The Stardust Revolution

by Jacob Berkowitz

  What made this discovery all the more interesting was that comets are considered to be the Solar System's ultimate time capsules, agglomerations of the primordial dust, ice, and molecules from which the Solar System formed, preserved since then in a permanent deep freeze out past Neptune, in the Solar System's Kuiper Belt. Thus, like light from distant galaxies that reaches Earth to reveal events from billions of years ago, these cometary samples tell a story not from today but from our planetary beginnings. The Stardust samples also revealed that comets and asteroids are much more similar creatures than previously thought, with overlapping origins. The material collected from Wild 2 revealed that the early Solar System had been a great mixing pot, with material gravitationally churned toward and away from the Sun as if by a mixing spoon. For example, the stardust samples contained a mix of mineral grains, some of which had been melted in the heat from the newborn Sun, and others that had never warmed and thus must have always been in the Solar System's outer reaches. Thus the material from Wild 2 resembled a carbonaceous chondrite, a chip off a dark asteroid, the most primitive and unaltered type of meteorite.

  The Stardust samples were the latest in a century and a half of carbon-rich objects from space to get scientists talking about the relationship between carbonaceous chondrites, comets, and life on Earth. In the Stardust Revolution, these streaking cosmic objects have gone from being seen as the age-old harbingers of death and disaster to offering tantalizing clues about the origins of life on Earth. We're at a remarkable stage with each comet sample and carbonaceous chondrite, and now, too, with a bevy of planned sample-return missions to comets and asteroids, adding a new piece to the story of our cosmic molecular origins.

  What we've found is that a supermarket-style tour through the organics section of a carbonaceous chondrite is the molecular equivalent of going into outer space and finding carrots, bananas, and broccoli. The soluble organic molecules found in meteorites—those that readily dissolve in water—form life's basic shopping list of ingredients. Carbonaceous chondrites contain not just the simplest organic building blocks, such as methane, ammonia, and formaldehyde, but also the relatively complex suite of molecules that come into play when we make decisions about what we'll have for breakfast. This suite includes proteins and fats, as well as the molecules that make DNA and give us the personality traits that allow us to choose between the sugary donut or the high-fiber bran muffin. According to astrobiologists, carbonaceous chondrites are molecular life kits that arrived filled with the ingredients needed for the emergence of the Earth's first organisms.

  And scientists are still learning how to read the cosmic stories held by carbonaceous chondrites. For example, most of what we know about the organic nature of the most famous carbonaceous chondrite, the Murchison meteorite—which crash-landed in Australia on September 28, 1969—has been gleaned only in the past decade, more than thirty years after the meteorite's arrival. Fifty years of technological advances has made a huge difference in the debate over carbonaceous chondrites. Now, rather than arguing about the generic nature of oily carbon materials—as was the case in the 1960s—stardust scientists are able to burrow down to parts-per-billion levels of detail for specific molecules. For the Murchison meteorite's fortieth birthday, a German-led team of researchers gave the meteorite a kind of organics general physical. They didn't search for particular molecules, as previous studies had done, but instead took an overall inventory of any organic molecules they could measure. The result was stunning. All previous research had identified about five hundred different organic molecules in Murchison, but, using a higher-precision chemical inventory method, the research group found more than fourteen thousand types of molecules. All this in only several milligrams of material taken from the meteorite's pristine interior. As a result, they estimated that Murchison could contain millions of different organic molecular structures—a cosmic big-box store of organics. Whereas the Earth was previously considered an organics oasis in a barren star field, this study provides a powerful hint that Earth's life-related chemical diversity might in fact be a small subset of an even richer cosmic diversity.

  One of the most common soluble organic molecules in carbonaceous chondrites is carboxylic acid. Chemists spot a carboxylic acid when they see a chain of carbon atoms that ends with a familiar carboxyl group: a carbon bonded to an oxygen atom on one side and an oxygen-hydrogen combination on the other. Most of us, though, know carboxylic acid when we taste it. Vinegar, or acetic acid, is the simplest form—just a two-carbon chain, including the carboxyl group, giving vinegar its sour taste. From our first day, we graspingly gulp it down in the form of caprylic acid, an ingredient in breast milk. As kids, many of us survived on it as arachidic acid, a component of peanut butter.

