The Stardust Revolution

by Jacob Berkowitz

  The revelation of Allende's age, and particularly that of the calcium-aluminum–rich inclusions, set astronomers and geologists on a quest to see what else they could learn about the Earth's prenatal stage. This was a truly remarkable thought—it was possible to perform hands-on cosmic archaeology and gain information about the Earth's origins not from terrestrial rocks but from rocks originating elsewhere in the Solar System. The Allende meteorite isn't alien material but rather is sibling material from the same stellar nursery. As an older sibling, it can tell us stories about the world before our arrival. For 4.567 billion years, the Earth and the Allende meteorite's asteroidal parent performed a mutual orbital dance around our Sun, until at some point the asteroid smashed into another body, ejecting a piece of asteroidal shrapnel that on a winter's night in 1969 became a shooting star. With its fiery impact, Allende brought to light the story of the Earth's birth.

  With the insights of the astronomer's periodic table—the ways that particular stars forge particular elements—astrogeologists turned to analyzing Allende's chemical makeup for clues to its and, by extension, the Solar System's origins. The Allende meteorite received the most intensive chemical fingerprinting of any rock ever. The most intriguing thing this analysis revealed was that those whitish calcium-aluminum–rich inclusions had oddly abundant levels of the isotope magnesium-26. This particular form of magnesium is a decay product of the radioactive isotope aluminum-26. Aluminium-26 in turn is formed in the blink of an eye during a supernova when enormous temperatures ignite the fusing of neon, producing a supernova spray of aluminum-26. But aluminum-26 has a half-life of only about seven hundred thousand years. This means that a rich burst of aluminum-26 seeded the solar nebula just before the calcium-aluminum–rich inclusions formed.

  In effect, magnesium-26, the fossil remains of aluminum-26, appears to be the smoking gun that proves that the shock from a nearby supernova might have triggered the gravitational collapse of the solar nebula that formed our Solar System. Although today the Sun and its stellar siblings are all far from their natal home, it's thought that the Sun, like most stars, formed as part of a densely packed stellar cluster—a family of hundreds of stars formed from the same dense molecular cloud, such as we see occurring today in the Orion Nebula. Amid this stellar family was at least one, and probably several, short-lived giant star, which, perhaps before the Sun had shed its first light, detonated, spewing energy and elements, including aluminium-26, for hundreds of light-years; the shock triggered the collapse of the dense molecular core from which emerged our Solar System. This evidence of supernova midwifery has stood the test of time and further scientific scrutiny. It appears that the death of one star sparked the birth of another, a phenomenon astronomers see played out in stellar nurseries across the Milky Way.

  In less than a decade of its arrival, the Allende meteorite had pushed back the veil of time to reveal turbulent lands and awesome events that were the equivalent of the moment of our Solar System's conception. However, Allende held deeper secrets, ones that would require even greater patience and persistence to tease out but that would take us back even farther in time, to reveal the direct stellar roots of our family tree.

  THE MEN WHO FIRST HELD STARDUST

  On his office desk in the University of Chicago's Enrico Fermi Institute, Edward Anders kept a set of red-capped, wooden Russian dolls; hollow figures that nestle inside each other, each one smaller than the next; the final, tiniest one being solid. Years later, Anders would reflect that the symbolic answer to his quest had been right in front of him as he'd made calculations and discussed strategy with his research team. But that observation was made in hindsight. At the time, Anders, an avowed master of meteorite studies, was stumped. There was something chemically strange about the Allende meteorite, but in the mid-1980s, after more than a decade of searching, Anders's team couldn't open the final layer of the Russian doll and see what it was.

  Anders was used to tough research questions and had made a name for himself as an impassioned intellectual fighter who'd spar with all comers. In 1964, he published a landmark paper that laid out the modern understanding of meteorites’ origins: they're chunks of wayward asteroids from the asteroid belt between Mars and Jupiter. The problem was that Anders's assertion was a full-on challenge to the view of leading planetary scientist and Nobel laureate Harold Urey, his former mentor, whose post at the University of Chicago Anders had filled in 1955. Urey argued vehemently that meteorites were blasted-off bits of the Moon rather than original Solar System material.

