Sagan and Chyba multiplied the higher rates of cosmic collisions during the Late Heavy Bombardment with the organic molecular content of carbonaceous chondrites and cosmic dust particles. They found that, rather than whacking life, the Late Heavy Bombardment delivered as much, if not more, organic material to the young Earth as could have been produced in its atmosphere or oceans through local chemical reactions. Sagan and Chyba had extended the famous Miller-Urey experiment into a cosmic context. They showed that the chemistry of life wasn't dependent on starting on Earth, but rather that the early Earth was literally seeded with life's core ingredients. They concluded that most of this carbon arrived not with a meteoritic bang but with a steady organic dusting, molecules drifting down as part of cosmic dust over millions of years, transforming the early oceans into a rich organic primordial soup, a broth of life.
But could a gradual, “green” rain over several hundred million years really deliver enough organic material to make all the fish, birds, trees, fungi, microbes, and, of course, people that make up terrestrial life? Subsequent studies have shown that the amount of organic material that splashed down on Earth during the Late Heavy Bombardment was actually many times more than currently on the Earth's surface; that our planet has in fact lost carbon to space in the form of volatile carbon molecules such as carbon monoxide or carbon dioxide, or that this organic material has been buried deep below the Earth's surface by geological processes.
The cosmic, postnatal delivery of Earth's organic layer is also supported by the fact that many astrobiologists think the evidence shows that a significant portion of the water that covers three-quarters of the Earth's surface similarly arrived via cosmic delivery in the form of water-rich asteroids and comets. The exact origin, timing, and delivery mechanism for Earth's water is still a hotly debated topic, with some researchers pointing to the possibility of off-gassing from water-rich molten rock, and others pointing to new evidence that some asteroids are much wetter than previously thought and that some comets bear the same isotopic signature as the water in Earth's oceans.
“Everyone used to talk about the origin of life as if it all happened on Earth,” says astrobiologist Scott Sandford. “A lot more stuff falls out of the sky. That could have played a big role. There's been a tendency for people to divide themselves into camps” on the origin of the Earth's carbon molecules, “but in reality that's probably crazy. It may well be that the net story for Earth could have been a very complicated and confused one. It may have been that when early life needed amino acids, it found them in reservoirs that contained both an extraterrestrial and terrestrial component. They were being made in both. Probably, early life needed some materials that by and large were heavily dominated by what was falling out of the sky, and other components that were largely being made terrestrially. In the end, life probably took whatever it needed and whatever it could find.”
The first tentative fossil and biochemical evidence of bacterial life on our planet appears in ancient ocean sediments in Greenland formed during the latter part of the Late Heavy Bombardment more than 3,850 million years ago. Those first marine cells contained a remarkably recent cosmic heritage. Their bacterial bodies contained molecules that only hundreds of millions of years before, or less, had been the stuff of a star-birthing cosmic cloud. The carbon had spun around its nascent star, clumped into dust, maybe into an asteroid or comet, and eventually arrived on the surface of a young world. Now as carbon-based molecules, they were reproducing, adding their own story to that of this cosmic carbon, shaping it into new proteins, sugars, and eventually long threads of the genetic molecules that would carry this cosmic molecular heritage into the creation of a new world.
TRACING OUR COSMIC CARBON ANCESTRY
With the knowledge of organic molecules in carbonaceous chondrites and comets and their cosmic delivery to Earth, one of the most intriguing, ongoing mysteries of the Stardust Revolution is the question of the cosmic origin of these carbon-based molecules. While fifty years ago astronomers didn't even think there was cosmic chemistry, today, in the Stardust Revolution, there are so many potential locales and chemical pathways for cosmic organic chemistry that the heated discussions are about which one, or ones, leads to us. In some situations, it's a case of the blind astrochemists and the cosmic molecular elephant. Different researchers literally see different evidence: radio astronomers see the spectroscopic signatures of molecules as gases around old stars; infrared astronomers try to deduce the nature of molecules frozen in ices and large clumps of carbon dust in interstellar space; and astrogeologists try to trace our carbon ancestry backward by analyzing the nature of organic molecules in meteorites. Is the molecule whose spectroscopic fingerprint is seen by a radio astronomer streaming away from a billowing red giant star the same molecule that dissolves out of a carbonaceous chondrite on Earth? Similarly, there's such a variety of forms of cosmic carbon-based molecules—from the biggest multi-ring aromatic molecules to the enormously abundant but simple carbon monoxide—that trying to trace our cosmic carbon ancestry is a case of looking at a vast genealogical database and having to work, and argue, just to know where to start looking for any particular genealogical lineage.
