12. Asteroids and meteorites


Space rocks

Most meteors, or shooting stars, are produced by tiny fragments of comets, which burn up in the air and never reach the ground, but occasionally a stone will fall from the sky, producing a brilliant trail of light flashing across the night sky. A rumbling sound and what appears to be a great burst of sparks may accompany it. These are fireballs and they are produced by tougher chunks of matter from space, resembling rocks.

Fig. .. 

Extraterrestrial chunks of rock and metal that survive the fiery descent through the atmosphere and reach the ground have been given the name meteorite. And strictly speaking a meteoroid is the solid object in space that appears as a meteor when it lights up in the atmosphere and becomes a meteorite if it reaches the ground.

The Antarctica lode

Fig. .. 

In 1969, a group of Japanese scientists discovered a bountiful source of meteorites on the blue ice fields at the bottom of the world, leading to a dramatic increase in the number of recovered meteorites. During the next three decades, about 20 thousand cosmic rocks were harvested from the Antarctic ice. The most productive areas were near the Allan Hills in Victoria Land and the Yamato Mountains in Queen Maud Land.

Typical meteorites

Fig. ..  Fig. ..  Fig. .. 

Most meteorites that have been seen to fall and then recovered are stones, rather than chunks of metal. About 94 percent of the fallen meteorites are stones, 5 percent irons and 1 percent stony-irons. About 90 percent of the stony meteorites are, in turn, classified as chondrites. So most of the meteorites that fall to Earth are chondrites. The name “chondrite” is derived from the ancient Greek word, chondros, meaning “grain” or “seed”. The other 10 percent of the stony meteorites are achondrites, which show signs of past igneous activity.

Rare and exotic finds

Fig. .. 

The frozen cargo of the Antarctica ice includes at least a dozen, greenish-brown meteorites that are strikingly similar to the welded highland rocks from the Moon. The abundance of various elements and gases in these meteorites are virtually identical to those found in lunar rocks; at the same time, they are unlike those found in any other known meteorite or terrestrial rock. These small stones were blasted off the Moon by impacting objects.

Out of the thousands of stony meteorites now found in terrestrial collections, roughly a dozen are believed to be pieces of Mars. They were blasted into space by impacting objects, with such force that they escaped the red planet’s gravitational pull and eventually reached Earth. One of them, dubbed ALH (for Allan Hills in Antarctica) 84001 contains controversial evidence for ancient microscopic life on Mars.

The asteroid-meteorite connection

There is little doubt that most of the meteorites have come from the asteroid belt. They are probably chips off wayward asteroids, and there are two pieces of direct evidence for this conclusion.

Fig. .. 

1. Orbits

Photography of meteorites as they descend through the Earth’s atmosphere can be used to determine their precise speed and direction of motion when they encountered the Earth. From these data, their orbits may be inferred, and many of the objects came from space beyond Mars, in the main belt of asteroids.

2. Colors

The surface composition of asteroids has been inferred by breaking down their reflected sunlight into its component colors. Such spectral displays are similar to those of meteorites, suggesting that the meteorites are the debris of colliding asteroids. The relative abundance of asteroid types are not like those of the fallen meteorites, but this may simply reflect the ease or difficulty in sending asteroid fragments to Earth.

In addition, there is some compelling indirect evidence

3. Crystalline structure

Fig. .. 

When the majority of iron meteorites are cut and polished and then are etched with acid, a delicate and complex pattern emerges. It is produced by regions of crystalline structure, depending on the local orientation of the crystals in the iron. The sizes and shapes of these crystals indicate that they grew very slowly, and that the meteorite must have been hot, almost to the melting point, for tens of million of years. It probably cooled at the rate of a few degrees in a million years.

Fig. .. 

So the crystalline patterns of meteorites also suggest an asteroid-meteorite connection. Additional suggestive evidence is the relationship among the sizes of asteroids, meteorites, and non-cometary meteoroids. The classes are not mutually exclusive, and there is considerable overlap. The simplest explanation of all the evidence is that meteorites are the debris of collisions among the asteroids.

Fig. .. 

(Summary Diagram)

(page 7 of 7)

Copyright 2010, Professor Kenneth R. Lang, Tufts University