  Anders's first paper proposing the asteroidal origin of meteorites was rejected by the leading astrophysical journal. He received a letter from a respected meteorite researcher telling him that his asteroidal model was ridiculous. In his 1964 paper “Origin, Age, and Composition of Meteorites,” Anders pored over, correlated, and stitched together thousands of pieces of varied meteorite evidence available at that time from the fields of chemistry, physics, mineralogy, and astronomy. His wife complained that he worked every day in December 1963 while he was on sabbatical at the University of Bern in Switzerland. Anders sealed the case, claiming that all the evidence pointed to the fact that the parent bodies of meteorites weren't other planets or the Moon but rather were asteroids. He was right. Martian and lunar meteorites may get lots of press, but these rocks actually account for only about 0.5 percent of all meteorites.

  While Edward Anders was describing the origin of meteorites, in the Soviet Union, the planetary geologist Viktor Safronov was using meteorites to piece together the seemingly unreachable origins of the Solar System. Safronov had taken the intellectual baton from his mentor, the theoretical geologist and mathematician Otto Schmidt. Purged from the Soviet scientific establishment by Joseph Stalin in 1942 because his father was German, the unemployed Schmidt turned to a detailed imagining of the cosmic origins of the Solar System. Although the United States won the race to set foot on the Moon, it was the Soviet Union, using Schmidt's pioneering work, that won the race to figure out how the Earth, the Moon, and rest of the Solar System formed. In 1969, Safronov published his book Evolution of the Protoplanetary Cloud and Formation of the Earth and Planets, the name of which in the original Russian sounds quite different, but the concept of which is identical to the 1972 English translation. In this book, Safronov synthesized more than a century of reflection into one critical insight: stars don't form alone. Stars are born, said Safronov, surrounded by a vast swarm of gas and dust that over time gravitationally settles down into a circumstellar disk, a thin band of material orbiting its star like the swirling, uplifted skirt of a spinning dancer. Planets, and everything on them, are made from these star-making leftovers.

  Since Safronov's seminal treatise, stardust scientists have filled in the details of this grand vision. At the core of their findings is a profound conclusion: the Earth emerged from stardust. Every mineral and molecule—from green olivine to formaldehyde—that coalesced to make the Earth and the other bodies of the Solar System was forged by existing stars and processed in the chemical mash-up that is the interstellar medium. Safronov provided a framework for extending terrestrial geology into space, for understanding our Solar System as if it were a single rock in whose multitudinous layers, minerals, and elements could be read the story of its origins.

  It was sentences of this ancient story that Anders was trying to read in bits of the Allende meteorite, but the message contained in the space rock's isotopic signature had Anders stumped. Isotopes of an element are like identical twins—exactly identical to most observers, but to someone who knows them well, they are different in important ways. All atoms of an element have the same number of protons and electrons—and, thus, the same chemical properties. But some atoms of an element are a little heavier than others, containing one or more additional neutrons. Chemists use a mass spectrometer to identify and sort isotopes of the same element based on differences in mass. The relative distributions of various elemental isotopes in a mineral or molecule—its isoto
pic fingerprint—can provide a powerful clue as to its origins. In the Stardust Revolution, this isotopic fingerprinting has become transformative in creating a detailed family tree of the ancestry of cosmic minerals and molecules—the field of stable isotope cosmochemistry. By the early 1970s, Anders had the world-leading cosmochemistry lab, attracting the most talented and ambitious graduate students and postgraduate researchers to his team, with Anders acting as the intellectual quarterback.

