On a clear night it is possible to see a few meteors—"shooting stars" —every hour. A meteor is a sudden streak of light in the night sky caused by friction between air molecules in Earth's atmosphere and an incoming piece of interplanetary matter—an asteroid, comet, or meteoroid. This friction heats and excites the air molecules, which then emit light as they return to their ground states, producing the characteristic bright streak shown in Figure 14.15. Note that the brief flash that is a meteor is in no way similar to the broad, steady swath of light associated with a comet's tail. A meteor is a fleeting event in Earth's atmosphere, whereas a comet tail exists in deep space and can be visible in the sky for weeks or even months. Recall from Section 6.5 that the distinction between an asteroid and a meteoroid is simply a matter of size. Both are chunks of rocky interplanetary debris; meteoroids are conventionally taken to be less than 100 m in diameter.
Figure 14.15 A bright streak called a meteor is produced when a fragment of interplanetary debris plunges into the atmosphere, heating the air to incandescence. (a) A small meteor photographed against a backdrop of stars. (b) An auroral display provides the background for a brighter meteor trail.
Before encountering the atmosphere, the piece of debris causing a meteor was almost certainly a meteoroid, simply because these small interplanetary fragments are far more common than either asteroids or comets. Any piece of interplanetary debris that survives its fiery passage through our atmosphere and finds its way to the ground is called a meteorite.
Smaller meteoroids are mainly the rocky remains of broken-up comets. Each time a comet passes near the Sun, some cometary fragments dislodge from the main body. The fragments initially travel in a tightly knit group of dust or pebble-sized objects, called a meteoroid swarm, moving in nearly the same orbit as the parent comet. Over the course of time, the swarm gradually disperses along the orbit, and eventually the micrometeoroids, as these small meteoroids are known, become more or less smoothly spread all the way around the parent comet's orbit. If Earth's orbit happens to intersect the orbit of such a young cluster of meteoroids, a spectacular meteor shower can result. Earth's motion takes it across a given comet's orbit at most twice a year (depending on the precise orbit of each body). Intersection occurs at the same time each year (see Figure 14.16), so the appearance of certain meteor showers is a regular and (fairly) predictable event.
Figure 14.16 A meteoroid swarm associated with a given comet intersects Earth's orbit at specific locations, giving rise to meteor showers at specific times of the year. We imagine that a portion of the comet breaks up near perihelion, at the point marked 1. The fragments continue along the original comet orbit, gradually spreading out as they go (points 2 and 3). The rate at which the debris disperses around the orbit is actually much slower than depicted here—it takes many orbits for the material to spread out as shown. Eventually, the fragments extend all around the orbit, more or less uniformly. If the orbit happens to intersect Earth's orbit, a meteor shower is seen each time Earth passes through the intersection (point 4).
Table 14.1 lists some prominent meteor showers, the dates they are visible from Earth, and the comet from which they are thought to originate. Meteor showers are usually named for their radiant, the constellation from whose direction they appear to come. For example, the Perseid shower is seen to emanate from the constellation Perseus. It can last for several days but reaches maximum every year on the morning of August 12, when upward of 50 meteors per hour can be observed.
| TABLE 14.1 Some Prominent Meteor Showers | ||||||||||||||||||||||||||||||||||||||||||||||||
*Every 33 years, as Earth passes through the densest region of this meteoroid swarm, we see intense showers that can reach 1000 meteors per minute for brief periods of time. This is next expected to occur in 1999. **Phaeton is actually an asteroid and shows no signs of cometary activity, but its orbit matches the meteoroid paths very well. |
Astronomers can use the speed and direction of a meteor's flight to compute its interplanetary trajectory. This is how certain meteoroid swarms have come to be identified with well-known comet orbits. For example, the Perseid shower shares the same orbit as comet 1862III, the third comet discovered in the year 1862 (also known as comet Swift—Tuttle).
Larger meteoroids—more than a few centimeters in diameter—are usually not associated with swarms of cometary debris. Generally regarded as small bodies that have strayed from the asteroid belt, possibly as the result of asteroid collisions, these objects have produced most of the cratering on the surfaces of the Moon, Mercury, Venus, Mars, and some of the moons of the jovian planets. When these large meteoroids enter Earth's atmosphere with a typical velocity of nearly 20 km/s, they produce energetic shock waves, or "sonic booms," as well as a bright sky streaks and dusty trails of discarded debris. Such large meteors are sometimes known as fireballs. The greater the speed of the incoming object, the hotter its surface becomes and the faster it burns up. A few large meteoroids enter the atmosphere at such high speed (about 75 km/s) that they either fragment or disperse entirely at high altitudes.