We're at the start of tracing our detailed cosmic carbon ancestry, and with each new observation, experiment, or meteorite sample, we are adding a little more color to the story—a process akin to knowing where a great-great-grandmother was born or that an even more distant relation was the one who'd made the great immigrant's journey from a distant land across the sea. Tracing our carbon ancestry has far-reaching implications for both understanding our own cosmic origins and the likelihood of life elsewhere. For example, if most key organic chemistry occurs around newborn stars, rather than all newborn solar systems being stocked with complex organic molecules from earlier cosmic processes, this fact would set boundaries on the likelihood of other organic-rich planets.
The weight of evidence that's emerging in the polyvalent effort to trace our cosmic carbon heritage is that the physical origin of the molecules that rain down on Earth largely predates the formation of the Earth. When we trace the genealogical heritage of the building blocks of life found in carbonaceous chondrites and meteorites, we find ourselves led back not to some little organically rich pond on the primordial Earth but to the cold molecular cloud from which our Solar System, and hundreds of others, emerged. What's clear is that the organic molecules that arrive on Earth were formed in a variety of astrophysical environments, from around dying stars, to within cold molecular clouds, and to within asteroids and comets.
Our carbon story begins with the carbon atoms that are born in the hearts of dying red giant stars from the fusion of three atoms of helium. One by one, the carbon atoms are dredged up in great convection currents to the star's outer atmosphere and blown outward on stellar winds to begin a journey that will involve a complex dance of molecular breaking, recycling, marriage, and familial bonding. Some astrochemists think that the vast bulk of the first generation of carbon-based molecules formed around these dying red giant stars are ripped apart in less than a century, the equivalent of a cosmic blink, by the intense ultraviolet radiation from the white dwarf star, the red giant's stellar remnant. However, University of Arizona astrochemist Lucy Ziurys sees evidence of abundant molecular survival around old stars, with molecules shielded by dust, water vapor, and other molecules. “It appears that molecules are surviving everywhere in old planetary nebulae,” the molecular remnants of dissipated red giant stars, Ziurys tells me. “We don't wipe the chemical slate clean with each stage. The origin of organic material on Earth—the chemical components that make up you and me—probably came from interstellar space. So one can say that life's origins really begin in the chemistry around [these old stars].”
One way or the other, big or small, after millions of years in interstellar space, carbon-based molecules eventually find themselves swept up into the cold, dense molecular clouds from which solar systems emerge. Through laboratory simulation
s, telescopic observations, and the analysis of organic materials in meteorites, what's become clear in the past decade is that these molecular clouds, and particularly the densest clumps from which stars emerge, are also the birthplace of organic molecules. Stars and complex carbon molecules fuel one another's creation.
For example, astrochemists have described what could be called cold cosmic flagpole chemistry in these dense molecular clouds. In this process, molecules form as a dance between ice-covered micrometer-sized dust and the small molecules that stick to these grains, like a child's tongue to a frozen flagpole. It's thought that methanol—the simplest alcohol, also called wood alcohol, the fuel used in drag-racing cars—is produced when carbon dioxide gas freezes onto a cold dust grain and chemically reacts with hydrogen molecules there. In a similar process, it's thought that the methanol later reacts to form formaldehyde, a molecule that readily joins in chains to form many larger, complex organic molecules. Thus the molecule that on Earth we associate with preserving corpses may, on a cosmic scale, be a key link in the chain from stars to life.