  With Allende, Anders was trying to crack the ultimate forensic cold case—what he called the meteorite's mysterious isotopically anomalous xenon component. This isotopic mystery involved one of the least common elements on Earth: xenon, from the Greek xenos, “strange.” In 1964, meteorite researchers discovered that Allende contained anomalous isotope levels of xenon. Of xenon's nine stable isotopes, the meteorite contained at least double the amount of both the lightest and heaviest isotopes found in terrestrial samples. Xenon is not just rare; it's also a “noble gas,” essentially a loner element, along with krypton, neon, argon, and helium, that has all the electrons it needs, and so it doesn't chemically react, or bond, with other atoms. It's also a gas at room temperature, and since the meteorite had been stored at this temperature, the fact that the xenon hadn't off-gassed meant that it must be physically trapped in the meteorite. Allende's isotopic mix was tantalizingly distinct from that seen in other meteorites, and this pointed to some astrophysical process that had shaped the meteorite, the memory of which was recorded in the xenon. There was a stranger in the meteorite. The question was, where was it hiding?

  That Anders was around to contemplate the origin of the xenon was itself an anomaly. Anders was born Edward Alperovičs on June 21, 1926, the summer solstice, in the town of Liepja, Latvia, where his grandfather Israel was head rabbi. When Nazi forces attacked the Soviet Union and entered Latvia in June 1941, there were approximately ninety thousand Jews living there. Only 2 percent of them survived the Nazi occupation. What occurred in Liepja was similar to what happened in other Latvian towns. In late December 1941, an SS Einsatzgruppe, or mobile death squad, arrived in Liepja. Jews were rounded up and marched to a former Latvian firing range on rolling sand dunes beside the sea. The first to arrive were forced to dig a mass grave. Then, over the course of three days, almost every Jew in Liepja was murdered. The scar of the shoreline mass grave is still visible from the air. Twenty-four members of Anders's extended family, including his father Adolph and brother Georg, were shot on those dunes. Of the Anders family, only Edward and his mother, Erika, survived—the result of a ruse whereby the family had agreed that Erika would claim she was an Aryan foundling raised by kindly Jews, and thus her children were only part Jewish. Surviving the war, Anders arrived in the United States in June 1949, working as a waiter in a downscale Jewish hotel in New York's Catskill Mountains until his acceptance at Columbia University. He quickly excelled there and became intrigued with meteorites, to whose study he devoted the rest of his research career.

  In 1972, Anders was joined by postdoctoral researcher Roy Lewis, who'd trained with the world's leading mass-spectroscopy expert and now brought his isotope-weighing skills to Anders's lab, taking over much of the lab's hands-on research, which by 1975 Anders had stopped, in favor of performing the reading, coaching, and paper writting of a senior scientist. In search of the location of Allende's xenon, Anders, Lewis, and several graduate students developed a procedure in which a piece of the meteorite was subjected to a series of progressively harsher acid and bleach baths that gradually dissolved most of the meteorite—particularly its rocky matrix and the chondrules. After each bath, the researchers used mass spectrometry to analyze the remaining acid-resistant solid residue for xenon. If it was present, they'd continue on a laborious, trial-and-error journey and choose a new corrosive solution, such as concentrated ammonia, chloric acid, or hydrogen fluoride, any one of which was strong enough to further eat away at the meteorite's remains. Even after a decade of on-and-off work, however, they couldn't identify exactly where the xenon was trapped.

  The breakthrough came serendipitously in the spring of 1986, when Anders, Lewis, PhD student Tang Ming, and postdoctoral researcher John Wacker found what they were looking for. They'd reduced a sample of the Allende meteorite down to a mere 0.5 percent and were left with a black, tarry residue that still contained the xenon. In an attempt to further reduce the sample, Wacker set up an overnight treatment in which he mixed some residue in a powerful acid, put the mixture on a hot plate at almost 300°F, and went home. But the thermostat on the hot plate got stuck, causing the sample to overheat, and when Wacker returned in the morning, his sample had turned white. At first, Wacker thought he'd ruined the experiment, since all known forms of carbon, except one, are black. But Lewis decided to run with what they had and to try to identify the white powder using x-ray diffraction.

