Sandford was among the ecstatic scientists waiting there. After months of microscopic analysis, the Stardust team extracted thousands of tiny cometary particles, all in all less than a milligram of cometary material, weighing less than a grain of table salt. That might seem like a poor return for more than a decade of planning and a $200 million mission to bring back the first samples from beyond the Moon. But Sandford and the other Stardust scientists knew that size didn't really matter. These tiny grains held secrets to a much, much larger story, one that encompassed the entire origin of our Solar System, us included, in the stars.
What's all the more remarkable is that Sandford didn't set out on a career to study the stars. As a child, he'd been intrigued by astronomy, owned two telescopes, and once did daily drawings of sunspots for a month to see the Sun's rotation for himself. But growing up in Los Alamos, New Mexico, where his father was an engineer at the nuclear lab, Sandford was particularly captivated by the Earth's rocky bones, evident everywhere in the desert and mountains surrounding his hometown. He dreamed of being a geophysicist and did his undergraduate work at the New Mexico Institute of Mining and Technology, spending summers working for a seismologist tracking earthquakes. But when Sandford applied to graduate school, his path turned skyward. He landed a fellowship at the McDonnell Center for the Space Sciences at the Washington University in St. Louis, Missouri, to study not terrestrial rocks but rocks from space.
Sandford began his research career just as the US space agency launched an interesting new program to collect dust from high in the Earth's atmosphere. The effort was spearheaded by Washington University geologist Donald Brownlee's radical contention that some of that high atmospheric dust wasn't Earthly effluent that had risen up but cosmic dust that was raining down. To test Brownlee's alien-dust idea, NASA agreed to bolt dust collectors onto the wings of some of its high-altitude U2 research planes—aircraft better known for their work as Cold War–era spy planes. “At that time, they had only been studying these grains returned from the U2s for a couple of years,” Sandford tells me in his office at NASA-Ames. “When I joined the group, there was still no real definitive proof these things were even extraterrestrial, although we thought that many of them might be. They could have been smokestack effluent or volcanic dust that got up into the stratosphere.” Working with colleagues, it was Sandford's job to prove the case.
To do this, he spent months of his graduate-school years in a lab clean room picking out bits of dust from the collectors. “St. Louis is often hot and humid,” he recalls, “so frequently I’d go to the university with a bathing suit under my clothes. Then I’d strip down into the bathing suit and put on the bunny suit [the classic sterile clean-room coveralls] just so I wouldn't die of heat stroke.” For hours on end, peering through a microscope, he used a micromanipulator to pick tiny bits of dust, whose size on average was ten to twenty microns (millionths of a meter) across—about the size of one of your red blood cells—out of the silicon oil in which they were trapped on the collectors. After he'd amassed this minute geological sample, Sandford used an electron-scanning microscope to take the sample's x-ray spectrum and to thus identify its elemental composition. For his PhD thesis in 1985, Sandford spent months comparing the infrared spectra, or light fingerprint, of the dust grains with the infrared spectra of comets and asteroids observed by astronomers. They were a near-perfect match—some with comets, others with asteroids. Brownlee was right; the dust grains Sandford was holding in his micromanipulator weren't of this Earth. They were bits of comets, asteroids, and maybe even the primordial stuff out of which the Solar System coalesced. At the time, it was a far-out idea.
“I remember the very first talk I gave at a science conference about this was attended by, I think, two people, because I was at a session with all the kooks who thought tektites [the glassy, rocky debris from huge meteorite strikes] were from the moon, which is wrong, and that the K/T boundary [the geological debris line associated with the end of the dinosaurs] marked an impact from an asteroid, which is probably right. But at the same time, none of those things were proven. So we were lumped in with the rest of the loony session, in a sense.”
