The Stardust Revolution

by Jacob Berkowitz

  This end-stage shrinking act begins when these stars have converted the bulk of their core hydrogen into helium. At this point, a layer of hydrogen begins burning around the star's helium core, marking the beginning of the red giant phase of the star's life. The “giant” part of the name comes from the fact that in these last moments the star begins to rapidly puff up, growing until an outer shell extends from ten thousand to as much as one hundred thousand times the star's original radius. With such a huge surface area, the star brightens enormously. As a result, the star's outer atmosphere is relatively cool and therefore appears red—hence, red giants. The best-known red giant star in the night sky is the bright star Mira, whose name, aptly, means “the wonderful.”

  In the star's next end-of-life stage, the asymptotic giant branch, helium in its core begins to fuse, or burn, transforming into carbon and oxygen. With each burst of core nuclear activity the star pulsates, a blast of solar wind carries the shell farther and farther away from the star's core. In the end, all that is left of the mighty star is a wizened white dwarf—a gravitationally trash-compacted carbon and oxygen stellar cinder, reduced to Earth-size, that continues to glow not from nuclear reactions but from its latent heat. About one in twenty stars is a white dwarf, and about 95 percent of stars are less than eight solar masses. Thus billions of stars in the Milky Way alone are going to lead geriatric red giant lives. Astronomers calculate that the Sun will take its turn as a red giant and end as a white dwarf in about 7.8 billion years.

  You might well be wondering, where does all that disappearing star material go? The bulk of the star's body is transformed into molecules and dust. It turns out that in their death throes, red giants give the gift of life. They become what Lucy Ziurys calls “the most remarkable chemical laboratories in the universe.” The vast majority of that chemistry is carbon, or organic, chemistry. If it seemed impossible to some theorists that stars might forge the elements, it was deemed equally impossible that stars were chemical factories. Until recently, astronomers thought that a star's outer shell, its circumstellar envelope, was a hellish, no-go zone in which all but the simplest and hardiest molecules, such as carbon monoxide, would be blown apart by the enormous temperature and the star's intense UV and particle radiation.

  But what Ziurys has discovered is that a red giant's circumstellar envelope is more analogous to a chemical tidal zone, an area of enormous chemical diversity nurtured by its location at the boundary of “sea and shore”—the boundary of the dying star and the expanse of the interstellar medium. Using a radio telescope and infrared observations, Ziurys has constructed a chemical flowchart that maps the changing conditions in a red giant's circumstellar envelope. As in a tidal zone, it's characterized by enormous gradients that spur a cascading flurry of chemical reactions. At the inner edge of the envelope, the temperature is close to 1,300°F, and the density is a hundred thousand times higher than it is at the envelope's outer edge, where the temperature has plummeted to a frigid -414°F. Around just one red giant, IRC+10216, astrochemists have identified a smorgasbord of more than fifty different chemicals. About four in five are carbon-based, from methane to bulky eight-carbon molecules. There are also metal-containing molecules, including sodium chloride, or salt, and molecules containing magnesium and aluminum, elements forged through atomic synthesis in red giants.

  As this dust and molecular soup expands outward, it forms bright planetary nebulae, with names like Cat's Eye and Helix Nebula—lit up like Christmas ornaments by intense ultraviolet radiation from the sizzling white dwarf. Gradually, over about ten thousand years, this cloud of stellar material dissipates, seeding the interstellar medium with a rich mix of organic molecules.

  Since Townes's discovery, the past decades have been a period of astrochemical revelation. Where once the cosmos appeared as an atomic landscape, barren of all but the simplest chemical complexity, it has now emerged as molecular. The discovery of water initially made it one of a tidy half-dozen known cosmic molecules, but since 1970 the molecular stock index has been on a steady rise. By 1973, the astromolecular count hit 27 molecules; by 1987, it had more than doubled to 64; and by early 2012, it had reached about 140, with no end in sight. This group molecular soup includes mixes of at least fifteen different elements, from carbon, oxygen, and nitrogen to metals such as iron and aluminum, as well as those elements familiar from the label of any energy drink, including sodium, potassium, and phosphorous. Each of these molecules has been identified through a stringent process (with scientists looking over one another's shoulders and more than happy to prove a colleague wrong) that involves perfectly matching the hundreds of lines, and relative intensities, in each molecule's light fingerprint. The absence of only a single line rules out a possible detection.

