Beyond matters of the soul, the inspiration for most art is in nature. For me, aesthetic meteorites are the closest approximation to being able to behold that which is in the heavens.
—DARRYL PITT
A BOLT FROM THE BLUE
It was a quiet night in the little town of Pueblito de Allende in Chihuahua, Mexico, on February 8, 1969. Everyone was fast asleep. Suddenly, at 1:05 in the morning, a huge fireball appeared out of the southwest and lit up the night sky and the ground brighter than daylight. It was a meteorite the size of a car, falling out of space at a speed of 10 miles per second (3,600 miles per hour). Residents were startled out of their sleep by the sounds of the flaming rock screaming down out of the sky and an immense explosion that rocked the earth as the meteorite hit. The impact scattered debris across an area spanning about 250 square kilometers, in a long oval 50 kilometers wide and 8 kilometers across, including thousands of small fragments of the meteorite itself, plus debris from the impact crater.
When dawn finally arrived and brought daylight to the landscape, the terrified residents emerged from their houses to see what had happened. They soon realized what had happened, as they picked up hundreds of pieces of the rock from space that had landed that night. Local officials and residents looked around for pieces of the meteorite, but luckily no one appeared to be injured or killed, and there was no serious damage.
Scientists responded as soon as the news spread and tried to reach the site. As recounted by Elbert King, a geologist at the University of Houston who specialized in meteorites, in his 1989 book Moon Trip: A Personal Account of the Apollo Program and Its Science:
While unsuccessfully searching for a meteorite fall close to Crosby, Texas, I heard on the car radio about a very bright fireball witnessed in southern New Mexico, Texas, and northern Mexico. I returned to my office and asked my secretary, who was fluent in Spanish, to place some phone calls for me. I first contacted a newspaper editor in Chihuahua City. We had a lengthy conversation about the phenomena accompanying the meteorite fall but no specimens had fallen near Chihuahua City. Finally, I asked him the right question: “Do you know anyone who has any pieces of the meteorite?” “Oh yes,” he said, and suggested that I call the newspaper editor in Hidalgo del Parral, much further to the south. My secretary located Sr. Ruben Rocha Chavez, editor of Correa del Parral. He recounted how a brilliant fireball had broken apart with a loud explosion in the middle of the night and had showered fragments over a large area near Parral. Chavez had several pieces of the meteorite on his desk and described them to me. There was no doubt—he had fragments of a freshly fallen stony meteorite! He invited me to visit Parral to see his pieces and to collect specimens. I thanked him for the information and his invitation and told him I would be there as soon as possible.
A quick check of airline schedules showed it was not going to be easy to get to Parral. I could fly to El Paso, but that was still more than three hundred miles north of Parral. It was the fastest way, however. My secretary promised to cover me with paperwork. I stopped by my house for a few clothes and headed for the airport.
The plane took off on time, but, as luck would have it, a faulty landing gear indicator light grounded us in San Antonio for five hours while it was replaced. By the time I arrived in El Paso it was already dark. I picked up a rental car, cleared through customs, and drove south. It was important to recover pieces of the meteorite right away in order to measure their short half-life radioactivities. This would be great practice for the Radiation Counting Laboratory of the LRL [Lunar Research Laboratory in Houston]. The Mexican highways were difficult to negotiate in the dark. The best technique was to follow a hundred yards behind a car with Mexican license plates. Some of the drivers were going 80 miles per hour, and when I saw brake lights or a cloud of dust, I knew the driver had spotted a burro on the highway. I arrived in Parral just after dawn. I checked into a hotel, washed up, drank some strong coffee, ate eggs and tortillas, and went to look for the newspaper office. I was waiting when the editor arrived. I was astonished when I saw the two big meteorite pieces on the editor’ s desk. One weighed more than 30 pounds.
The greatest surprise was the meteorite type—a rare carbonaceous chondrite. Chondrites are stony meteorites that contain chondrules, small spheres of silicate of disputed origin. Carbonaceous chondrites are chondrites that contain abundant carbon and organic compounds. While I was standing in Chavez’s office, the telephone rang. The editor handed the receiver to me. It was a colleague from the Smithsonian who wanted information about the meteorite. He had called my Houston office, where my secretary gave him the number of the newspaper office. I told him what little I knew. I asked the editor about his plans for the two specimens on his desk. He said they were reserved for the National Museum. I agreed this was perfectly appropriate, but I was eager to recover some additional specimens. The editor said I must visit the local municipal president or mayor. I was going to be treated as an official NASA representative.
The mayor, Sr. Carlos Franco, was extremely gracious, and though my Spanish was meager and he spoke little English, we had an amiable meeting. I explained, through the editor as translator, how scientifically important meteorites are in general and that this particular one was a very rare type. Sr. Franco was eager to help me, and he assigned me one of his policemen and an official car for as long as I needed them.
