METEOROID DEBRIS
As much as 1000 tons of cosmic debris-billions of microscopic particles- pepper the earth daily. We are aware only of those weighing a significant fraction of a gram, which produce the so-called shooting stars that flash across the sky. All but a few are too small to leave luminous trails. These solid particles are called meteoroids before they encounter the earth. Those large enough to survive flight through our atmosphere and land are called meteorites. And the luminous trails of the smaller particles that are completely vaporized in the atmosphere are called meteors. In order of increasing size and brightness, meteors are classified as (1) telescopic and radio meteors, (2) visual meteors, and (3) fireballs, or bolides.
Our atmosphere slows incoming meteoroids and transforms their kinetic energy into radiant and thermal energy. A meteoroid passing through the atmosphere leaves a wide, dense column of electrons stripped from the atoms and molecules in its path. As the ionized atoms regain their electrons, they deexcite, emitting photons that make the momentary luminous trail we see from the ground as a meteor, or shooting star.
Anything that remains of the meteoroid slowly filters down through the air as dust and solidified droplets of melted meteoroid.
When meteoritic particles encou nter the earth, they are moving anywhere from about 10 up to 72 kilometers per second, depending on their direction and the angle at which they stri ke the earth. The velocities convince us that meteoroids belong to the solar system, moving in independent orbits around the sun.
The normal observed rate for meteors is about 10 per hour over the entire sky. Why do we see fewer meteors before midnight than after midnight? During the even ing hou rs we are on the back side of the earth, facing the direction opposite to earth's orbital motion, and we see only the swift meteoroids overtaking us from the rear. During the morning hours earth's rotation has turned us so that we are facing in the same direction as its orbital motion. Hence we see those meteoroids that we overtake and those that meet us head on.
RECOVERED METEORITES
Most meteorites are discovered accidentally years after they fall. Of some three dozen meteorite falls weighing more than a ton only a few were seen descending. Not many falls are ever recovered: Most meteorites land in the oceans or in unoccupied places, where their fall is not likely to be observed. No known record tells of a community destroyed or an individual killed by a meteorite in spite of some close calls. Approximately 3000 meteorite specimens have been recovered and catalogued for study.
Meteorites striking the earth have probably formed thousands of craters, but only 200 or so have been found. One great collision in 10,000 years is a conservative estimate, and at that rate at least 50,000 giant meteorites must have fallen on the earth in the past 500 million years. But the fossil craters left by many of these may lie buried and unnoticed in the earth's crust. Probably most of them have been obliterated by weathering, erosion, and geological processes.
One that we know about, near Winslow, Arizona, is the Barringer meteorite crater, created by a meteorite weighing at least 30,000 tons. It struck the earth about 24,000 years ago and must have devastated all plant and animal life within a large area. The crater is over 1 kilometer across. Thirty tons of shattered iron fragments have been picked up within about 6 kilometers of the crater.
At 7 A.M. on June 30, 1908, a tremendous fireball flashed across the sky in Siberia. A great fall of flame brighter than the sun was seen leaping from a forested region near the Tunguska River. The sight of the fire was followed by the sound of an explosion powerful enough to level trees within 50 or so kilometers. Earth tremors were recorded on seismographs throughout Europe yet no large crater was formed, only many small ones. The most plausible explanation for the event is that a small comet (possibly part of comet Encke) or a large, fragile, stony meteorite struck the earth, dissipated its kinetic energy on the forest and the ground, and completely vaporized.
Three classes of meteorites have been established based on their chemical and metallurgical properties:
also These generally have a relatively smooth, brown or grayish, fused crust indented with pits and cavities. Buried inside all but a small fraction of them are small pieces of glassy minerals, called chondrules, that apparently formed from molten droplets, presumably during the formation of the solar system.
- One subgroup of the stones is the carbonaceous chondrites, which contain large amounts of carbon, water, and other volatiles that would have been driven off with the slightest heating above about 500 K. Therefore these are the most primeval samples of matter from the early solar system that we have. They are doubly interesting because they contain organic compounds, such as hydrocarbons, amino acids, and lipids. These biologically important compounds evidently formed in the primordial solar nebula without the assistance of living organisms.
- Stony-iron meteorites are a mix of stone and iron. Their brownish crust sometimes contains pockets of the yellow mineral olivine. Inside the meteorite the iron may have a veinlike or globular structure.
- Iron meteorites are almost exclusively composed of iron, with some nickel. They are easily identified by their characteristic pitted, brownish exterior and high density. Cut, etched, and polished, they usually have a peculiar crystalline pattern unlike any in terrestrial iron. They show evidence of melting and signs of other heating and cooling processes.
Stones are the most brittle kind of meteorite, and they are more fragile than the irons. Even though most falls are stones, more of the recovered meteorites are irons because they are relatively easy to identify and they resist weathering.
Those meteorites that have been dated by thei r natural radioactivity average tens of millions of years for the stones and 600 million years for the irons. These are their ages only since the breakup of the larger mass of which they were probably a part. The most ancient specimens are about 4.6 billion years old, the same age as the earth. The chemical and mineralogical sequences in the different classes of meteorites indicate that they share the same heritage as that of the rest of the solar system.
We are still not sure of the origin of meteorites. Are they the remains of comets? Perhaps, but the supporting evidence for this idea is not strong. Another line of speculation is that most meteorites may be descended from a few chemically differentiated asteroids, whittled down by repeated collisions early in the planetary system's history. In such a case stony meteorites come from the original crusts, the stony irons from the intermediate parts, and the irons from the core. Regardless of our ability to understand their origins, it is evident that asteroids and meteorites are representatives of the unused building material from which the terrestrial planets formed at the birth of the solar system.
METEOR SHOWERS
Several times a year we can see meteor showers, the swarms of shooting stars that dart from a small area in the sky. These showers can persist for hours or days. On such occasions the earth is passing through a large group of particles moving in ribbonlike fashion along an orbit around the sun. Perspective makes their tracks seem to diverge from a small spot in the sky called the radiant. The shower is named after the constellation in which the radiant appears. Some of the better-known showers are listed in Table 8.3.
Long ago astronomers found that some meteoroids travel in orbits much like those of some comets. They had found a link between meteor showers and the short-period comets (to be discussed in Chapter 9). The particle swarms may be debris left by evaporation and tidal disruption of comets. For example, on the night of November 13, 1833, watchers in the southern part of the Atlantic seaboard were awestruck as over 100,000 shooting stars per hour plummeted from the constellation Leo for 3 hours. The great display was produced when the earth encountered a swarm of meteors orbiting the sun in a period of 33 years and associated with comet Tempel (1866 I). The comet itself has long since vanished leaving the meteor shower as a remainder of its existence. The meteoric displays of 1866, 1899, and 1932 were progressively weaker; then on November 17,1966, a fairly spectacular meteor shower was observed in the southwestern part of the United States.
With the passage of time the meteor streamwhich is made up of conglomerates of fine dust, ices, and ice-covered particles- is strung out along the comet's orbit. This ribbon of particles typically averages about 50,000 kilometers in cross section. Thus the earth must come fairly close to the meteor stream in order for us to see a meteor shower.