The more massive meteoroids (at least a ton in mass and a meter across) do make it to the surface, producing a crater such as the kilometer-wide Barringer Crater shown in Figure 8.17. From the size of this crater, we can estimate that the meteoroid responsible must have had a mass of about 200,000 tons. Only 25 tons of iron meteorite fragments have been found at the crash site. The remaining mass must have been scattered by the explosion at impact, broken down by subsequent erosion, or buried in the ground.
Currently, Earth is scarred with nearly 100 craters larger than 0.1 km in diameter. Most of these are so heavily eroded by weather and distorted by crustal activity that they can be identified only in satellite photography, as shown in Figure 14.17. Fortunately, such major collisions between Earth and large meteoroids are thought to be rare events now. Researchers believe that, on average, they occur only once every few hundred thousand years (see Interlude 14-1).
Figure 14.17 This photograph, taken from orbit by the U.S. Skylab space station, clearly shows the ancient impact basin that forms Quebec's Manicouagan Reservoir. A large meteorite landed there about 200 million years ago. The central floor of the crater rebounded after the impact, forming an elevated central peak. The lake, 70 km in diameter, now fills the resulting ring-shaped depression.
The orbits of large meteorites that survive their plunge through Earth's atmosphere can be reconstructed in a manner similar to that used to determine the orbits of meteor showers. In most cases, their computed orbits do indeed intersect the asteroid belt, providing the strongest evidence we have that they were once part of the belt before being redirected, probably by a collision with another asteroid, into the Earth-crossing orbit that led to the impact with our planet. (See also Interlude 14-3.)
Not all meteoroid encounters with Earth result in an impact. One of the most recent meteoritic events occurred in central Siberia on June 30, 1908 (Figure 14.18). The presence of only a shallow depression as well as a complete lack of fragments implies that this Siberian intruder exploded several kilometers above the ground, leaving a blasted depression at ground level but no well-formed crater. Recent calculations suggest that the object in question was a rocky meteoroid about 30 m across. The explosion, estimated to have been equal in energy to a 10-megaton nuclear detonation, was heard hundreds of kilometers away and produced measurable increases in atmospheric dust levels all across the Northern Hemisphere.
Figure 14.18 The Tunguska event of 1908 leveled trees over a vast area. Although the impact of the blast was tremendous and its sound audible for hundreds of kilometers, the Siberian site was so remote that little was known about the event until scientific expeditions arrived to study it many years later.
One feature that distinguishes the small micrometeoroids, which burn up in Earth's atmosphere, from larger meteoroids, which reach the ground, is composition. The average density of meteoritic fireballs too small to reach the ground (but which can be captured by high-flying aircraft) is about 500—1000 kg/m3. Such a low density is typical of comets, which are made of loosely packed ice and dust. In contrast, the meteorites that reach Earth's surface are often much denser—up to 5000 kg/m3—suggesting a composition more like that of the asteroids. Meteorites like the ones shown in Figure 14.19 have received close scrutiny from planetary scientists—prior to the Space Age, they were the only type of extraterrestrial matter we could touch and examine in terrestrial laboratories.
Figure 14.19 (a) A large meteorite on display at the Hayden Planetarium in New York serves as a jungle gym for curious children in this photograph from the 1930s. (b) The Wabar meteorite, discovered in the Arabian desert. Although small fragments of the original meteor had been collected more than a century before, the 2000-kg main body was not found until 1965.
Most meteorites are rocky in composition (Figure 14.20a), although a few percent are composed mainly of iron and nickel (Figure 14.20b). Their basic composition is much like that of the inner planets and the Moon, except that some of their lighter elements—such as hydrogen and oxygen—appear to have boiled away long ago when the bodies from which the meteorites originated were molten. Some meteorites show clear evidence of strong heating at some time in their past, most likely indicating that they originated on a larger body that either experienced some geological activity or was partially melted during the collision that liberated the fragments that eventually became the meteorites. Others show no such evidence and probably date from the formation of the solar system.
Figure 14.20 (a) A stony meteorite often has a dark fusion crust, created when its surface is melted by the tremendous heat generated during passage through the atmosphere. (b) Iron meteorites, much rarer than stony ones, usually contain some nickel as well. Most such meteorites show characteristic crystalline patterns when their surfaces are cut, polished, and etched with weak acid.
Most primitive of all are the carbonaceous meteorites, so called because of their relatively high carbon content. They are black or dark gray, and they may well be related to the carbon-rich C-type asteroids that populate the outer asteroid belt. (Similarly, the silicate-rich stony meteorites are probably associated with the inner S-type asteroids.) Many carbonaceous meteorites contain significant amounts of ice and other volatile substances, and they are usually rich in organic molecules.
Finally, almost all meteorites are old. Direct radioactive dating shows most of them to be between 4.4 and 4.6 billion years old—roughly the age of the oldest lunar rocks. Meteorites, along with some lunar rocks, comets, and perhaps the planet Pluto, provide essential clues to the original state of matter in the solar neighborhood.