We drove to places where specimens had been found. Recovering additional specimens proved to be easy. Everyone had small pieces of the meteorite, but I wanted some larger ones. I purchased these from the local people, with the policeman acting as interpreter and handling the negotiations. We documented several sites where specimens had been found. The stones had showered over a large area. One large stone had missed the post office in Pueblito de Allende by only 30 feet. Meteorites normally are named after the nearest post office. This one almost named itself. We listened to many tales of the fireball, its direction of travel, the loud claps of thunder, stones falling everywhere, and people running to the church in the middle of the night. I picked up 13 pieces of the meteorite, including two large ones—enough samples for the time being.
King was followed by many other scientists from museums and universities around the world, and soon the search was on for as many fragments of the Allende meteorite as could be found. It came at a particularly important time in the study of meteorites, because the field of planetary science was growing and well funded thanks to the Apollo missions. Two of the Apollo 11 crew members (Neil Armstrong and Buzz Aldrin) were the first humans to land on the moon, and their moon walk happened only a few months after the Allende impact. Altogether, thousands of pieces totaling over 3 metric tonnes were collected, and people are still finding small pieces today, some 48 years later. In fact, small fragments only a gram or two in weight are still sold over the Internet and are actually quite affordable. Thanks to the interest surrounding it, Allende is by far the best-studied meteorite in history.
TRACES OF THE EARLY SOLAR SYSTEM
The Allende meteorite was part of a class of meteorites known as carbonaceous chondrites, which are among the rarest and most important meteorites. Allende was very unusual in that most of the meteorites known to be in this class had been collected many years earlier and had been sitting in museum drawers. Prior to Allende, the best-studied of the carbonaceous chondrites was the Orgueil meteorite from France, which had landed back in 1864. A number of other smaller examples were known but poorly studied. All of the Orgueil meteorite’s short-lived isotopes had long since decayed, and some of the meteorites were weathered and altered by having lain out in the field a long time before they were collected. By contrast, Allende was a fresh fall, so the material could be analyzed mere days after it had fallen, and there had been no time for weathering or contamination.
Chondritic meteorites are a special class of meteorites that formed from the early solar system and that predate the planets. They are made of the dust and debris of the original solar dust cloud, or solar nebula, or from smaller planetary bodies that never became large enough to have a separate core and mantle, so they are valuable clues to early solar system evolution. They get their name from the tiny blobs of material found in them (figure 9.1), called chondrules, which are even older bits of the early solar system that clumped together when the meteorite coalesced.
Figure 9.1
A slice of the Allende meteorite, showing the densely packed blobs of early solar system material known as “chondrules.” (Courtesy of Wikimedia Commons)
Although small chondrites are not rare (about 2,700 specimens in collections or about 86 percent of total meteorites are chondrites), certain types of chondrites are very rare. Among these are the carbonaceous chondrites like Allende, which make up less than 5 percent of the total number of chondrites. They get their name because they have a relatively high carbon content compared with other meteorites and often still contain water-bearing compounds from the primordial solar system. It is thought that this is because they formed farther away from the sun than other meteorites and were not heated enough to wipe out their carbon or water content.
Once scientists had samples of Allende in their labs, they mined those remnants for every possible bit of information. The overall composition of the matrix surrounding the chondrules gave a good idea about the composition of the initial dust ring of the solar system. Other scientists focused on the chemistry of the chondrules included within the meteorite. The most interesting of these are known as “CAIs,” or calcium-aluminum-rich inclusions. They have a truly unusual composition, rich in not only calcium and aluminum, but also silicon, oxygen, iron, and other elements. Their composition is completely unlike the rest of the early solar system, so they are thought to have formed from a high-temperature (greater than 1,300 K) protoplanetary disk of matter in the earliest stage of the solar system, before most of the rest of the material had condensed out.
In addition to giving us insights into the earliest history of the solar system, carbonaceous chondrites also give us dates for the time when the solar system formed. Allende has chondrules (including CAIs) that gave a uranium-lead date of 4.567 billion years in age. This is 30 million years older than the formation of Earth, and about 200 million years older than the oldest rocks and minerals on Earth. Another CAI in a different carbonaceous chondrite from northwest Africa gave an age of 4.56822 ± 0.00017 billion years, making it the oldest object ever dated, and a good estimate for the beginning of solar system formation.
Studies of the Allende meteorite and other carbonaceous chondrites continue to be published as scientists think of new things to analyze and newer and better techniques come along that weren’t available in 1969. In 1971, scientists discovered tiny black markings (up to 10 trillion per square centimeter) that were evidence of radiation damage. This proves that the meteorite did not start out near Earth (which is screened from radiation by its magnetic field), but formed far from Earth and before it had a magnetic field, when objects (including the oldest lunar rocks) were being intensely bombarded by radiation. Using the same Caltech lab that first analyzed the lunar rocks (waggishly nicknamed the “Lunatic Asylum”), scientists in 1977 found that the Allende meteorite contained new forms of the elements calcium, barium, and neodymium, as well as krypton, xenon, nitrogen, and other rarer elements that apparently came from the shockwave of a supernova that may have helped trigger the formation of the solar system.
