Introduction to Stones from Space
The asteroid was 795,000 kilometers (494,000 miles) from Earth when astronomers at the Spacewatch Telescope at Kitt Peak Observatory in Arizona spotted it moving across the night sky on Jan. 18, 1991. By measuring the brightness of the rocky object, the astronomers determined that the asteroid was fairly small—only about 5 to 10 meters (16 to 33 feet) in diameter. Twelve hours later, the asteroid, in orbit around the sun, made its closest approach to Earth. Traveling at about 20 kilometers (12 miles) per second, the asteroid passed 170,000 kilometers (106,000 miles) away—less than half the distance between Earth and the moon.
Although this asteroid never collided with Earth, other pieces of matter from space slam into Earth's atmosphere all the time. Most of these particles are no bigger than a pea. Others, however, have been large enough to create huge impact craters.
These visitors from space interest scientists for a number of reasons. Their impacts helped shape the early Earth and, scientists have recently learned, may have had a major effect on the history of life on this planet. Some of these rocks from space are the only intact survivors from the birth of the solar system. And, as I and other researchers have found, they hold precious chemical clues to processes that occurred 4.6 billion years ago, just before the sun and planets formed from a swirling disk-shaped cloud of gas and dust known as the solar nebula. Moreover, scientists in the late 1980's and early 1990's found that some of these space rocks contain star dust, mineral grains that formed around stars tens of millions of years older than our own sun. This star dust is the only direct evidence we have of the processes that fuel the nuclear fires that burn within stars, of the stars' chemical makeup, and of the raw material dying stars contributed to the solar nebula.
Where Meteorites Come From
Most of the solid objects that enter Earth's atmosphere are pieces of asteroids, small planets no larger than 1,000 kilometers (620 miles) in diameter. Asteroids should not be confused with comets, balls of ice and dust that shed dust particles and gas molecules and that form bright halos and long tails when their orbits take them close to the sun. Most asteroids orbit the sun in a region between the orbits of Mars and Jupiter—known as the asteroid belt—that contains millions of pieces of rock left over from the formation of the solar system.
In the crowded asteroid belt, there are occasional collisions between asteroids, causing them to break into smaller pieces. Scientists usually call pieces with a diameter of about 1 kilometer (0.6 mile) or less a meteoroid. Some of the asteroids and meteoroids kicked out of the asteroid belt during collisions go into a new orbit that crosses Earth's orbit. Eventually—perhaps not for millions of years—Earth and the asteroid or meteoroid will meet at the crossroads of their orbits, and the two will collide.
When an asteroid or meteoroid plummets through Earth's atmosphere, friction causes its outer surface to heat up and glow. If the object is so small that it is completely vaporized (changed to gas) during its descent, it is called a meteor. Meteors cause streaks in the sky called shooting stars when they occur at random and meteor showers when relatively large numbers of them appear on the same date each year.
Meteorites are objects large enough to survive the intense heat of their passage through the atmosphere and reach the ground. Some meteorites strike Earth with such force that they explode and create huge depressions called impact craters. At first, scientists were slow to accept that such depressions were, in fact, created by impacts. But today we know that the impact of even a relatively small object from space can create a huge crater on Earth's surface. Meteoroids and asteroids travel through space at speeds of about 25 kilometers (16 miles) per second. Friction with the atmosphere does not even slow objects weighing more than 1,000 metric tons (1,100 short tons). And at these speeds, even meteoroids with small diameters have such large amounts of kinetic energy (energy of motion) that they may create large craters.
Meteor Crater in Arizona, for example, is about 1,265 meters (4,150 feet) wide and 174 meters (571 feet) deep. Scientists believe it was created 50,000 years ago by the impact of a meteorite that was only 60 meters (200 feet) in diameter but that landed with an explosive force 1,000 times greater than that of the atomic bomb dropped on Hiroshima, Japan, in 1945.