Today this daily cosmic dusting isn't considered crazy but is common sense. Most meteoritic material falls to the Earth not with a fireball's bang but as an invisible daily powdering. Every year, about forty thousand tons of these cosmic dust particles, averaging around twice the width of a hair, make landfall or splash imperceptibly into the sea. This cosmic dust comes primarily from two sources: meteoroids (the name given a meteorite while it is still in space) and interplanetary dust particles. If a meteoroid is smaller than your fist, the violence of its six-thousand-mile-an-hour entry into the Earth's upper atmosphere shreds it into a stream of cosmic dust more than thirty miles above the ground, and the dust gradually drifts down to the Earth's surface. Interplanetary dust is a combination of dust produced by pulverizing asteroid collisions in the asteroid belt between Mars and Jupiter and the minute debris of comets that evaporate or break apart as their orbit takes them close to the Sun, such as that collected by the Stardust mission. The Earth acts as a gravitational dust buster, sucking in these particles as it orbits the Sun. The result is that every day, on any sidewalk-square-sized area of the Earth's surface, whether in Manhattan or Mongolia, a tiny piece of outer space touches down.
Sandford was one of the first people to see this firsthand. Through his extraterrestrial dust research, the young man who'd wanted to study the dynamics of the Earth's rocks was initiated into a new field of the Stardust Revolution: astrogeology, or lithic astronomy. Astrogeologists watch stars not through telescopes but through microscopes. A century and a half earlier, the French philosopher of science Auguste Comte had dismissed astronomy's future as a science because, he argued, you'd never be able to actually hold a piece of star in your hand and do experiments on it to determine its fundamental nature. But Scott Sandford has done this, and now geology is joined directly with the stars. “We normally think of everything around us being ultimately four and a half billion years old because that's when it all came together,” he says.
But of course every atom itself was outside the Solar System and came together when it formed. Even more intriguing is that we didn't just get a delivery from the interstellar medium in the form of atoms. We got molecules and grains, and some of them survived relatively unscathed. As individual entities, they are older than the Solar System. If you want to understand how our Solar System formed, there is a limited amount you can learn from planetary materials like Earth rocks because they have all been completely reprocessed. Because of planetary geological processes, there are no rocks on the Earth that witnessed the birth of the Earth. If you're looking for samples of primitive primordial material, the starting stuff that all of us began with, you need to look to meteorites and comets.
The first great Stardust Revolution story that meteorites would tell was of the birth of the Earth.
THE BIRTH OF THE EARTH
Taking a walk with a toddler, you get a lot of time to think about how amazing rocks are. When my son was a two-year-old, we couldn't walk more than ten steps before he was bending down, picking up a nondescript bit of crushed gravel, and holding it up to me with a look of deep joy. “Rock!” he'd exclaim. We all had this sense of wonder about the rocks around us, though most of us have lost it. But if there was one rock in the twentieth century that really made planetary geologists and astronomers excited—that had them holding it up to the rest of the world and shouting, “Look!”—it was the Allende meteorite, a rock that opened a new chapter in the Stardust Revolution.
Meteorites are often named after the closest post office by which they're found. With the Allende meteorite, coming up with a name was simple. Around one in the morning on February 8, 1969, in Pueblito de Allende, in the northern Mexican state of Chihuahua, one of the chunks from a massive shattered space rock missed crashing into the local post office by just thirty feet. Allende was no make-a-wish shoo
ting star. It collided with the Earth with the force and momentary terror evoked in a Hollywood cosmic-collision apocalyptic blockbuster. The sedan-sized meteoroid entered the Earth's atmosphere at supersonic speed, creating a series of thunder-like sonic booms that awoke sleepers from southern Arizona into northern Mexico. Its blue-white fireball “was so bright we had to shield our eyes,” said Guillermo Asunsolo, the editor of a Chihuahua newspaper. “The light was so brilliant we could see an ant walking on the floor.” The terror evoked by this midnight intruder sent many rural Mexicans running for the local Catholic churches. “The people, especially the people in the small villages are very alarmed,” Asunsolo reported. “They say that this is an announcement that the world will soon end.”
The Allende meteorite, however, turned out to be an amazing heaven-sent messenger revealing details not about the Earth's end but about its beginnings. For the first time, scientists would realize that they were holding stardust in their hands and gaining an understanding from this stardust of the forces that shaped the Earth's birth—and of a time when the Earth was part of a cosmic cloud of dust, gas, molecules, and ices holding all the potential of its next billions of years, yet it was not even a blue glimmer on the cosmic scene.