  When it comes to life's ingredients, the spiral arms of the Milky Way aren't a need-to-go-shopping wasteland; they're chock-full of the organic stuff of which we're made. The cosmos has gone green. Look into the darkness of a star-filled night sky, and some of the light you see has been tinged by passing through stellar clouds of sugars, fats—even alcohol. Reading alcohol might make you think of a shot of whiskey, a glass of wine, or a bottle of beer. What astrobiologists are seeing would keep all Earthlings tanked from here to eternity: clouds of diffuse cosmic booze millions of light-years across.

  The mix of molecules discovered is a combination of, by Earth standards, the familiar, the rare, the exotic, and the truly bizarre. Given the astronomical extremes of temperature, density, and energies around stars and in interstellar space, there's a true astrochemistry that's different than anything seen on the relatively quiescent Earth—and in some cases it is so extreme that no one has yet been able to replicate it in a terrestrial lab. Out amid the stars are tongue-twister molecules like cyano-octatetra-yne and the butadiynyl radical, molecules that form in space but not on Earth.

  Thus, astrochemistry is definitely its own chemistry. Yet what astrochemists have discovered is that this broader cosmic chemistry is different in particulars but not in fundamental processes from terrestrial chemistry. While there's enormous cosmic molecular diversity, what astrochemists have unveiled over almost half a century of cosmic molecular mining is that at their core, these cosmic molecules are ones we associate with us, with chemistry on Earth.

  In a terrestrial context, it's not exactly a chemical stew you'd want to play in. The most common familiar cosmic molecules could be described as a pot of poison with a dash of sugar and a pinch of salt. Though toxic individually, what makes them deadly also makes them potent in life—they're highly reactive; they want to bond and form more complex molecules. After molecular hydrogen and water, the most common cosmic molecule is carbon monoxide, on Earth, an odorless, colorless gas released in combustion—from tailpipes and furnaces—that will quickly kill you if not vented properly. There's abundant ammonia, well-known as an ingredient in toilet-bowl cleaners; formaldehyde, also known as embalming fluid; and more than a touch of acetone, a common solvent used in nail-polish remover. There's also plenty of hydrogen cyanide, the toxic gas used in the Nazi death-camp gas chambers. Not to mention the abundant methane, a carbon with four hydrogen atoms, which, on Earth, is the extremely flammable primary component of natural gas. Amid this hoary chemical cocktail is a more welcome mix of substances. It turns out that interstellar space is awash with a molecule made up of two parts carbon, six hydrogen atoms, and a single oxygen atom to top it off. Chemists call it ethyl alcohol. Most of us celebrate it as vodka. There are the salts sodium chloride, the stuff of salt shakers, and potassium chloride, a key component of many agricultural fertilizers. Also acetic acid, named for the Latin word for vinegar, acetum, and the main ingredient of vinegar. To balance out all this alcohol and salt, there's also lots of simple sugar in the form of glycolaldehyde, an eight-atom molecule that's a building block for more complex sugars such as glucose, one of the sugars that makes table sugar.

  One of the gradually building realizations of this cosmic carbon-chemistry awareness i
s that this chemistry also produces a flurry of big, complex carbon molecules, ones that on Earth are associated with barbecued steak and diesel exhaust. During the past three decades, as astrochemists cataloged bigger and bigger carbon molecules using radio-wave vibrations, over in the infrared there was evidence that something else was present in the interstellar stew. That something else now goes by the name polycyclic aromatic hydrocarbons, or PAHs, which are special for three reasons: they're big; they're rings; and they're everywhere, infiltrating every nook and cranny of the cosmos like the fine soot they resemble.