When Meteorites Strike Earth
By analyzing the rock at and near craters and by laboratory experiments, geologists have been able to determine what happens when a meteorite crashes into Earth. In a typical impact, almost all of a meteorite's kinetic energy is released as a shock wave that transmits heat and pressure into the surface rocks at the point of impact. The shock wave compresses the surface rocks, squeezing some downward and lofting others outward at speeds of several kilometers per second. Some of the surface rock is vaporized and some melted in place. Some rock is crushed and the pieces are almost instantaneously cemented together again, forming breccia, a type of rock containing angular fragments.
As the shock wave travels outward from the point of impact, it lifts, overturns, and folds the surface rock, forming a bowl-shaped crater with a raised rim all the way around it. The impact of a large meteorite also may result in the formation of a peak in the center of a crater, created by rock compressing and then rebounding. Meteorite impacts can also form numerous concentric rims around the crater.
Identifying impact craters is easiest when a crater's bowl-shaped basin, raised rims, or central peak are preserved. Unfortunately, most impact craters on Earth's land surface are so old that they are difficult if not impossible to recognize. Over thousands of years, erosion, the advance and retreat of glaciers, and other geologic processes have worn away the craters' raised rims and central peaks. Some have been filled in with and even buried by sediments. Even the presence of breccia is not proof that a circular structure is, in fact, an impact crater, because other geologic events, such as landslides and volcanoes, can produce breccia.
The most conclusive evidence that a circular depression is an impact crater is the presence of shocked rocks. Such rocks may contain shatter cones (rocks fractured in conical patterns). Or they may have distinctive patterns of microscopic cracks in certain minerals, which indicate that they were suddenly subjected to high temperatures and pressures. Geologists can also identify impact craters by the chemical composition of the rocks. Meteorites leave chemical calling cards—high concentrations of nickel, platinum, and iridium, elements generally found in low concentrations in Earth rocks.
Meteorites and Mass Extinctions
One major reason for studying impact craters is to learn more about the history of our planet. An intriguing recent development in the study of meteorite impacts, for example, has been the discovery of evidence suggesting a link between at least one impact and a period of mass extinction, a biological crisis of major proportions that occurs when a large number of species become extinct over a short time period.
Some scientists believe that a large meteorite impact may have been responsible for a mass extinction that occurred about 65 million years ago at the end of the Cretaceous Period and the beginning of the Tertiary Period. Scientists had long known from the study of fossils that many species became extinct at that time. These species included all marine and flying reptiles, the last of the dinosaurs, and almost all species of small marine organisms called plankton. Some groups, however, such as land plants, snakes, and mammals, were hardly affected.
In 1980, a team of scientists led by physicist Luis Alvarez of the University of California at Berkeley reported finding high concentrations of iridium in a layer of clay in northern Italy. The layer dated from the end of the Cretaceous Period and the beginning of the Tertiary Period, which scientists refer to as the K-T boundary. Since iridium is not abundant in Earth rocks, the scientists proposed that it had been deposited during a meteorite impact. The discovery of high iridium concentrations in sediments at the K-T boundary in Denmark and New Zealand indicated that the process responsible occurred throughout the world. Today, scientists generally agree that Earth was indeed struck by a meteorite about 65 million years ago.
They believe that the meteorite was probably about 10 kilometers (6 miles) in diameter. A meteorite of that size hitting Earth would have released energy 5 billion times greater than that of the Hiroshima bomb and produced a crater with a diameter of about 200 kilometers (120 miles).
How A Meteorite Collision Could Cause Mass Extinctions
Advocates of the theory that a meteorite caused the Cretaceous extinction contend that such an impact would have ejected so much dust into the upper atmosphere that the entire Earth would have been shrouded in darkness and cold for at least several months. The lack of sunlight would have shut off photosynthesis during this period, killing off all marine plants and, ultimately, nearly all marine animal species. Land plants would have died also, but they could have regenerated a few years later from seeds and spores. Dinosaurs, which required large amounts of food, would have starved or frozen. But smaller mammals probably survived by eating decaying vegetation and insects.