The analysis of the organic materials found in meteorites confirms that some of this material predates the Sun. When astrochemists took the isotopic fingerprints of distinctive, globule-shaped clumps of organic molecules in the Tagish Lake carbonaceous chondrite—whose charcoal-like chunks crash-landed in the Yukon, Canada, in January 2000—they found they all bore the same label: made in a cold cosmic cloud. The meteoritic molecules’ isotopic signatures revealed that they were formed at cosmic deep-freeze temperatures, either in the cold molecular cloud from which the Sun formed or at the frigid outskirts of the solar nebula, after which they were incorporated into the asteroid that spawned the Tagish Lake meteorite. Similarly, the amino acids in the Murchison meteorite also have isotopic signatures that peg them as having formed in a cold molecular cloud.
Indeed, at NASA-Ames, Scott Sandford and his colleagues have verified that supercold is no barrier to abundant organic chemical synthesis. In their cryovacuum system, they've cooked up thousands of organic molecules, only a small fraction of which have been identified. Those positively fingerprinted include the amino acids glycine and alanine; the nucleobases uracil and cytosine; quinones, an important class of biologically active molecules, including 1,4-naphthoquinone, which is the core of vitamin K; and a flood of simple organic molecules such as formaldehyde, methanol, and methane.
“It turns out that a lot of our chemistry happens during the warm-up,” Sandford explains.
So probably what happens is you shoot the photons in, they hit all these molecules, and they break a lot of bonds. So you break the water into OH and H, and you break the methanol into CH3 and OH and so on. These are all highly reactive molecular species. They would love to do chemistry. But they can't because they are frozen in the ice. So they just sit there with all this chemical potential. But the minute you start to warm up and the ice can rearrange, then all these things find each other and they start reacting like crazy. It's a very bizarre chemistry, not the kind you do in freshman chem lab.
Yet it could well be the chemistry that occurs around newborn stars. The heat from a gestational star's friction, and then initial nuclear reactions, warms up and softens icy grains in which previously formed molecules gain the energy to move and interact, forming more complex, larger molecules. This warming, melting, and molecular moving and mixing also occurs in asteroids and comets, turning them into orbiting chemistry labs, each one on its own chemical journey. There's evidence from the Tagish Lake meteorite that, while some of its organic bulk was formed in cold molecular clouds, other molecules, including some amino acids, were formed in the asteroid itself from common precursor molecules during periods in which the asteroid contained liquid water.
We know that pre-solar grains of stardust such as nano-diamonds and silicon carbide survive the journey from their birth in a long-ago star's atmosphere to modern-day Earth. Is it possible that Earth is also pelted with organic stardust, pristine molecules that formed around dying stars and that have remained intact across time and space until they crash-land on our pale-blue dot? Some stardust researchers believe that at least part of this complex insoluble organic material in carbonaceous chondrites comes directly from stars. The evidence, according to the foremost proponent of this view, Hong Kong–based stardust scientist Sun Kwok, is in the enigmatic spectroscopic infrared fingerprints of molecules around dying stars. The spectroscopic fingerprints of dying red giant stars contain a large swath of unidentified infrared emission bands. There's something there; the question is: What? Kwok believes that the emissions are from nanoparticle-sized particles of complex mixes of rings and chains of large carbon-based molecules. It's a structure very similar to that identified in the interlocking weave of branched, ringed carbon-based molecules extracted from the Murchison meteorite. If this view is correct, it means that organic stardust older than the Earth, and largely unchanged over billions of years, rains down on our planet. Thus, in some cases, our carbon ancestry is perhaps but one degree of separation, a single molecular generation, away from its birth star. In this case, when we talk of being stardust, the lineage is stunningly direct.
For astrobiologists like Scott Sandford, the dream today isn't to understand the cosmic context of just life on Earth but of life on planets in general. “The universe seems to be hardwired to generate real molecular diversity, including some of these astrobiologically relevant molecules,” he says. “So, in a sense, the universe is a big organic chemist. Not a very efficient one, but the beakers are so big, and the time scales are so long, that in the end you can produce an awful lot of material…I think you can pretty much count on the likelihood that when a new star forms, if it's got planets around it, the surfaces of those planets will in fact have a complex organic mix dumped on them.”