  Rather than a single molecule, PAHs refers to a broad class of these molecules. PAHs are the molecular carbon giants of the interstellar medium. Whereas most cosmic molecules have few carbon atoms, the average PAH has fifty; some have many more. This places PAHs at the molecular borderlands, claimed as huge molecules by some astrochemists and as small dust by others. It's an academic hinterland all the more contested because PAHs are identified by the equivalent of a smudged molecular fingerprint—a broad band of infrared signals that points to an overall generic structure rather than to specific individual molecular ones.

  Yet what defines them isn't so much their size as their shape. PAHs are thought to form from groupings of the ring-shaped carbon molecule benzene. Benzene is dubbed an “aromatic” molecule because when these ring-shaped carbon molecules were first identified by sniffing chemists in the nineteenth century, it was by their mildly sweet smell, which they'd only learn later is carcinogenic. Each carbon atom in the benzene ring, as well as clasping its carbon neighbors, is also adorned with a hydrogen atom, making the molecule a hydrocarbon—a molecule made up entirely of carbon and hydrogen. On Earth, when we hear hydrocarbon, we think petroleum and natural gas. The cosmic benzene rings join to form snowflake-like “polycyclic” structures, the PAHs.

  Just as they're churning out other molecules, stars are pumping out PAHs. These big, sweet molecules are the most abundant organic molecules in the universe, accounting for up to one-tenth of all carbon atoms out there. They're everywhere, in almost every astrophysical condition, from the hottest to the coldest: PAHs are seen around dying stars, in the diffuse interstellar medium between stars, and in the ultracold dense molecular clouds that give birth to new stars. Viewed in the infrared, galaxies spew out vast clouds of PAHs into the intergalactic medium.

  What makes PAHs all the more special is that they're far from being only extraterrestrial. On Earth, PAHs are usually thought of as pollution, formed from the incomplete burning of fuels. We cook up and eat PAHs in the carbon-y surface of whatever we're grilling when we fire up the barbecue, and we breathe them in when we're sitting in traffic behind a poorly tuned truck belching clouds of sooty exhaust. But thinking of PAHs only as pollution obscures a deeper truth: these cyclic carbon molecules are essential to life. Hydrocarbons come in two basic structures: rings and chains. The long chain-shaped hydrocarbons, such as the eight-carbon octane, are a key part of gasoline. It's when a chain of carbon atoms circle in on themselves to form stable rings that much of the most interesting biology takes place. For example, chlorophyll, the green pigment that enables plants to convert sunlight into sugars, is a large polycyclic molecule that includes several additional nitrogen atoms. Astrochemists have seen nitrogen-containing PAHs being spewed from dying stars, and many think there could be other, more complex PAH structures awaiting discovery. Even more essential, three of the nucleobases that form the backbones of DNA and RNA—cytosine, thymine, and uracil—are benzene-like ringed molecules in which two nitrogen atoms have replaced two of the carbon atoms. Thus, PAHs are more than a vast organic carbon repository; they're abundant evidence that the fundamental structural chemistry that underpins biology—the carbon-ring structure—is intimately wed to the cosmos. As a result, many astrochemists think that PAHs are a cosmic chemical feedstock—that these large molecules are eventually broken down by ultraviolet radiation and heat to provide the raw materials for further cosmic chemistry.

  In our molecular searching, we are still at sea, still learning to interpret cosmic molecular fingerprints. All indications are that we have seen only the tip of the iceberg in terms of the molecular and mineral makeup of the interstellar medium, of what's actually out there between the stars. The strongest evidence for this is the existence of the enigmatic diffuse interstellar bands, the DIBs—one of the longest-standing mysteries in astronomy. When astronomers take the spectra of starlight that has traveled through clouds of dust and gas in the interstellar medium, they see not just the well-defined absorption and emission lines of identified atoms and molecules but also a forest of unidentified broad-absorption patterns, the DIBs. The term diffuse refers to the spectroscopic fact that these unidentified regions are broad valleys rather than thin, distinct lines. Paul Merrill, the Mount Wilson astronomer who would later provide the keystone observation that stars make elements, discovered the first DIBs in 1934. They've been a vexing, unsolved problem ever since. From the initial discovery of these mysterious light fingerprints, astronomers have identified more than three hundred DIBs in visual light, as well as dozens in the infrared region of the spectrum. The closer they look, the more they find.