Some scientists, however, argue that other effects of the impact may have been as significant as the loss of sunlight. For example, the heat generated by the meteorite impact may have ignited massive wildfires that burned up vegetation and created huge clouds of soot. The vaporization of rock by the impact may have produced chemicals that poisoned the atmosphere.
The location of the crater left by the Cretaceous impact remains a question. Many possible sites have been proposed, but none has won universal scientific acceptance. In 1990, geologists Alan R. Hildebrand and William V. Boynton of the University of Arizona in Tucson suggested that the meteorite may have landed on the Yucatan Peninsula of Mexico. Although evidence of an impact has been found at the site, some scientists believe that the crater was formed earlier than 65 million years ago.
How Often Have Meteorites Struck Earth?
Studying impact craters has also provided scientists with clues to the frequency with which Earth has been struck by meteorites in the past. Although the rate of bombardment has fallen dramatically since the planet's formation about 4.6 billion years ago, large meteorites still enter the atmosphere from time to time.
How great is the danger from meteorite impacts? Only those asteroids and meteoroids in Earth-crossing orbits have a chance of colliding with Earth. Until recently, scientists relied solely on cameras attached to telescopes to search the sky for these objects. But these devices can perceive only those meteoroids with a diameter greater than 1 kilometer (0.6 mile). In 1990, however, scientists at the Spacewatch Telescope at Kitt Peak began searching the sky with more sensitive electronic detectors capable of seeing objects as small as 10 meters (33 feet) across.
Early reports from the Spacewatch project indicate that there are many more relatively small objects in Earth-crossing orbits than scientists had believed. In fact, the Spacewatch scientists have calculated that objects 50 meters (160 feet) wide collide with Earth about once every century. Such a collision rate is surprisingly high and has sparked serious scientific discussion about how we could protect ourselves from such an impact.
Unfortunately, locating and determining the orbits of large numbers of near-Earth objects would be a massive task. Blowing them up in space would probably not help, because many of the fragments—including larger ones—would still hit Earth. Some experts suggest that detonating a nuclear warhead or other explosive device near the object could cause meteoroids or asteroids to veer away from the Earth.
The Different Types of Meteorites
While some scientists investigate impact craters to learn more about the history of our planet, other scientists study meteorites themselves. Scientists have been analyzing meteorites since the early 1800's, when people began to accept the idea that meteor trails in the sky were made by stones entering the atmosphere from space. Today, scientists classify meteorites according to the relative amounts of rock and metal they contain. Iron meteorites consist almost entirely of a metallic iron-nickel alloy, something rarely found naturally on Earth. Stony-iron meteorites consist of about half silicates—the chief minerals in soil and rock on Earth—and half metallic iron-nickel. Stony meteorites are made mostly of silicates but also have significant amounts of metallic iron-nickel.
Stony meteorites are further classified as achondrites or chondrites. Achondrites were formed by volcanic action. Most of them come from asteroids, but a few are pieces of the moon, and some are believed to be from Mars—in both cases thrown into space by a meteorite impact.
Chondrites are the rarest and most scientifically valuable type of meteorite. They contain tiny spheres of crystals and glass called chondrules that have never been found on Earth. Scientists believe that chondrites are probably the only objects that have survived almost unchanged since the solar system formed. All other objects in the solar system were altered by heating processes such as those that accompanied the formation of the planets. Studies have revealed that the material in this class of meteorites probably formed before any other type. This discovery in the mid-1950's helped scientists determine that 4.6 billion years is the age of the solar system.
Meteorites Hold Clues to How the Solar System Formed
Analyses of chondrites have provided clues to the puzzle of why different planets in the solar system have different chemical compositions. All the planets formed from the same solar nebula, a swirling cloud of gas and dust. So we might expect that the planets would have the same composition. But they don't. The inner planets—Mercury, Venus, Earth, and Mars—are made of rocky material. The outer planets—Jupiter, Saturn, Uranus, Neptune, and Pluto—consist chiefly of gases. Some process must have been responsible for the separation of the elements as the hot nebula began to cool and matter began to clump together to form the planets.