Whether or not we know if this is what happened on Earth will largely depend on finding another living planet, a point of cosmic comparison, the great dream of the Stardust Revolution: to find an alien Earth.
The province of the student of astrophysics may be said to end with an understanding of the production of a planet like the Earth.
—George Ellery Hale, The Study of Stellar Evolution, 1908
NEW FRONTIERS
At first glance, Michel Mayor could easily be one of the late-summer tourists stopping at Wyoming's Jackson Lake Lodge to get a bite and enjoy the lodge's unparalleled view of the gray, angular Teton Mountain Range rising precipitously from the flat plain ten miles to the west. The sixty-nine-year-old Mayor's relaxed casualness—red sweater, beige slacks, Velcro®-buckled sandals over socks—isn't that of a tourist but of the emeritus astronomer. For Mayor, the astronomy conference he's here to attend is a gathering of the clan, a homecoming of sorts for astronomers from around the world—with all its rivalries, friendships, and black sheep. Mayor is the clan's great elder, the one who finally broached a seemingly impassable frontier and opened the way to new, unimagined worlds. While tourists snap pictures of the Tetons, there—sitting in an upholstered, wingbacked chair, head down while working on his laptop in the Jackson Lake Lodge atrium—is a Galileo of our times.
The setting is a gustatory purgatory for the Geneva-based Swiss astronomer. The lodge's diner, with its classic linoleum-topped counter, is a testament to Americans’ penchant for speed over aesthetics.But the organizers of the Extreme Solar Systems II conference chose the locale for its symbolism, not its food. Wyoming is synonymous with the US frontier of western settlement, a place where many people still wear cowboy boots and Stetsons®, and there's even an outside chance they need both for working on the range. For the twenty-first-century cosmic explorers meeting here for the conference, few places evoke such a strong sense of both exploration and the Earth as a dynamic planet. Jackson Hole gets its name from the fact that it's sinking. The Teton Mountains are North America's youngest, most active mountain range. With each jarring earthquake, they strain skyward on one side of a geological fault, while on the other side,
gravity pulls down the slab of the Earth's crust that is Jackson Hole. This geological activity is even more pronounced about seventy miles north, up the road in Yellowstone National Park, where each year, millions of visitors arrive in geological pilgrimage to see Old Faithful, a geyser whose hot, sulfurous eruptions give vent to the unseen tumult of forces below the Earth's surface.
This is a land of emergent possibility that speaks to Mayor's career. The astronomer has spent a lifetime painstakingly exploring and mapping the hidden crevices of the cosmos, probing deeper and deeper into the unknown, revealing a universe many thought unattainable. He hadn't planned it this way; as a master's student in Geneva in the late 1960s, he studied theoretical particle physics, a field in which a single atom holds the equivalent of a cosmos of activity. One day, he and a friend stopped in front of a bulletin board that announced two PhD research opportunities, one in statistical mechanics—a branch of quantum mechanics—and the other in astrophysics at the Geneva Observatory. His friend expressed interest in the first; Mayor settled for the second.
“I had no special interest in astronomy,” Mayor recalls. “I have exactly the same interest for a lot of domains of science.” Add to this deep, enduring curiosity the qualities of patience and persistence. With these attributes, Mayor began what's now a five-decade career watching and measuring how stars and planets gravitationally tug on each other. When he began in the early 1970s, nine planets were known in the entire universe: those of our Solar System, from scorched, rocky Mercury out to dim, icy Pluto. But in 1995, from an observatory in Southern France, Mayor and his graduate student Didier Queloz ended millennia of speculation and philosophizing: they discovered the first planet revolving around a star other than our Sun—an exoplanet. It was only one more planet in the cosmos—about half Jupiter's bulk, orbiting its star in just over four days—but the exoplanet 51 Peg b represented a whole new universe.
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