Allende had arrived with perfect timing. In preparation for the Moon rocks to be returned by the Apollo 11 mission just months later, NASA had prepared a national network of laboratories with the latest geochemical-analysis tools to study the first lunar samples. Before Neil Armstrong and his crew returned the first lunar rocks, the Allende meteorite delivered a free payload of cosmic material that would turn out to be as scientifically important as the Moon rocks. Within hours of Allende's impact, meteorite scientists were rushing to the site to gather possible fragments. The first to arrive at the impact site was Bert King, a NASA scientist who'd been busily preparing the Lunar Receiving Laboratory in Houston, and who would soon become the first lunar sample curator. King could be away from work only for little more than a day. He didn't sleep for thirty hours, consumed as he was in a meteorite-collecting spree that had him buying pieces of the Allende meteorite from locals with the help of a Mexican policeman as interpreter and negotiator.
What was most important, though, was his reconnaissance for those who'd follow. Colleagues at the Smithsonian Institution in Washington managed to track him down by phone, and he told them of the wealth of meteoritic material that lay strewn across the remote Chihuahuan semidesert. During the next several weeks, Smithsonian scientists enlisted a local search party, including many schoolchildren, to collect meteorite fragments. Together they collected more than two thousand pounds of the Allende meteorite, about 2,100 pieces, ranging in weight from as little as several ounces up to a meteoritic whopper of 242 pounds. The pieces were strewn over a debris field of nearly two hundred square miles, the result of a literal meteorite shower when it had exploded.
The large number of pieces was crucial to what would follow. With an abundance of material, the Smithsonian readily distributed bits of the meteorite to interested researchers. They were able to destroy pieces in their analysis, if necessary, without worrying about running out of priceless research material, as is sometimes the case with rare samples. As the results of the Allende meteorite analysis arrived, they showed that this rock from space packed more than a shock for just the Mexican villagers who'd experienced its spectacular arrival. It has continued to surprise scientists for half a century.
For Bert King, the greatest surprise was the meteorite's type. To put his excitement in perspective, it's important to know that there are three categories of meteorites, each defined by composition and asteroidal origins: iron, stony iron, and stony meteorites. And just by its composition, each of these types of meteorites tells a story. “When geologists look at a rock they don't just say ‘That's a sandstone’ or ‘That's a granite,’” meteorite scientist Richard Herd tells me when I visit Canada's National Meteorite Collection in Ottawa.
What happens is that immediately the wheels start turning, and they try and figure out how the rock was made, under what conditions. Is this a fluvial sandstone formed in a river; is it a beach deposit; is it a desert sandstone? Because a sandstone is literally a rock made out of sand. So the kind of material that a rock is made of and the way that material is put together—whether it's igneous, metamorphic, or sedimentary—tells you something about the environment of formation. That way we can read the history of the Earth. Meteorites are rocks that reflect the processes that they've been through, just like the Earth rocks. They have a history and provenance that can be deciphered. These geological processes tell you a story.
Iron meteorites get their name from what they are made of, which is readily apparent when you hold one in your hands and feel a quick sense of preparing to hold something heavier than it looks: solid, heavy, slag-like chunks of iron and various amounts of nickel and other siderophile (iron-loving) elements. As such, they'd be better named steel. Cut one crosswise, and you end up with a slab that looks as if it could have been rolled straight out of a steel mill, but that lump of iron is a sampling of a shattered asteroid's core. The asteroid was large enough that, as occurred in the Earth, its interior melted (in the asteroid's case, through radioactive heating early in its life), and its heaviest elements, iron and nickel, formed a metallic core that slowly solidified.