In the late 1960's, I and a number of other scientists began exploring the theory that differing temperatures of condensation (the process by which a gas changes to a solid) could account for the differences. In other words, we thought that some elements in the solar nebula must have condensed and formed mineral grains while temperatures were still so high that other elements remained in a gaseous state. These first grains became the building materials for some of the larger bodies called protoplanets that collided, broke up, and recombined to form the planets and asteroids. Grains that condensed later became the building blocks for other protoplanets. I had calculated, for example, that as the solar nebula began to cool, aluminum, calcium, and titanium would have been among the first elements to form mineral grains. But we had no way to simulate the condensation of the solar nebula in the laboratory.
We thought that carbonaceous (carbon-containing) chondrites, the oldest objects in the solar system, might hold some evidence that would confirm our theories. However, this type of meteorite was very hard to come by. Carbonaceous chondrites are still very rare today. Twenty years ago, researchers could get only small samples—pieces about the size of a fingernail—from museums.
Then, in February 1969, the Allende meteorite, a carbonaceous chondrite weighing nearly 2 tons, landed in Mexico. This enabled scientists to remove and study large slices of this kind of meteorite. We found that the Allende meteorite contained irregularly shaped particles. When scientists analyzed these particles, they found that they consist of aluminum, calcium, and titanium, in just the mineralogical forms predicted by my calculations. This finding indicated that elements had indeed condensed from the solar nebula at different temperatures and so verified the theory of condensation, accounting for the different compositions of the planets.
Star Dust In Meteorites
In the late 1980's and early 1990's, scientists discovered star dust in carbonaceous chondrites—a finding that has revolutionized research into the nature of stars. Before this discovery, scientists wishing to study stars could do so only indirectly and at great distances. For example, they could use instruments called spectrographs to measure the patterns of light emitted by stars. These patterns, which vary according to the mix of elements in a star, can be used to determine a star's chemical composition and the temperature on its surface. Scientists could also use powerful atom-smashers, more accurately called particle accelerators, to study the properties of isotopes created in stars. (Isotopes are varieties of an element. The different isotopes of an element have the same number of subatomic particles called protons but different numbers of neutrons in their nuclei [cores].)
Scientists used their findings to develop theories that would enable them to predict which isotopes would be found in what proportions in various types of stars. They also theorized about how nuclear reactions within stars had produced the elements and isotopes in the proportions found in the solar system. Before we could confirm these theories, however, we had to know the composition of the matter in the solar system. We gained this knowledge by analyzing the elements and isotopes in meteorites, rocks on Earth, and, later, moon rocks. But the problem with this approach was that these rocks represent a composite of the different elements and isotopes from all the ancient stars that contributed material to the solar nebula.
How Star Dust Forms
The center of every star is a nuclear “furnace,” where the nuclei of atoms of lighter elements and isotopes fuse to form heavier ones. The isotopes produced by a particular star depend on the chemical material from which the star formed and on the nuclear fusion reactions taking place within the star. As a result, each star has a distinctive “signature,” a particular mix of elements and their isotopes.
The outermost regions of some stars also contain “star dust,” mineral grains that formed because the gases there were cool enough to condense. The ratios of isotopes of a given element in these mineral grains match those of the gases from which they formed.
Over time, stars lose gas and dust in several ways. But most star dust comes from supernovae, the violent explosions of massive stars that have run out of nuclear fuel. The gas and dust ejected from numerous exploding, dying stars accumulate in interstellar space, forming enormous, cold clouds. Eventually, gravity pulls the molecules in a cloud together, and the cloud collapses, heats up, and breaks into fragments. Scientists believe that our solar nebula formed from such a fragment.
The center of the solar nebula would have been so hot that all the dust grains there were completely vaporized. As individual dust grains disappeared, so too did the record of the unique isotopic signatures of the stars around which they formed. As a result, the gas in the inner solar nebula became a well stirred, isotopically uniform mixture. The gas in this part of the nebula eventually cooled and condensed into new mineral grains whose isotopes were identical to those of the gas.
