Passage IV

NATURAL SCIENCE: This passage is adapted from the article “On Impact" by lan Frazier (@2007 by Conde Nast Publications).

A meteorite is a meteoroid that falls to earth. Most meteoroids don't, but burn out in the atmosphere as meteors, or shooting stars. Given the Earth's surface composition, meteorites have a seventy-one-per-cent chance of landing in water. Those odds, plus the atmospheric filter, keep the number of known meteorites down. Meteorites are given names based on the place of their discovery. Cambridge University publishes a catalogue that lists twenty-two thousand five hundred and seven meteorites, a number that includes the nearly twenty thousand, most of them small, that have been found in the cold desert of Antarctica and the hot deserts of Africa. Visibility is, of course, a factor; in some states and countries, no meteorite has ever been found.

Most meteorites come from the asteroid belt, about a hundred million miles away. The asteroid belt is made up of fragments left over from the formation of the solar system; these fragments, some of them hundred of kilometres in length and some with their own moons, occupy a solar orbit between Mars and Jupiter. The big fragments are called asteroids. Scientists sometimes refer to the asteroid belt as the junk yard of the solar system, and, as in any junk yard, things rattle around Collision, the process by which the planets were assembled, has slowed in the intervening billions of years, but it is still going on. Objects crash into one another, collisions lead to other collisions; as fragments fly into new orbits, those orbits approach Earth's, coincidences happen, and gravity pulls the fragments in.

Meteorites are pf three kinds: stones, irons, and stony irons. Each corresponds to a different part o planet formation. The stones resemble pieces of planetary crust and mantle, the irons the planet cores, and the stony irons the transitional area between core and mantle. In some stony irons, translucent crystalline structures interpenetrate with opaque mantle rock. Collectors sometimes cut stony irons into thin slices and illuminate them from one side for a gorgeous stained glass effect. Only one per cent of all meteorites are stony irons. Five per cent are irons, and the remaining ninety-four per cent are stones.

There are also meteorites from Mars. In the nineteen-seventies and eighties, several of them were found in Antarctica. Comparison with information about Mars rocks sent back by the Viking spacecraft in 1975 confirmed these meteorites' origin. Evidently cratering impacts on Mars a hundred million to three hundred million years ago knocked pieces clear of that planet's gravity, and they later ended up on Earth after the pattern of asteroid-belt meteorites. In the nineteen nineties, astrogeologists examining the Mars meteorites found what seemed to be traces of primitive bacterial forms of life. This made news because of the speculation that life on Earth could have begun with life forms arriving aboard a meteorite. Since then, further investigation has shown that the intriguing traces are probably not related to any kind of life.

Similarly, a number of meteorites have been identified as having come from the Moon. Chemical matchups with lunar rocks brought back by the Apollo astronauts provided the proof. Among collectors, lunar and Martian meteorites are the most highly prized by far. Asteroid-belt meteorites sell by the gram; lunar and Martian meteorites sell by the milligram.

The fact that fragments of Mars have wound up on Earth does not imply, in all likelihood, that the reverse is true. Probably there are no Earth meteorites on Mars. The reason for this is that the general gravitational flow of the solar system is toward the sun; for pieces of Earth to reach Mars they would have to travel upstream, gravitationally. Chances are greater that Earth meteorites have reached Venus and Mercury, because those planets are between the sun and us. Of course, we can be sure that certain objects of earth origin are now on Mars, because we shot them there——including the aforementioned Viking, the Mars Pathfinder in 1997, and in 2003 the Mars rovers Spirit and Opportunity. On the Moon, our manned and unmanned missions have left all kinds of relics of ourselves——cameras, nuclear-fuel casks, discarded plastic rock-sample bags, golf balls, a color photograph of the astronaut Charles Duke and his wife and their two children, and many astronaut footprints, preserved for ever in the Moon's absence of atmosphere. The bulk of what we launch into space orbits the Earth for some unspecified period of time and then comes back down.