  There have been dozens of theories trying to explain what is producing DIBs. They have ranged from dust to molecules and from gas to solid materials. What all theorists agree on is that these deep spectroscopic prints can't be pinned on a single molecular suspect. Today, most astrochemists and cosmic-dust researchers are putting their money and telescope time on the chance that DIBs are produced by a range of large, complex carbon-based molecules. Other astronomers think that these “mysterious” carbon molecules might not be that mysterious, or at least not foreign, but that they are created by chains of amino-acid-like molecules, or proteins. Whether bloodlike pigments, muscle-like proteins, or the stuff of carbon-fiber bicycles, the DIBs represent a terrain of deep valleys and mountains within which lie further secrets of the Stardust Revolution.

  The first astrochemists were struck by the possible relationship between cosmic chemistry and life on Earth. Charles Townes's discovery of water and ammonia alongside methane and carbon dioxide in interstellar space recalled the four molecules of a by-then famous experiment on the possible molecular origins of life. A decade earlier, Stanley Miller at the University of Chicago had mixed just those four molecules together, had zapped them with electricity, and, to his amazement, had produced a sludge loaded with amino acids. Townes and his colleagues saw that what Miller had done in the lab could perhaps occur throughout the cosmos. “These observations have an interesting connection with biology,” the astronomers wrote in 1971, reflecting on the emerging view of a molecular cosmos.

  But as much as this molecular cosmos resembled our molecular home, it was still out there—similar but removed by light-years from the messy business of life on Earth. Yet, while some astronomers were using radio telescopes to reveal a molecular cosmos, others had found that they didn't need telescopes at all. The next step in the Stardust Revolution would join out there and right here by using a whole new approach to stardust—not looking at it through a telescope but holding it in our hands.

  Would you rather look at stars through a telescope or hold stardust in your hands?

  —Richard Herd, curator of the

  Canadian Meteorite Collection, 2011

  THE SPACE-ROCK EDUCATION OF SCOTT SANDFORD

  In astrobiologist Scott Sandford's cluttered office at NASA's Ames Research Center, twenty-five miles south of San Francisco, the walls are festooned with pictures of his scientific adventures. Some show him bundled up in a heavy parka during one of his Antarctic expeditions in search of meteorites. In his three-decade career, Sandford has stooped down to pick up pieces of the Moon—he found one of the largest lunar meteorites ever, MacAlpine Hills 88105—and was part of the team that from Antarctic ice retrieved a dark meteorite, Allan Hills 84001, identified as a long-traveling piece of Mars that some would later claim contained ancient fossilized Ma
rtian microbes. Yet for all this meteoritic success, Sandford is not in the picture of which he's proudest. He couldn't be. It's a simple black-and-white image of what look like tiny grains of sand—which is what they are, except that they were formed in our Solar System's nascent protostellar cloud more than 4.5 billion years ago, long before the existence of Cancun, Cape Cod, or any other terrestrial beach.

  The grains were collected by NASA's Stardust mission, launched in 1999, for which Sandford was a leading member of the scientific team. Stardust was the beginning of a dramatically new kind of Solar System prospecting: collecting samples from their source. Rather than wait for asteroidal samples of the Solar System to arrive on Earth as meteorites, Stardust was sent out to sample a comet in its natural habitat—the first space probe to do so. By 2004, Stardust had caught up with comet Wild 2 at about Mars's distance from the Sun, where the probe sped to within one hundred and fifty miles of the two-and-a-half-mile-wide nucleus of the comet—close enough to pass through the comet's coma, the fuzzy halo of dust and ice ejected, geyser-like, from the comet's icy core as it approached the Sun. There, Stardust's aerogel collector—made of a material as light as air yet rigid and strong enough to stop microscopic comet shrapnel—swept up some of this microscopic cometary debris. In 2006, after an almost three-billion-mile round trip, the little explorer dropped off its extraterrestrial cargo in the Utah desert in a special return capsule.