Stardust geology—a subdiscipline of astrogeology—is the study of rocks at the smallest, oldest, and most primordial level possible; it's the study of grains belched directly from a star. Unlike metaphysical stardust, these stellar crystals aren't all pretty and glistening. They look more like bits of beach sand that you'd shake out of your shoe after an oceanside walk: angular, pocked cubes of silicon carbide or crusty balls of graphite. What's remarkable is that the same geological principles apply in understanding stardust as they do in understanding a chunk of coal from Appalachia or the freshest piece of volcanic basalt. Each grain's structure and composition, its shape and what it's made of, are the results of the formative processes—the heating and cooling, pressures, and elemental merging—that made it. It's just that, rather than telling the terrestrial story of fine sediments accumulating over some tropical reef or of the gradual heating and crushing of limestone deep in the Earth to form marble, stardust reveals the story of the particular nuclear reactions inside a star and then the turbulent processes as these elements spew forth, bond, merge, and gradually cool into the first rocks: stardust. And just as geology reveals the history of the Earth, this lithic astronomy reveals the history of the cosmos—but before the Earth existed.
Pre-solar grains are the cosmos’ oldest artifacts. Each is between five and seven billion years old. It's difficult to pinpoint the ages more precisely, because each grain is so small one has yet to be radioactively dated. What does age mean for a rock? On one level, everything in the cosmos is the same age, all of it originating with the big bang. A discussion about the age of terrestrial rocks, stardust grains, or ancient Egyptian artifacts refers to how long something has existed in its current state. For example, your age isn't the age of your atoms but rather is the age of the time since they joined to form you. The same concept applies to stardust grains. Cosmic-dust scientists think that the vast majority of stardust is reprocessed numerous times before forming part of a solar system. For example, just as the Earth's continents go through a gradual cycle of movement and mixing—the plate tectonics that cause earthquakes, volcanoes, and tsunamis—stardust goes through an endless process of physical change. It's transformed by heat, cold, pressure, dramatic dust collisions and mixing, and repeated cycles of all these elements. It's obliterated—vaporized by supernova blast waves—then locked into planets for billions of years, only to be recycled into the interstellar medium with a star's death—either slow and billowing or with blink-of-an-eye explosive force. Although it certainly doesn't sound as impressive on an inspirational poster, we are made not so much directly from stardust as we are from heavily reprocessed interstellar dust. In contrast, pre-solar stardust grains are truly the unchanged remains of our ancestors, and, as such, they hold the deepest memories of galactic history and our cosmic origins.
Stardust geologists work on a previously inconceivable scale. For these stellar rock hounds, the emblematic tool of the trade isn't the geologist's rock hammer but rather is the nanoSIMS—nano Secondary Ion Mass Spectrometer. In the nanoSIMS process, a stardust grain is bombarded with a high-energy ion beam that gradually erodes the sample, as occurs in sandblasting, sending atoms hurtling off from the sample and into the mass spectrometer, a sophisticated scale that measures atomic masses by tallying the way they respond to electric and magnetic fields. The end result is that nanoSIMS produces a grain's detailed atomic makeup, its atomic fingerprint. What's critical are the isotopic details of this stardust fingerprint. They're distinct from the mixed material of our Solar System in almost every element. To a stardust scientist, isotopic anomalies, like Anders's xenon anomaly, are the hard evidence that prove that a bit of microscopic dust is indeed the stuff of a star.
While all this is awe-inspiring, what's even more remarkable is that with this nanoSIMS grain-by-grain isotopic fingerprinting, stardust scientists can trace a grain's parentage not just to the stars but to particular kinds of stars at particular stages of their lives. It's one grain, one star type. The isotope differences start with the parent star, and at each stage of its life, a star has a distinctive isotopic fingerprint. Depending on the temperature, pressure, and elemental mix in a star, different atomic nuclei are formed. A supernova produces a different mix of isotopes than does an old red giant. Eventually, these stellar isotopic characteristics are frozen in stardust, which in turn, billions of years later, holds its story accreted inside a meteorite. Differences in isotope ratios can range from a modest parts-per-thousand to in-your-face thousandfold differences. For example, the average ratio of oxygen-18 to oxygen-17 in the interstellar medium is different from that ratio as it exists in the Solar System, a fact that provides cosmochemists with a clear isotopic signature that reveals whether a mineral or a molecule was formed in the interstellar medium or after the formation of our solar nebula.