The second class of meteorites are the stony irons. As their name implies, these meteorites combine the qualities of iron meteorites with terrestrial rocklike characteristics. The stony parts of these meteorites are the remains of the crustal, rocky portions of asteroids. Asteroids larger than about fifty kilometers in diameter—called planetesimals, denoting their mini-planet-like natures—developed into differentiated, layered mini-worlds with molten cores and solid crusts, just like Earth. During its hot, formative early years, an asteroid's geology had many similarities to terrestrial geology. The stony, crustal meteorite bits are cosmic cousins to both terrestrial basaltic rocks (those formed by lava) and granites (those igneous rocks formed when molten rock slowly cools below the surface). The stony irons contain the complex story of when asteroids collided, producing impact-generated mixes of interior and crustal rocks.
The stony meteorites are the third group of these rocks from space. It's a subclass of stony meteorite, the chondrites, that are the core and the future of the Stardust Revolution. Chondrites are the sedimentary rocks of the solar system. They're agglomerations of cosmic sediments, the original particles present in the solar nebula and protoplanetary disk. Remove the Sun's hydrogen and helium, and the mix of elements you'd be left with would resemble those of a chondrite. Chondrites are characterized by containing chondrules, a word derived from the ancient Greek word for grain. Chondrules are millimeter-to-centimeter-sized glassy, metallic beads that formed from rapidly cooling metallic gas, free-floating molten droplets, in the emergent blast furnace of dust and gas surrounding the early Sun. They are the Solar System's first igneous rocks.
What gets stardust scientists really excited are the rare carbonaceous chondrites, meteorites that are rich not only in the elements that make the Earth's core and crust, but also in the element that's central to life: carbon. Some carbonaceous chondrites are so rich in carbon-based molecules that they resemble charcoal briquettes or even peat. What makes the carbonaceous chondrites even more special is not how they've changed but how they've stayed the same. They are the least-altered meteorites, the ones that preserve the nature of the early protoplanetary disk from which all that is the Earth formed. The asteroids, of which meteorites are a chip off the larger block, formed within the first ten million years of the Solar System's birth. That's about fifty million years before the Earth-Moon is thought to have formed from the massive collision and fusing of a couple of other planetary embryos. Each carbonaceous chondrite is a tiny sampling of the natal protoplanetary disk material from which life emerged. As such, each carbonaceous chondrite is greeted with profound interest by stardust scientists, who seek to tease every det
ail about the origins of life from each new immigrant from outer space.
Looking at the thirty-pound chunk of the Allende meteorite on a newspaper editor's desk in the village of Hidalgo del Parral, Bert King knew he was looking at just such a carbonaceous chondrite. The fresh chuck of Allende that King saw was so rich in carbon he could smell its oily, organic nature. Today these meteorites make up only about 4 percent of all collected meteorites, and Allende still holds the record as the largest carbonaceous-chondrite meteorite shower ever.
The biggest surprises were waiting in the labs of King and other geologists. From broken pieces of the meteorite, King could clearly see that it contained a mix of distinctive white circular and fuzzy-edged blobs amid the meteorite's darker matrix, separate from and less abundant than the chondrules. These whitish inclusions, most of them much less than a penny's diameter in size, turned out to be mother lodes of calcium and aluminum; thus they were dubbed calcium-aluminum–rich inclusions. What makes these inclusions truly special is their age. Using uranium-lead dating, geologists determined that the calcium-aluminum–rich inclusions were older than the Earth.
Here was a seemingly impossible idea. How could something be older than the Earth? The Allende meteorite is so special because it's an amalgam of materials that coalesced thirty million years before the Earth formed, when the nascent Sun was surrounded by the still-developing protoplanetary disk. The calcium-aluminum–rich inclusions formed even before that. They were the first solid objects to emerge in the hottest areas of the solar nebula, marking the beginnings of the solid phase of our Solar System. Whereas most materials were still molten or gaseous, the calcium-aluminum–rich inclusions had a high enough melting point, almost 2,600°F, that they were the first objects to condense out of the cooling solar nebula. In some of the most detailed uranium-lead dating ever, scientists have pinpointed the age of some calcium-aluminum–rich inclusions to 4.567 billion years old, give or take half a million years. The Allende meteorite contains some of the first solid materials to form in our Solar System; tiny time capsules from the very beginning of what we call home.
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