  Although the discovery of carboxylic acid doesn't make great news headlines, the discovery of amino acids in carbonaceous chondrites does. These building blocks of proteins are often referred to as shorthand for life. Every muscle in your body—from your heart to the little muscles that move your eyes—owes its structure to long strings of amino acids joined into a peptide chain. These chains provide both protein structure and function. Some proteins are enzymes, the body's chemical matchmakers. Enzymes bring together other molecules and speed up chemical reactions. Without them, the chemical reactions in our bodies wouldn't happen quickly enough to keep us alive, and we'd experience a fast death from chemistry that was too slow. When you see a buff bodybuilder's flexed biceps, you're seeing the cumulative effect of trillions of amino acids. They also bulk up carbonaceous chondrites.

  Life on Earth uses twenty different amino acids, each one a variation on a central three-part chemical theme that includes a carbon chain core with a nitrogen-containing component at one end (the amine part) and a carboxylic acid part at the other, hence amino acid. But more than eighty amino acids have been isolated from the Murchison meteorite, including eight of the twenty used to build us. Our bodies manufacture some amino acids, but others—“essential” amino acids—we get in our foods. Without them, we're not able to manufacture the full range of proteins that our bodies require. Three of these essential amino acids—leucine, isoleucine, and valine—are all found in the Murchison and other carbonaceous chondrites: you could pop a valine out of Murchison meteorite and exchange it for one in you. Few facts in the Stardust Revolution speak so eloquently to our cosmic ancestry. Our bodies both manufacture and require amino acids identical to those contained in meteorites that, until recently, were at home in the asteroid belt between Mars and Jupiter. Almost half (and probably more, as new carbonaceous chondrite samples appear) of our twenty amino acids are a fingerprint not only of terrestrial life but also of cosmic organic chemistry.

  One of the critical achievements of stardust scientists in the past decade has been a conclusive confirmation that these meteoritic amino acids arrived in the meteorites rather than being terrestrial contamination. The evidence for this extraterrestrial origin of amino acids is based on a trio of facts. First, there's a much greater diversity of molecules in carbonaceous chondrites—known as “extraterrestrial analogs”—than on Earth. Many of these molecules are either extremely rare or nonexistent on Earth—as shown by the eighty Murchison amino acids—so they must have come from somewhere else. The second clue is that the meteoritic molecules have distinctly alien isotopic signatures, an indication that they were formed in the ultracold environment of space.

  Finally, the most intriguing evidence relates to the fact that some parts of the cosmos are more chemically ambidextrous than here on Earth. Some molecules, including amino acids and many sugars, are chiral—their structure is such that they have naturally forming left-handed and right-handed versions, mirror-image molecules that, just like your hands, can't be superimposed. Even though the atomic contents of the left- and right-handed molecules are exactly the same, the left- or right-handed configuration is pivotal to how, and whether, the amino acid can bond with another molecule. Just as you can't put a right-handed glove on your
left hand, you can't mix left- and right-handed amino acids. Amino acids in life on Earth are totally southpaw. We've evolved such that our amino acids are exclusively left-handed, and all our sugars, with which the amino acids bind, are right-handed. However, in carbonaceous chondrites there's a mix of both right- and left-handed amino acids, though often with a slight balance in favor of the lefties. This meteoritic evidence for the extraterrestrial formation of amino acids was further supported with the discovery of the amino acid glycine in the material collected by the Stardust mission from comet Wild 2, the first amino acid detected in a comet. Thus, their diversity and isotopic and structural chemistry combine to seal the case that these meteoritic amino acids are indeed otherworldly.