Most of the work on tying stardust back to parent stars has been done with the best-studied stardust type, silicon carbide, a crystalline combination of carbon and silicon. Silicon carbide is one tough material. It was the second stardust mineral that the University of Chicago researchers isolated and was able to survive numerous rounds of alternating acid-bleach corrosive attacks. On Earth, silicon carbide rarely forms naturally, but it is synthesized for its diamond-like hardness and durability; for example, as the grit on sandpaper and part of the bullet-stopping material in body armor. This ability to withstand forces that dissolve or abrade other minerals makes it a great cosmic storyteller. Stardust scientists have isotopically fingerprinted thousands of individual grains of silicon carbide from meteorites and, in the process, have linked these particles back to two types of dying stars: supernovas and red giants. Nine in ten silicon carbide stardust grains are thought to emanate from red giants. Similarly, some silicon carbide grains carry their supernova origins in their isotopes. Some of these grains have large excesses of calcium-44 relative to calcium-40. Once again, the reason is radioactive decay. Supernovas produce large amounts of the radioactive isotope titanium-44, which quickly decays to calcium-44. Only exploding stars forge abundant titanium-44, and thus the presence of its calcium decay product is the evidence that a grain of stardust formed in the cooling outflow of a cosmic-scale firecracker.
Today, stardust researchers have amassed a catalog of more than ten thousand pre-solar grains that have been isotopically fingerprinted back to their natal star type, including at least a dozen different pre-solar minerals. Along with nano-diamonds and silicon carbide, these pre-solar minerals include titanium carbide, globular balls of graphite and grains of various silicate minerals—otherwise known as sand, which are formed in the atmospheres of red giant stars and are the type of mineral from which most of the Earth's crust is made. From the diversity of stardust grains, Edward Anders once ballparked a guess that our Solar System formed from the stellar remains of at least one thousand dead ancestral stars. We see only part of the evidence in pure stardust. Most of the atoms from those stars have been thoroughly recycled, their pedigree lost in the general mix of our Solar System. The story of our origins is being pieced together stardust grain by stardust grain.
These were the kinds of pre-solar grains from which the Earth formed. As you read this, careers are being spent—days and weeks of computer modeling, months of space-satellite observations, painstaking hours with electron microscopes probing otherwise invisible bits of stardust—trying to piece together the exact steps by which stardust grew to become the Earth and other planets. What's clear is that, in rough outline, the process was a case of a great cosmic clumping. The story of our cosmic origins is one of coagulation, of the coming together of, for all intents and purposes, an infinity of otherwise negligible cosmic bits and pieces. It began when, inside a giant molecular cloud, some gravitational instability, a ripple in space-time, caused the cold dust and molecular gas—all the gas is molecular when it's at just a smidgen above absolute zero—to start to clump and spiral in on itself. It was the first contraction on the way to the birth of a solar system. Cosmic matter spr
ead over a diameter of perhaps ten light-years—a bubble sixty trillion miles across—collapsed down to a mere five billion miles across, the approximate distance from the Sun to the outermost reaches of our present-day Solar System. About 99 percent of this cloud was gas, most of it hydrogen, with a smaller mix of helium. The bulk went into forming the Sun, but there was also a solar nebula, a cloud of surrounding material of between 1 and 7 percent the mass of the Sun. Most of this material was also gas—but a tiny fraction, just 1 percent, was molecular and mineral dust. As this gas and dust swirled around the gestating Sun, it formed into a flattened frisbee-like disk around the star, what astronomers call an accretion disk, providing the first inkling of the Solar System to come. Through microscopic dust collisions, slightly larger particles took shape. It might have taken millions of years of orbiting for a stardust grain to grow to the size of a marble and for these marbles to merge into boulders, but time is all that was needed. As the grains grew, the process accelerated. More massive chunks of material were gravitationally drawn toward one another and toward the center of the accretion disk, creating a denser cluster of orbiting traffic. The collisions were complicated by the baby stars’ growing-pain outbursts of radiation; caught up in currents of radiation and magnetic fluxes, grains were melted, fused, evaporated, sublimated, irradiated—repeatedly heated and cooled in a maelstrom of creation.
