by David Toomey
All these liquids are “cryogenic,” this being the term applied generally to things with low temperatures, and each has been shown to support complex chemistry. What of biochemistry—the chemistry, vastly more complex, that works in living organisms? Enzymes, which we might think of as molecules edging toward self-organization and replication, are known to work in solutions of methionine and ethylene glycol chilled to –100°C. Admittedly, it is a long way from such molecules to the breathtakingly complex structures and chemical systems that make a living cell, and it may be that at cryogenic temperatures such complexity is simply impossible. But a handful of scientists suspect otherwise. Inspired by the examples provided by extremophiles, they have imagined exotic biochemistries, including chemical pathways and catalysts, that would work in the very cold. They have also imagined organisms—some simple, some not so simple—that might use those pathways and catalysts. Without intending to, these scientists have given us a bestiary that lies somewhere between the known and the mythical, somewhere between the Encyclopedia of Life and Margaret Robinson’s Fictitious Beasts. In other words, a bestiary of weird life.
* * *
* Water, however, has not responded in kind. Since water is a nonliving and nonevolving chemical compound, we should not be surprised. We might be surprised, though, to learn that for all the ways water benefits life, it also does life harm by degrading proteins and actually damaging DNA. (National Research Council, Limits of Organic Life, 27)
* The protoplasm is the fluid content of the cell, composed mainly of nucleic acids, proteins, lipids, carbohydrates, and inorganic salts. In the nineteenth and early twentieth centuries (and at the time of Wells’s writing), the term referred to a substance that endowed the cell with the ability of self-replication.
* Using radio and infrared astronomy, however, scientists have found that silicon is rare in deep space, especially by comparison with oxygen, carbon, and nitrogen.
* Recall that the traditional habitable zone around a star is termed, colloquially, the “Goldilocks zone.” Presumably, the fair-haired heroine of the children’s story was made of DNA, amino acids, and proteins. Suppose that somewhere out there in a small moon’s subsurface ocean, or in the clouds of a giant planet, there are other Goldilockses—or at least microbial versions—and they are made of other stuff entirely. The zones they find habitable might be greater than ours by several orders of magnitude. Even within our local piece of real estate—the Solar System—there are more than 160 moons orbiting the outer planets, four dwarf planets, and billions of comets. If you believe most who tell the story of Goldilocks, its lesson is to respect the possessions and privacy of others. There is, though, a deeper truth in the story; it is that despite what Goldilocks thinks, the world is not made for her. If advocates of the search for weird life are right, then we will learn, once again and in ways we had not imagined, that it is not made for us either.
CHAPTER FIVE
A Bestiary of Weird Life
Thermometers in the inner regions of Antarctica typically drop to –70°C in winter, and the official record for the coldest temperature on Earth, measured at the Russian research facility Vostok Station, is –89°C. Strange to say, such temperatures would not be unusual for a summer morning on Mars’s Chryse plain, the landing site of Viking 1—a fact that, depending on your point of view, makes Antarctica seem very far away or Mars seem not so distant. Of course, there are much colder places in the Solar System, but if we wish to visit them we must leave Earth and Mars far behind, and venture outward nearly 800 million kilometers to the vicinity of Jupiter, where the Sun seems pale and shrunken. For the moment let’s turn our attention not to Jupiter itself, but to three of the planet’s so-called Galilean moons—Europa, Ganymede, and Callisto. Each is large enough to be called a world.
It’s cold here. Page through polar explorers’ descriptions of Arctic winters or jilted lovers’ descriptions of hearts gone cold, and you won’t find words adequate to describe the chill. The surface temperature of Europa, at its equator, averages –160°C. Its smooth surface is covered with great sheets of fractured ice, making for a crust that may be tens of meters or tens of kilometers thick, but beneath which almost certainly lies a vast, dark ocean. Many scientists believe there may be life in that ocean, taking warmth and energy from hydrothermal vents.*
In the view of many, Europa has overtaken Mars as the place in the Solar System most likely to harbor extraterrestrial life. In fact, although a Europan ecology is decidedly hypothetical, NASA scientists worried that Galileo might someday impact Europa and contaminate it. So in September 2003, when the Galileo orbiter had completed its eight-year reconnaissance of Jupiter and its moons, they steered the spacecraft into Jupiter’s atmosphere at a speed of 174,000 kilometers per hour, thus ensuring that it, and any terrestrial microbes it might have carried, would be incinerated.
