a melt, and these provide evidence in favour of early melting events among the larger asteroids.
Iron meteorites are distinctive in appearance. They consist of alloys of nickel and iron, and are taken to represent samples of the core of a large asteroid that was broken apart by a collision.
Stony-iron meteorites contain mixtures of the two components, and are interpreted as having originated near the core–mantle boundary of their parent bodies.
A few meteorites have been identified whose ages, textures, and isotopic compositions match those of rocks collected from the Moon. They are evidently fragments of ejecta thrown out by crater-forming events on the Moon at sufficient speed to escape the lunar gravity. Even rarer meteorite types are believed to have come from Mars in the same way. These
martian meteorites are fine- or coarse-grained basaltic rocks, showing variable amounts of alteration caused by the passage of aqueous fluids. The crystallization ages of most of these meteorites fall in the range 0.2–1.0 Ga, but the oldest sample (ALH84001), on which most of the arguments for life on Mars are based, is much older, having been dated at about 4.3 Ga.
Comets
Comets represent fragments of primitive material from the outer Solar System in the same way that asteroids represent fragments of material from the inner Solar System. Because they were formed further from the Sun, comets are mostly ices of various sorts, with small proportions of rocky and sooty dust. Most are only a few kilometres or tens or kilometres across.
The majority of comets spend most of their time in the Oort cloud, which is a spherical shell some thousand bil-lion kilometres from the Sun (far beyond Neptune's orbit at 4.5 × 10
9 km). When a comet passes through the inner Solar System, heat from the Sun vaporizes some of the ice, liberating gas and dust to form a tail, shining by reflected sunlight, that may be hundreds of millions of kilometre in length. The tail is pushed outwards from the Sun by the solar wind and radiation pressure, and so the popular conception that a comet's tail streams away behind it is wrong.
The orbit of a comet that passes close to Jupiter or another of the giant planets may become shortened into an ellipse with a period of a few years or hundreds of years. The comet then becomes a periodic comet, such as Halley's comet (Fig. 2), whose successive close passes near to the Sun rapidly deplete its supply of volatiles so that its fate is to become an inert, dusty lump. This being so, the distinction between comets and asteroids is not so clear-cut as it seems at first sight. Many of the small asteroid-like objects orbiting beyond Jupiter may well be most appropriately thought of as dead comets, as may be the small irregularly shaped satellites of the outer planets.
Collisions by comets are responsible for producing the impact craters that are visible on the surfaces of the solid bodies in the Solar System that were not formed by asteroid impacts. Comets may hit giant planets too, as was the case when comet Shoemaker–Levy 9, previously torn into fragments by Jupiter's tidal forces, struck the planet in 1994.
Kuiper-belt objects
Many icy bodies other than Pluto and Charon are now known to circle the Sun beyond the orbit of Neptune in what is known as the Kuiper belt. The largest are only a few hundred kilometres across, and most are much smaller; they are therefore exceptionally difficult to study from Earth.
David A. Rothery
Bibliography
Beatty, J. K., Peterson, C. C., and Chaikin, A. (eds) (1999) The new Solar System (4th edn) Sky Publishing Corporation and Cambridge University Press, Cambridge.
Rothery, D. A. (2000) Teach Yourself Planets. Hodder and Stoughton, London.