For all its cosmic proportions, the birth of the Solar System recognizable to us today occurred within the blink of a geological eye. Within ten million years of the first stellar contractions, the almost-nothingness of interstellar grains had merged into sixty-mile-diameter balls, like the asteroids Ceres and Vesta. Planetary geologists call these country-sized masses planetary embryos, for it was their cataclysmic collisions that formed the rocky planets from Mercury to Mars, as well as, many planetary geologists believe, the cores of the gas-giant planets Jupiter and Saturn and their more distant icy neighbors, Uranus and Neptune. While rocky, metallic objects formed closer to the Sun, it's thought that comets formed out past the Solar System's ice line—the point at which it was cold enough that water in the protoplanetary disk existed as ice, combining with a mix of rocky material to form dirty ice balls.
Astrogeologists estimate that within the first million years of the Sun's ignition, about 4.567 billion years ago, more than a hundred Mars- and Moon-sized planetesimals were jockeying for orbital position within the space that's now the Earth's distance from the Sun. This crowd of early planetary contenders wasn't so much whittled down as it was forced to play together. For between fifty million and one hundred million years, these planetesimals collided, merging through molten collisions into the rocky planets we know today.
What emerged from this great coming together of stardust was a molten mass of a planet. Not a pale-blue dot but a reddish, squirming orb—hot, energized, and angry from the trauma of birth, and yet, as we are testament to today, a planet that somehow cried with the essence of life. But how could a cosmic rock come to life? Rocks from space are once again revealing the secrets of an ancient story.
SAGAN'S DREAM
From a glass-fronted cabinet in one of the NASA-Ames astrochemistry labs, Scott Sandford pulls out an unusual but historic piece of lab glassware: an empty champagne bottle. The bottle's original label is plastered over with a new one, a white piece of paper bearing a scientific graph with a single horizontal line stretching from left to right, interspersed with three sharp, spiking peaks. Those spikes are the reason the bottle is empty. Whatever the champagne's original vintage, on March 14, 2000, Sandford and colleagues emptied it and relabeled it to celebrate a cold cosmic cloud vintage with a wonderful bouquet of amino acids. Like explorers making their mark on newfound land, the scientists tagged the label with their signatures. Among them were the names of Lou Allamandola, the astrochemistry lab's founder and one of Sandford's mentors; Max Bernstein, a former Ames research colleague and now a NASA administrator; and Sandford himself, his signature directly under the spiking graph. The occasion would have also warranted the handing out of cigars (if smoking were allowed in the lab), for the scientists were celebrating a birth of sorts.
As the current label indicates, the scientists had mixed water, methanol (rubbing alcohol), hydrogen cyanide, and ammonia—a classic foursome of common interstellar molecules—and zapped the mixture with ultraviolet light from a hydrogen lamp in the lab's cryovacuum system. The cryovacuum system's heart is a shiny metallic chamber in which, in a volume smaller than that of a pound-block of butter, the Ames researchers routinely simulated the conditions in cold cosmic clouds—the kind from which solar systems are born. In the lead-up to popping the champagne cork, the scientists had activated the system's vacuum pump to suck out air and drop the internal pressure to less than one-millionth of the Earth's atmospheric pressure at sea level. Meanwhile, the system's cryocooler chilled the system's core to -441°F. When the chamber was completely otherworldly, they'd sprayed in their simple gaseous mix. Before you could say “chilly,” the molecules had frozen out on a metallic plate inside the chamber. But the water didn't freeze into the organized crystals associated with ice cubes and snowflakes. Rather, it became haphazard amorphous ice—water molecules so cold they just stopped moving, without time to even get organized. Embedded in that ice were the three other molecules the researchers had injected, and soon the iced-over metallic plate was turned to face the full brunt of an ultraviolet shower from the system's hydrogen lamp, a stand-in for a star. Ultraviolet photons ricocheted into the amorphous ice mixture, shattering molecular bonds and creating molecular fragments and orphaned atoms.