When scientists analyzed an entire chondrite using a technique called mass spectrometry, they found that the overall mixture of isotopes in the meteorite was similar to that in Earth rocks. However, there were slight differences in the isotopic ratios of noble gases, particularly xenon. (Noble gases do not react readily with other elements and so may remain essentially unchanged for long periods.) When scientists heated pieces of a chondrite, they found that the ratios of xenon isotopes released at certain temperatures were very different from those of Earth rocks. Some scientists suggested that the minerals in the chondrites in which the xenon was locked might be older than the solar system. They might even be the unchanged dust of dead stars. If any grains of star dust existed, however, they were present in extraordinarily small amounts. In order to find them, scientists would have to find a way to separate them from the material that makes up the bulk of the meteorite.
Finding Star Dust In Stones From Space
In the early 1970's, a team of scientists at the University of Chicago headed by chemist Edward Anders began working on this problem. Between 1987 and 1991, they announced that they had found three types of star dust in a sample of carbonaceous chondrite—diamonds, graphite, and silicon carbide.
Using several strong acids and other chemicals, the scientists had dissolved more and more of the chondrite until only about one-fifth of 1 per cent of the sample remained. Then they added ammonia to the sample. When they did, some of the very finest particles clumped together. These particles turned out to be diamonds. The diamond grains were extremely small, only about 50 atoms across. The scientists then determined that other solid particles in the sample were grains of graphite. These grains were about 600 times larger than the diamond grains. Finally, when the scientists added two more acids to the remaining sample of the chondrite, they found grains of silicon carbide about 30 times larger than the diamond grains.
When Anders and his colleagues heated the three types of star dust, a variety of isotopes of xenon and of neon were released. The relative proportions of the isotopes of each of these gases were so enormously different from those found in the solar system that the scientists had no doubt that the grains were star dust.
Anders and his colleagues theorized that some xenon isotopes found in the diamond grains were formed when matter in the core of a star was exposed to a sudden, massive burst of subatomic particles called neutrons. The other xenon isotopes were created when matter from deep in a star was blasted through a thick outer layer of hydrogen. Both processes are believed to occur in supernovae. The scientists suggested that the diamond crystals had probably condensed in the cool outer atmosphere of the star before it exploded. The force of the explosion implanted the freshly created xenon isotopes into the tiny diamond crystals.
The scientists also theorized that the distinctive proportions of xenon and neon isotopes found in the silicon carbide and graphite crystals formed in a red giant star, a star that has exhausted its hydrogen fuel and has begun to expand and turn reddish in color. They suggested that blobs of cooler material in the surface layers of a red giant sank, forcing upward hotter material that had formed in the interior. This hotter material, which contained the neon and xenon isotopes, became trapped in grains of silicon carbide and graphite as they condensed from the outer atmosphere.
How Star Dust Could Survive In A Meteorite
Why had this star dust survived? Theories answering that question bear on conditions that existed in the solar nebula. For example, scientists theorize that the grains may not have been vaporized and mixed in the solar nebula because they happened to be in the outer part of the cloud, where temperatures were cooler than in the center. Or perhaps the entire nebula cooled relatively quickly, before the grains could evaporate completely, like hot tea cooling before all the ice cubes dropped into it melt.
However this happened, the scientists also suggested the grains were incorporated into protoplanets that never became part of a planet. Because they were not subjected to the heat that accompanied the formation of the planets, they remained cold and unaltered for the entire history of the solar system. As a result, the isotopic signatures of their parent stars were preserved.
Studies of interstellar grains have thus yielded the first direct information on the isotopic composition of the stars that contributed matter to our solar system. These data have allowed astrophysicists to refine their theories about the nuclear reactions that take place within stars. Scientists also have been able to use the grains as clues to the physical conditions, such as temperature, density, and circulation patterns, inside the stars that produced them.
It could be that a meteorite may ultimately influence our fate. Meanwhile, however, studies of these stones from space continue to reveal fascinating new insights into our beginnings.