 | Introduction |
| Meteorites |
| Asteroids |
| Comets |
| Giant Impacts |
Introduction
Meteorites, asteroids and comets are debris left over from the era of planetary formation. As such they provide valuable information about the nature of the solar nebula. In particular, we can learn about the age of the solar system and its physical and chemical characteristics during the era of planetary formation.
Meteorites
The solar system is filled with small particles called meteoroids, which constantly strike the Earth's atmosphere at speeds between 10 and 40 km/s. The friction between the atmosphere and the meteoroids causes them to heat up, glow and eventually vaporize as meteors streaking across the sky.
Sometimes a meteoroid is large enough not to be fully vaporized and a fragment reaches the ground. A meteoroid that reaches the ground is called a meteorite.
Sometimes Meteorites fall into two broad categories:
Iron meteorites made of solid chunks of iron and nickel.
Stony meteorites made of silicates.
Iron meteorites contain large crystals of nickel-iron alloys, which suggests they cooled very slowly, about a few degrees per million years.
Stony meteorites have been divided into three classes depending on how hot they became:
Carbonaceous chondrites, which contain glassy beads of rock, called chondrules, as well as volatile (easily evaporated) compounds. This class of meteorites could not have been heated much otherwise they would have lost their volatile compounds. Therefore, it is likely that they are the least altered of the stony meteorites.
Chondrites, which contain chondrules but no volatile compounds. Therefore, this class of stony meteorite must have suffered greater heating than the carbonaceous variety.
Achondrites, which contain neither chondrules nor volatile compounds. These meteorites must have suffered the most intense heating, intense enough to melt the chondrules.
Since meteoroids bombard the Earth constantly and the largest ones land as meteorites we are able to study them directly. An enjoyable way to do this is to observe meteor showers, like the Leonids shower (Nov. 14-19).
In a meteor shower the meteors appear to emerge from one direction in the sky. The meteors of the Leonids appear to emerge from the direction of the constellation Leo.
Meteor showers are convenient because they occur year by year. Moreover, some showers can yield up to hundreds of meteors per hour. From studies of meteoroids orbits astronomers have concluded that most meteoroids, and therefore meteors, are debris from comets. Basically, they are comet dust.
However, meteorites are larger and stronger than comet dust. Therefore, they most probably have a different origin. Indeed, by studying the orbits of those meteoroids that hit the ground astronomers have discovered that the orbits trace back to the asteroid belt, which lies between the orbits of Mars and Jupiter.
This provides evidence that meteorites are debris, not from comets, but from asteroids.
Asteroids
Asteroids are small planetary-like objects the vast majority of which lie in the asteroid belt. The largest is Ceres, which is just under one third the diameter of the Moon.
Ceres is spherical, but most asteroids have an irregular shape, being too small for gravity to have crushed them into a spherical shape.
In late 1990 the Galileo spacecraft flew by the asteroid Gaspra. As expected, from measurements of the fluctuation of reflected light from Gaspra the asteroid is irregular in shape.
Comets
Giant Impacts
Every so often the Earth and other planets of the solar system are struck by an asteroid or comet. Indeed, in 1994 the fragments of the Shoemaker-Levy comet slammed into Jupiter with a force of millions of megatons. We can estimate the kinetic energy of the impact using the formula
E = (1/2)mv2
where m is the mass of the object and v is its speed.
There is evidence that such catastrophic impacts may have led to mass extinctions, such as occurred 65 million years ago when the dinosaurs were wiped out.
The most recent major collision between the Earth and another object occurred in 1908 in Siberia. This impact is called the Tunguska event and is somewhat mysterious in that no crater was left by the impact, even though there is evidence that the impact must have been very energetic.