When this cold cosmic chemistry experiment was complete, the cryovacuum system was allowed to warm up to room temperature. As it began to warm, the ice sublimated, going directly from solid to gas; this is the same process that happens to ice cubes in a frost-free freezer that gradually disappear when not used. And there, left as a barely visible residue on the metallic head, were the end products of what astrochemists imagine goes on today in cold molecular clouds—and what went on in the cold molecular cloud from which our Solar System emerged. When Sandford and his colleagues analyzed their astrochemical haul, they detected hundreds of different molecules, most at very low (parts-per-million) concentrations. The new champagne bottle label marks two special molecules they had made: glycine and alanine, two of the most common amino acids, the building blocks of proteins. The NASA-Ames scientists had mixed four simple molecules under conditions that decades ago were unthinkable for chemistry, and they'd produced not just any end products but two molecules that are considered cornerstones of life—and they'd done it by simulating the birthplace of solar systems.
With this experiment and many others, Sandford and his colleagues of the past twenty years have been piecing together one of the greatest current puzzles of the Stardust Revolution: to understand the cosmic origins not of the Solar System's minerals but of its organic molecules. They're creating a detailed molecular family tree that traces our specific carbon ancestry from the molecules of terrestrial life to the stars. The NASA-Ames group is one of the few in the world that's able to tie all the required pieces together: the collection of detailed telescope observations of what's actually out there; laboratory simulations of possible chemistry under diverse cosmic conditions; and hands-on astrogeology, in the form of cometary and meteorite samples, to trace the genealogy of the organic, or carbon-based, molecules that arrive on Earth in carbonaceous chondrite meteorites, comets, and interplanetary dust particles.
In mimicking the coming together of stellar dust grains, water, organic molecules, and starlight, Sandford is realizing the great dream of astrochemist Carl Sagan. Back in the early 1950s, Sagan, then an astronomy undergraduate student at the University of Chicago, sat in on Harold Urey's legendary lectures on planetary formation. At the same time, Urey's graduate student Stanley Miller was conducting the famous primordial-soup experiments that showed that it's possible to make the amino acids central to life from the s
implest of molecules, ones thought to have been abundant on the early Earth. For Sagan, Miller's discovery was an inspiration rather than an answer. Smitten by the implications of Miller's result, Sagan decided that his life's mission was to extend Miller's research beyond the Earth, to the cosmos: to understand the cosmic primordial soup. At NASA-Ames, Scott Sandford and colleagues are doing just that. Theirs are the twenty-first-century cosmic versions of Stanley Miller's experiments. Sandford is part of a new generation of astrobiologists who are combining astrochemistry observations, hands-on analysis of extraterrestrial samples, and experimentation to piece together a new story of the origins of life on Earth in a cosmic context. When Scott Sandford first held stardust thirty years ago, he thought it was all about rocks. Today he's a rock hound who's turned prospector for cosmic life.
DNA FROM SPACE
When the Stardust probe returned to Earth from its rendezvous with comet Wild 2, Sandford wasn't waiting to collect bits of rock swept up from the vapor trail of this dirty cosmic ice ball. He was after molecules. What the Stardust researchers confirmed when they parsed Stardust's microscopic payload was that, as suspected from infrared telescopic observations, comets aren't just gigantic, dirty snowballs; they're gigantic, dirty, organic snowballs. The molecular evidence swept up by Stardust showed that if you'd been traveling in its wake and had metaphorically lowered your rocket windows, you'd get a very diffuse whiff of something resembling diesel exhaust. The Sun's heat and wind weren't blowing off just bits of cometary dust and water but also the ringed carbon molecules called polycyclic aromatic hydrocarbons—key components of diesel exhaust—as well as a rich stew of other oxygen-, nitrogen-, and carbon-bearing molecules.
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