by David Toomey
If you could peel Venus’s atmosphere like the skin of an orange, stretch it out, and flatten it against the planet Jupiter, it would occupy only about the area, relatively speaking, that India occupies on the surface of Earth. The atmosphere of the larger planet is vast. It is also deep. In 1995, NASA’s Galileo atmospheric probe fell nearly 5,000 kilometers through Jovian air, measuring the wind, temperature, composition, clouds, and radiation levels all the way, and ceasing to function only when it reached an altitude where the pressure was twenty times that of Earth’s at sea level. But during that long fall it taught us a great deal.
Jupiter’s upper layers are a clear thermosphere and stratosphere of hydrogen and helium. Beneath these is a troposphere of clouds and hazes of ammonia, ammonia hydrosulfide, and water—all smeared by the planet’s swift rotation into bands whose edges are frayed into wisps the size of continents. Beneath that, things get very strange. Jupiter has no solid surface. Instead, somewhere beneath the troposphere, at a level far deeper than the Galileo probe penetrated, the atmospheric pressure increases to the point where the hydrogen and helium become a “supercritical fluid”—a nonclassical state of matter with the properties of both a liquid and a gas.
Twenty years earlier, scientists knew far less of Jupiter’s chemistry and cloud systems, but they knew enough that Sagan and astrophysicist Edwin Saltpeter could hypothesize Jovian life in considerable detail. The result, a 1976 article published by the American Astronomical Society, counts as one of the few attempts outside science fiction to describe not merely weird organisms, but an entire weird ecology.23
In that article, Sagan and Saltpeter imagined cell-sized organisms they called “sinkers.” They looked like tiny hydrogen-filled balloons, and they could drift in the upper troposphere for weeks or months before they fell to lower levels, where the high temperatures would be fatal. If their species were to survive longer than that, at least some would need to reproduce before they fell. Sinkers, so the paper continued, might reproduce asexually by exploding seeds or spores, or they might coalesce much as raindrops do, with others of their kind, thereby becoming a single, larger organism.
The authors knew that the charge against the hypothesis for the Venusian organisms might be used here as well, and they had prepared an answer. The hydrogen gas makes a sinker buoyant (slightly lighter than an equal volume of Jupiter’s hydrogen-helium atmosphere), while the thin skin weighs it down. Since, as with coalescing bubbles, the volume-to-surface (or hydrogen-to-skin) ratio increases with every coalescence, enough coalescing will make a sinker unsinkable. It will become another organism, of the type Sagan and Saltpeter called “floaters.” Naturally, floaters would mate with other floaters. Over time, genetic variation and natural selection would produce floaters with sense organs and floaters that could direct their flight. Nothing would limit their growth, and nothing would prevent them from reaching dimensions befitting the vastness of their habitat. Sagan conjectured that they might become “kilometers across, enormously larger than the greatest whale that ever was, beings the size of cities.”24
Life in Jupiter’s clouds would be anything but dull. Since pure hydrogen gas is all that would keep a floater from a fiery death, it would be regarded as a valuable commodity. And since it might be easier to steal hydrogen from a floater than to separate it out of the atmosphere, some floaters might evolve into a third type of Jovian: “hunters.” The hunters, too, might grow to enormous sizes. Sagan and Saltpeter conjectured that all three types—sinkers, floaters, and hunters—might be stages in a single life cycle. In any case, there was the possibility of a dynamic and dramatic ecology.
Sagan had a particularly visual imagination. A few months before the paper on life and death in Jovian skies appeared, he had suggested that Viking’s scanners image Earth from the Martian surface (they were insufficiently sensitive). In 1989, he would propose that the Voyager 2 spacecraft, at a distance of 4.8 billion kilometers from the Sun and leaving the Solar System, should image our home star’s retinue of planets (it did). It is not surprising, then, that he noted that floaters and hunters, if they existed, would be of such size that they might be resolved by the imaging system that would be installed on the twin Voyager spacecraft. The Voyager mission planners made no special effort for that system to target such creatures, and alas, nothing in the Voyager images, or any images made of Jupiter since, might be mistaken for them.
* * *
* A possible model for that ocean is Antarctica’s Lake Vostok, a body of liquid water the size of Lake Ontario, lying beneath more than 3 kilometers of ice. It may be as much as a million years old, and its waters may have been separated from the rest of Earth’s water for far longer. As this book goes to press, Russian scientists (following considerable controversy arising from concerns that they might unintentionally contaminate the lake) have obtained a water sample.
* http://www.nasa.gov/mission_pages/cassini/multimedia/pia08117.html.
* In February 2009, NASA and ESA officials agreed to continue pursuing studies of a mission to Jupiter and its four largest moons, and to plan for a mission to Titan. NASA and ESA agreed that the Jupiter mission was the most technically feasible to do first, but ESA’s Solar System Working Group recommended, and NASA agreed, that both missions merited implementation. By early 2012, however, budget cuts forced NASA to suspend plans for both missions. ESA is proceeding on its own, developing a version of the Jupiter mission.