Even more importantly, the Allende meteorite was rich in the rare isotope magnesium-26, which is formed from the decay of radioactive aluminum-26. This decay takes place very rapidly and must have occurred soon after the solar system had formed. But the abundance of magnesium-26 in this meteorite suggested that it was once abundant in all solar system rocks, including those that formed the earliest earth, and answered a long-standing question: What heated the early earth and caused it to melt, separating its core from its mantle? The answer? Abundant aluminum-26 in the early earth generated more than enough heat by its decay to melt the earth many times over.
LIFE IN METEORITES?
In fact, 1969 turned out to be a banner year for meteorite studies. On September 28, 1969, another carbonaceous chondrite (figure 9.2) fell near Murchison, in Victoria, Australia. Local people first saw a fireball at about 10:58 A.M., followed by the sound of its descent through the atmosphere, and then felt its impact, a tremor that occurred about 30 seconds after the fireball was sighted. The meteorite broke into three large pieces as it fell, then broke up even more after impact, forming a strewn field with an area of more than 13 square kilometers. Hundreds of fragments were found, totaling over 1,000 kilograms in weight. Many weighed more than 7 kilograms, and the largest weighed 680 kilograms and broke through the roof of a barn, landing in a pile of hay.
Figure 9.2
One of the larger pieces of the Murchison meteorite, now on display at the National Museum of Natural History, Smithsonian Institution. (Courtesy of Wikimedia Commons)
The Murchison meteorite turned out to be even more important than most other carbonaceous chondrites, because it contains organic compounds not found in any previous meteorite. The original studies found 15 amino acids, and more recent research with more sensitive techniques has found up to 70 amino acids and many more complex compounds. The discovery of amino acids was a shock, because amino acids are the building blocks of life, and it was thought that they could only be produced in a warm little pond on Earth. Back in 1953, the famous Miller-Urey experiment simulated the early earth’s atmosphere and ocean in a lab apparatus. Stanley Miller and Nobel Prize–winning chemist Harold Urey showed that simply by heating a mixture of ammonia, methane, nitrogen, and water (but no free oxygen), the early earth could have produced most of the amino acids used by life. Now the Murchison meteorite showed that the process must have been widespread and had actually happened, and that amino acids were formed all over the early solar system, long before the earth formed. In fact, many scientists argued that life on Earth was seeded from amino acids that rained down from space, so in a sense, all life is extraterrestrial in origin.
Even more importantly, the amino acids in the Murchison meteorite were a mix of both right-handed and left-handed forms. This a property of chemical compounds in which molecules are asymmetrical and one form is the mirror image of the other. This showed that even if life started on Earth from a shower of amino acids brought by meteorites (or if it formed by itself in a warm little pond on Earth), there is only one common ancestor to all of life, because all biologically important molecules (except certain sugars) are left-handed—a property they must have inherited from a single early living form that happened to use only left-handed molecules.
So the next time you visit a museum and see a carbonaceous chondrite on display (especially if it’s a piece of the Allende or Murchison meteorite), show it some respect. It is probably the oldest object you will ever encounter, and it’s a piece of the earliest solar system before the planets formed. Even more, it may have carried the seeds that launched the origin of life on Earth in the first place.
FOR FURTHER READING
Bevan, Alex, and John De Laeter. Meteorites: A Journey Through Space and Time. Washington, D.C.: Smithsonian Books, 2002.
Chambers, John, and Jacqueline Mitton. From Dust to Life: The Origin and Evolution of Our Solar System. Princeton, N.J.: Princeton University Press, 2013.
Dalrymple, G. Brent. The Age of the Earth. Stanford, Calif.: Stanford University Press, 1994.
——. Ancient Earth, Ancient Skies: The Age of the Earth and Its Cosmic Surroundings. Stanford, Calif.: Stanford University Press, 2004.
Gargaud, Muriel, Hervé Martin, Purificacíon López-García, Thierry Montmerle, and Robert Pascal. Young Sun, Early Earth, and the Origins of Life: Lessons for Astrobiology. Berlin: Springer, 2013.
Hedman, Matthew. The Age of Everything: How Science Explores the Past. Chicago: University of Chicago Press, 2007.
Macdougall, Doug. Nature’s Clocks: How Scientists Measure the Age of Almost Everything. Berkeley: University of California Press, 2008.
Nield, Ted. The Falling Sky: The Science and History of Meteorites and Why We Should Learn to Love Them. New York: Lyons, 2011.
Norton, O. Richard. Rocks from Space: Meteorites and Meteorite Hunters. Missoula, Mont.: Mountain Press, 1998.
Smith, Caroline, Sara Russell, and Gretchen Benedix. Meteorites. London: Firefly, 2010.
Zanda, Brigitte, and Monica Rotaru, eds. Meteorites: Their Impact on Science and History. Cambridge: Cambridge University Press, 2001.