  Although the discovery that carbonaceous chondrites are packed with the stuff of proteins has energized the Stardust Revolution, the one-two punch is that they contain not only the building blocks of life but also the blueprint materials. The latest confirmed extraterrestrial organic bounty in carbonaceous chondrites is the presence of nucleobases, the stuff of DNA. Nucleobases are the largest components of nucleotides, the letters of the DNA and RNA alphabets. The four DNA nucleobases, or just bases, in their abbreviated forms are sometimes known as A, G, C, and T, for adenine, guanine, cytosine, and thymine. In RNA, a sister molecule of DNA that acts as an intermediary in the building of proteins, thymine gets bumped in favor of uracil, U.

  Nucleobases are the structural and functional foundation of our genetic code, the bits that bind to form the long chains of DNA that are our genetic blueprint. On their own, they carry little information, but put billions of them together, and they become the letters of our genetic alphabet. What meteorites reveal is that this is a cosmic alphabet. Meteorites carry the essence of our genetic code, one that formed abiotically in space before there was life on Earth—before there even was an Earth. In 2008, British astrochemist Zita Martins performed a kind of cosmic genetic forensics. Through stable isotopic fingerprinting—the same kind used to show that meteoritic amino acids have extraterrestrial origins—she showed that the nucleobase uracil that she'd extracted from the Murchison meteorite had formed in space rather than from terrestrial contamination. In 2011, a NASA-led group extended this line of research to show that Murchison and ten other carbonaceous chondrites contain a wide range of extraterrestrial nucleobases, including the core parts of adenine and guanine.

  The importance of the discovery of these extraterrestrial nucleobases to our cosmic heritage goes beyond its link to our genetic code. All the nucleobases are formed from either a single or a double ring of carbon atoms, with other atoms branching off these central rings. These cyclic rings of carbon are not only central to our information system; they're also the backbone for energy-transfer molecules and those that perform a suite of other functions. The central molecule in energy transfer in all animals, adenosine triphosphate—more commonly called ATP—is based on a nucleobase-like molecular skeleton. As a final indication of just how important this cosmic carbon-ring framework is to the proper functioning of our daily lives, consider this: the double-carbon-ring-based molecule xanthine was also extracted from the Murchison meteorite. If you chemically accessorize xanthine just a touch with the addition of a couple of simple methyl groups—a carbon and three hydrogen atoms—you get a truly marvelous molecule: caffeine. It's thus quite possible that somewhere out there in the asteroid belt orbiting our Sun is the cosmic equivalent of the jolt that enlivens your morning java.

  Most of a carbonaceous chondrite's organic carbon, up to 95 percent, is in the form of a sticky, tarry mass that reminds meteorite researchers of terrestrial kerogen, the decomposed organic matter found in sedimentary rocks. On Earth we search and fight endlessly for the chemical by-products that kerogen produces when it is heated inside the Earth: natural gas and crude oil. Kerogen in meteorites, as on Earth, is made up of an interlocked, complex network of rings of carbon, with a spicing of hydrogen, oxygen, nitrogen, and sulfur. Some researchers think this abundant tarry carbon is the end product of accumulated polycyclic aromatic hydrocarbons, the abundant, large ring-shaped carbon molecules thought to be pumped out by dying stars. But since this glob-like part of carbonaceous chondrites is insoluble, it remains a kind of black hole that holds its individual secrets in an often-impenetrable mass.

  In exploring carbonaceous chondrites, stardust scientists have found that they contain all the ingredients of life. Yet “ingredients” doesn't fully capture the scope of molecular diversity found in carbonaceous chondrites and comets. We see an entire molecular system. There are the information molecules in nucleobases; a wide array of structural components, including long chains of carbons and complex carbon rings; there are the nitrogen- and phosphorus-bearing molecules key to protein building and energy transport in cells; and there's a mix of water-soluble and not readily soluble carbon molecules—ones that are ready to react, and others that form a kind of molecular carbon warehouse awaiting the right conditions to release their wealth.

  For the scientists at the leading edge of the Stardust Revolution, the bounty of organic molecules in meteorites prompts one key question: What's their direct relationship to life on Earth?