CHAPTER SIX
Life from Comets, Life on Stars, and Life in the Very Far Future
The Space Science and Astrobiology Division at NASA Ames is housed in a concrete-and-glass structure of 1960s vintage, and you might mistake it for a classroom building at a community college, until you enter the lobby. On the far wall is a large mural depicting an imaginative history of life in the universe. From left to right, star clusters are braided into DNA molecules that morph into cells, leading across billions of years and ever-larger plants and animals, to finish with a pod of blue whales cavorting in water shot through with sunbeams. It’s a vision breathtaking in its richness and grandeur, and it bespeaks a seriousness of purpose in the people who work here.
The vision is offset slightly by a somewhat smaller object of art in the receptionist’s office. On a small table near the door, sitting among brochures and issues of Astrobiology, is a small 10½-ounce can with an iconic red and white label, which, upon close inspection, reads “Campbell’s Primordial Soup.” It’s a science joke, and a good one, although its referent—the idea that life on Earth began in a warm brew of amino acids—may have passed its expiration date. One flight up and just down the hall, two scientists have other ideas about the conditions that led to life, and they are working to simulate those conditions—as it happens, in a chamber about the size of a soup can.
ALLAMANDOLA
Lou Allamandola is a senior scientist in the Space Science and Astrobiology Division, and the founder and director of NASA Ames’s Astrophysics & Astrochemistry Laboratory. He is tall and bespectacled, and were you to see him outside his lab, you might guess him to be a college basketball coach. His office is sunlit, its several tables and bookcases agreeably cluttered with drafts of papers to be proofed, a well-thumbed copy of The CRC Handbook of Chemistry and Physics, an orrery (that mechanical model of the Solar System, like a Calder sculpture, with marble-sized planets moved by gears), and many pictures of family. On one bookcase, curiously, are corked champagne bottles, with scribbling on their labels. Dr. Allamandola explains that when someone on his research team makes a discovery, the team celebrates, and the writing on the label is the name of whatever it was they discovered. Listening to Allamandola speak, an untutored ear might guess his origin to be Brooklyn or Queens—a fair try, as it happens. He was raised in the Italian section of Greenwich Village, and his inflections have survived unscathed through many years in California and a long stint at Leiden University in the Netherlands.
As Allamandola explains, a great deal has changed in our understandin
g of what is called interstellar chemistry—beginning with the realization that there is such a thing. As recently as twenty years ago, most astronomers thought that interstellar space, except for an atom of hydrogen here and an atom of helium there, was barren. Some scientists conjectured about the existence of interstellar ice, but few took those conjectures seriously. Astronomers could see interstellar dust, but no one knew what it was made of, and most were quite sure it could not be large molecules. In those vast spaces, there simply would be no way for atoms or small molecules to find each other, and even if they did, they’d be torn apart immediately by ultraviolet radiation from nearby stars.
Now, though, we have a better understanding of what’s in space and how it got there. The story goes something like this. The elements necessary to familiar life—carbon, nitrogen, and oxygen—are forged deep inside certain stars, and late in the star’s life they are thrown off the star’s surface. By that time they’ve been sufficiently mixed to form simple molecules like acetylene and carbon monoxide, as well as dust particles of carbon and silicates. All these are pushed into space—more specifically, the near vacuum that astronomers call the interstellar medium, an extremely diffuse gas of hydrogen and helium. There the molecules and dust particles are bathed in ultraviolet radiation, bombarded with gamma rays and subjected to more chemical reactions. Some of the molecules accrete on the surfaces of the dust particles.
The ultraviolet radiation destroys the smaller molecules, but others, along with the larger molecules and dust, are pushed into molecular clouds—these being vast regions of relatively dense gas and dust some light years across. Within the clouds, cold refractory grains block out enough ultraviolet radiation that simple molecules can survive and continue to form. It is cold inside a molecular cloud—a chilly 10–50 degrees above absolute zero—and the molecules condense onto the surface of the grains as ices. Confined to the very small surfaces and crowded against other molecules, they have another chance to engage in chemistry, and many do.
Meanwhile, on a much larger scale, part of the cloud itself becomes unstable, and areas with greater concentrations of matter begin to condense and collapse. Once a concentration has broken free from the other parts of the cloud, it is called a protostar. As more matter is drawn inward and releases kinetic energy, the star begins to burn. Some accreting matter is pulled into a slowly turning disk around the star, where it slowly coalesces into planetesimals, and then planets, moons, asteroids, and comets. Mixed with that matter are the grains coated with ices. They adhere to others like themselves, in time growing into ever-larger mixes of ice and organic material, and become comets. Early in a solar system’s history they are plentiful, and many strike planets and moons, delivering great quantities of water and organic material, and setting the stage for life.