  Small samples of a crystal can often be identified by x-ray diffraction; each mineral bends or diffracts light in a characteristic way to create a kind of diffraction shadow—the rough equivalent of how a human skeletal pattern appears on an x-ray. When Lewis subjected his minuscule space-rock samples to this x-ray examination, the diffraction shadow that appeared was of an uncannily well-known terrestrial material, one he'd never imagined finding in a meteorite: diamond. There were trillions of them—what we now call nano-diamonds—each one too small to be measured in carats; rather, they could be measured only in terms of carbon atoms, averaging about two thousand atoms. “They would barely make engagement rings for bacteria,” Anders quipped.

  No matter how small they were, they were still diamonds—something that, Anders and his coauthors wrote, “no one had seriously considered a constituent of the cosmos outside the Solar System.” Geologists thought that diamonds could be created only by the high temperatures and pressures inside planets. In fact, astronomers using infrared telescopes had first detected what looked like the infrared light fingerprints of interstellar diamonds as early as the late 1960s. They had discounted the detections as impossible. Clearly the diamond-formation theory needed tweaking: Where could this collection of cosmic jewels infused with anomalous amounts of xenon in a meteorite come from?

  If the diamonds were a surprise, the answer to their origins was even more bewildering. So much so that, even years later, scientists reflecting on the discovery noted that it could reasonably seem to strain credulity. Anders and his colleagues saw only one feasible source for these cosmic diamonds: pure stardust. At the time, astrophysicists thought it was impossible that raw stardust could survive from one generation of stars to the next. Common sense indicated that this primordial dust would inevitably be mixed and lost in the maelstrom of collisions, heating, nuclear reactions, and intense radiation of star birth. But the nano-diamonds in their samples weren't reprocessed protoplanetary debris that had been morphed, melted, or otherwise changed. They were the pure, original output of stars. And not just any stars. Now the xenon made sense. The diamonds had condensed out in the carbon-rich atmosphere of a red giant, a medium-sized star in its bloated death throes. Later, when the star's remnant white dwarf core exploded as a Type 1a supernova, the stellar diamonds were infused with a spray of xenon atoms, which they'd carried ever since.

  Anders and his colleagues hadn't just dissolved a meteorite to find stardust; they'd dissolved time. The sequential acid baths had mimicked our Solar System's accretion process in reverse. Instead of building up from stardust, the process used by Anders's team had worked back to it. They'd taken a meteorite, itself a chunk of asteroid, and winnowed it down to its most fundamental essence: stardust. The billionth-of-a-meter-sized diamonds were the oldest minerals anyone had ever discovered, let alone ever imagined. They weren't just stardust. They were stardust from stars that lived and died and whose dust was dispersed through the galaxy long before our Solar System was born. This was pre-solar stardust. In their lab glassware were the remains of long-extinguished stars that had burned bright somewhere across the Milky Way. Their light was gone, b
ut their diamond dust remained. As a scientist, Anders was renowned for an elephantine memory, but this was a whole new kind of remembering. The teenager who'd survived some of the bleakest, most murderous days of the Holocaust had helped find slivers of preserved ancient starlight amid cosmic darkness.

  “As a young graduate student I was bowled over by the sight of meteorites that came from one or two astronomical units [the Earth–Sun distance] beyond Earth,” Anders later recalled. “Now, as an old man, I am awed by the sight of stardust from a few hundred light-years beyond the Earth; ten million times further. Such is the magic of meteoritics.”

  STARDUST MEMORIES

  Anders and his colleagues had forged a new field of the Stardust Revolution: hands-on astrophysics. Just as the emerging nuclear physics of the 1940s and ’50s intersected with the understanding of nuclear processes in stars, stardust geology is merging with astronomy to provide what geologists call the ground-truth of stellar processes: you don't just look or talk about it; you can actually “hold” and examine up close a piece of stardust. Meteorites, Anders said, were the poor man's space probe, able to deliver samples directly from a star.

  “Until recently people could study stars only by remote means,” he told a reporter just after the discovery. “Astronomers who have been studying stardust for years with their telescopes could make only some general statements about the chemistry and the grain size and so on, considering the nearest sizable quantity of stardust is a hundred light years away. No one had looked at any samples. But we have made it possible to study stardust with all the techniques of modern science.”