  FROM ETERNITY TO HERE

  When Apollo mission astronauts stepped, jumped, putted, and drove across the Moon, most television viewers were focused on the astronauts and their experience of being on another world. The first generation of planetary geologists, however, had their eyes on the rocks. The lunar-surface samples returned by the Apollo missions were geologists’ first opportunity for lunar “ground truth”—the geologist's term for the clearer picture of reality that comes from actually visiting a locale and collecting samples. One of the key questions geologists asked those Moon rocks was: How old are you? The answer, gained through radioactive dating, provided ground truth not just about the Moon but also about a lost period in the Earth's formative history, one that's as much about life as rocks.

  The dating of rocks from the various craters that were the Apollo mission landing sites showed that much of the Moon's pockmarked surface is the result of a relatively brief lunar developmental period called the Late Heavy Bombardment. This was a period of several hundred million years following the Earth and Moon systems’ initial formation from the protoplanetary disk, when the Solar System was strewn with copious planet-building leftovers: from bits of dust to asteroids to comets and enormous planetesimals hundreds of miles across. It was an era that would bring joy to those who love crash-bang movies. For millions of years, the natal Earth collided with these other bodies, a period of heavy bombardment from Earth's perspective, but if you asked the other drivers, you'd get another answer. On Earth, the evidence of this period has been eroded by tectonic forces, the movement of continental plates. But the crater-pocked surfaces of the Moon, Mars, and Mercury bear testament to these collisions during our Solar System's early, formative years. In retrospect, the Late Heavy Bombardment appears as a logical tailing-off of a bumper-car-like process of collisional accretion that formed the Earth and other rocky planets. Rather than anomalies, cosmic collisions are the nature of Solar System development, and the Late Heavy Bombardment was the tail end—though not the end, since the process continues today—of an essential formative process.

  Initially, biologists envisioned this period of intense cosmic collisions as a brake on the emergence of terrestrial life—that just as some primordial reproduction was heating up, the Earth would have been pounded by a planetesimal, asteroid, or comet that vaporized oceans and left the natal Earth cloaked in a cold, choking haze of dust. But Stardust Revolution scientists are rethinking this period, not as one of early life's brutal bombardment but instead as a critical period of seeding the Earth with the cosmic molecules of life.

  This new way of seeing was inspired initially by thinking not about life's extraterrestrial start but about its end, by the realization of the role of past asteroid and comet impacts in mass extinctions on Earth. While today the image of a terrified dinosaur l
ooking up at an incoming fireball is the stuff of kindergarten books, our understanding of the role of cosmic impacts on the snuffing out of past life is a remarkably modern one. It wasn't until the early 1960s that geologists generally agreed that many distinctive circular craters found around the world are the remains of ancient cosmic impacts, rather than, for example, ancient volcanic rings. Similarly, the first evidence for an end-of-dinosaur-era impact didn't arrive until 1980, in a paper by Luis Alvarez, who reported the discovery of the K-T boundary—a sixty-five-million-year-old planet-wide geological layer that contains a high level of iridium, a metal rare on Earth but more common in asteroids, and tektites, glassy shards caused by the melting and re-forming of rock during a massive impact. Here was the smoking gun that proved how the end of the dinosaurs came about. The evidence for a cosmic collision's impact on ancient life was sealed in the early 1990s, when the impact was linked to the “crater of doom,” the Chicxulub crater, an impact crater more than one hundred and ten miles across, discovered buried beneath the Yucatán Peninsula and dated to the same time as the K-T boundary.

  For Carl Sagan, and for Christopher Chyba, his Cornell colleague at the time, this new view of the asteroidal demise of the giant lizards was a clue to an even more ancient story about the role of cosmic collisions in the history of life on Earth. The combination of the awareness of the role of cosmic collisions in the story of life on Earth, the revelation of the Late Heavy Bombardment, and the growing evidence of the carbon-rich content of carbonaceous chondrites all came together to tell another tale. In 1992, four years before his death, Sagan, along with Chyba, proposed a new cosmic-delivery view of the origin of Earth's primordial molecules of life. The Earth didn't form with its thin life-sparking organic layer intact; life's ingredients arrived later via cosmic delivery.