Many details of this picture are new. As recently as the 1970s, Allamandola says—despite the common experience of breath condensing on cold glass and the common knowledge that raindrops get their start by forming around particles of dust or salt—no one thought ices would condense on grains. When asked the reason for the doubt, he pauses. For a moment he seems about to speak of long experience with the intractability of the scientific establishment. But then he just shrugs and says, “People are people.”1
A RECIPE FOR A COMET
Astronomers know what interstellar clouds are made of because light passes through them; and by using interferometry and analyzing the spectrum of that light, they have detected many types of molecules. Comets, though, present a more difficult problem. Many of us have heard astronomers call large comets “dirty snowballs,” fragments of ices and rock a few miles across, covered with a dusting of organic matter. As it happens, that’s only a rough description, as no one is sure exactly what kind of ice and exactly what kind of organic matter. Allamandola says, “We don’t know yet, really, what comets are made of.”2
Finding out won’t be easy. Being mostly solid bodies, comets resist interferometry. In 2003, it occurred to Allamandola and his colleague Doug Hudgins that there was another way. If they could make a comet from scratch, they could study it. They took a sample chamber, removed most of its atmosphere, and froze what was left to a temperature near absolute zero, thus creating a fair representation of deep space. They introduced into the chamber a few simple molecules that might be found in a star’s outflow, and turned on a lamp (representing nearby stars) that bathed the molecules in ultraviolet radiation. Then they waited. They were not expecting much, and they were certainly not expecting what happened. The molecules combined, split, and recombined, and before long the chamber contained some very complex molecules, many of which were prebiotic.
Astronomers using interferometry have learned that among the large molecules spread through the interstellar medium are polycyclic aromatic hydrocarbons (or PAHs). Under an electron microscope, PAHs look like pieces of chicken wire; and being resistant to ultraviolet radiation and other unpleasantness, they are at least as hard to pull apart. Since they seem to be almost everywhere in space, there was every reason to expect they would find their way into comets. When Allamandola and Hudgins included them in the mix, even more types of molecules were produced. They were much more complex than those made without PAHs, and again, many were prebiotic.
Allamandola wondered how much chemistry might go on inside a comet as it traveled along its orbit around the Sun. It was possible, he thought, that as surface layers of ice and organics boiled off, producing the comet’s tail, ices inside might melt, refreezing as the comet departed the Sun’s vicinity, melting again on its return, and so on, thereby enabling still more complex chemistry. By way of testing the hypothesis, Allamandola and biochemist David Deamer (of UC Santa Cruz) took ices formed in the chamber and dropped them into liquid water, and then put a drop of that water under a microscope slide. To their amazement, they found circular structures that looked like red blood cells. Of course, they were not cells, and Allamandola and Deamer were careful to call them “vesicles,” but they bore more than a passing resemblance to cell membranes, with several lipid layers separating inside from outside. There was another surprise. Under ultraviolet light, the light that they used to mimic conditions in space, the vesicles fluoresced.
The vesicles themselves were significant, of course, because they suggested the possibility that such structures were precursors to cells, and—more provocatively still—that those structures originated not on Earth, but in interstellar space. The fluorescence was significant because it meant that the vesicles might absorb more energy in the ultraviolet range of the spectrum than they release in the visible range. If they do, they would enjoy an energy surplus that, like the bacteria taking advantage of the sulfate-producing reaction in the vicinity of hydrothermal vents, they might put to other uses, like synthesizing molecules. That fluorescence would have been doubly useful early in Earth’s history, before an ozone layer shielded its surface from ultraviolet radiation, when it might have performed a neat biochemical jujitsu, redirecting some of that energy toward its own uses and at the same time rendering it harmless.
Most of the compounds that Allamandola and his team have made in their chamber have been found in meteorites. In fact, scientists had thought that the aromatic hydrocarbons in meteorites were created when simpler hydrocarbons were heated to high temperatures by entry into the atmosphere. The evidence now is that they, with much else, were made elsewhere. And the upshot is a real Kuhnian paradigm shift.
Many scientists had long assumed that the chemistry that enabled life on Earth began here, as it were, “from scratch,” and many had assumed that prebiotic molecules could be formed only on a warm planet with an atmosphere and water. But what Allamandola and Hudgins found in their sample chamber made them (and many others) reconsider those assumptions. It was entirely possible that when comets brought water to Earth, they also brought the chemical compounds necessary for life. “Perhaps Darwin’s ‘warm little pond,’ ” wrote Allamandola and Hudgins, “is a warmed comet.”3 But only
perhaps. Allamandola is careful to say that they have a long way to go to show that the ingredients of the primordial soup, let alone life, may have been delivered here via comets. He is also careful to say that there is no evidence that life itself originated in molecular clouds.4
Another scientist, though, has argued, if not that life began in such clouds, then that at some moment in an unimaginably distant future, they may be the place where it makes its last stand.
LIFE IN A DISTANT FUTURE
In 1923, British geneticist J. B. S. Haldane published a paper called Daedalus: or, Science and the Future; and in 1929, John Desmond Bernal published a monograph called The World, the Flesh and the Devil: An Enquiry into the Future of the Three Enemies of the Rational Soul. At about the same time, Jesuit priest and philosopher Pierre Teilhard de Chardin was developing his own account of the long unfolding of the material cosmos, an unscientific (albeit quite poetic) description of the long ascendancy of life. All these works, appearing in the first half of the twentieth century, were predictions of the future of humanity and life. In 1977, physicist Jamal Islam published an article that predicted the future of the physical (and nonbiological) universe. It was left for a physicist with a philosophical bent to pull these strands together.