Well before Galileo met its timely end, there had been numerous proposals for unmanned missions with a focus on Europa: one for a spacecraft that would orbit the moon and map its sub-surface with ice-penetrating radar; another for a spacecraft that would launch a projectile to impact the crust and generate a plume that the spacecraft would fly through and analyze; yet another—the most ambitious—for a lander that would drill and or melt its way through the ice and into the dark ocean beneath, where it would release an autonomous submersible that would cruise those depths seeking life. At present, study teams are preparing a proposal for a scaled-back Europa orbiter that might stay within NASA’s recently diminished budget, and NASA’s European counterpart, ESA, is proceeding with its own plans for an independent mission.1
In the meantime, astrobiologists are making do with hypotheses of Europan life. The difficulty here is that those hypotheses depend on unknowns like the thickness of the ice crust, the depth of the putative ocean, the energy sources available, and, of course, the chemistry of the water. Some have suggested a water-ammonia mix.2 Enough ammonia—say 30 or 40 percent—would mean a pH high enough to denature the DNA of any familiar life, and William Bains makes a case that in such an environment no terrestrial cell would have the energy for the chemical reactions necessary to maintain a balanced pH inside itself. Life in such an ocean would need an “ammoniated” biochemistry, making it unlike any life we know.3
Others, however, suspect that Europan life might greatly resemble familiar life. Galileo’s mission scientists found that Jupiter’s magnetic field fluctuates around Europa, thus suggesting that water in its ocean is highly conductive. Kevin Hand, a planetary scientist at NASA’s Jet Propulsion Laboratory, is one of many who suspect that the ocean is saturated with salts, mostly magnesium sulfate. “There are terrestrial halophiles, salt-loving microbes,” says Hand, “that could survive in the ocean we propose.”4 In fact, it is possible that terrestrial halophiles are there already. In a series of computer simulations, a team of researchers from the National Autonomous University of Mexico simulated meteor impacts on Earth and found that rock fragments launched with sufficient speed by such impacts could enter orbits that eventually brought them to Jupiter—not quite Europa, but very near it.5 As noted several chapters back, certain microorganisms are capable of surviving such journeys. This means that there is a possibility—it should be emphasized a very remote one—that Europan life does not merely resemble familiar life. It is familiar life.
Europa and her sister moons are each several thousand kilometers in diameter—the size of small planets. Bodies much smaller than this, less affected by tidal heating, were once thought unlikely to hold liquid water. One such is a satellite of Saturn called Enceladus, a small snowball of a moon (with a diameter of 500 kilometers) that might have come straight out of The Little Prince. When in 2005 the Cassini spacecraft returned images of the moon, mission scientists were properly stunned to see it pinwheeling geysers of water ice and ammonia crystals into the vacuum—by what internal forces they could only guess. Since the discovery, planetary scientists have suspected that Enceladus does experience tidal
heating after all, and that the geysers arise from long-lasting fissures and cavities holding liquid water along with organic molecules, nitrogen, and mineral salts. Such places might be agreeable abodes for life, and as with possible life in Europa’s ocean, it might resemble certain Earthly extremophiles.
Enceladus may have more accommodations for life than fissures and cavities. In 2011 John Spencer, a Cassini mission scientist, told a journalist, “Basically, I suspect we have an ocean.”6 To be able to say things like “basically, I suspect we have an ocean” as part of your day job, you might think that 1.2 billion kilometers is not too far to send a spacecraft. But you might also think it seems a long way to go to find microbes like those we know, and certain astrobiologists would agree. Europa and Enceladus are no doubt intriguing. But if we are inclined to look for rather more exotic life—life that, say, could use liquid methane as its solvent—we might look elsewhere, and at one place in particular.
A WORLD FOR WEIRD LIFE
In the seventeenth century, a Dutch natural philosopher named Christiaan Huygens contributed to studies of motion and gravity, proposed a wave theory of light, and invented the pendulum clock. He also designed several new sorts of telescopes, and spent enough time looking through them to earn a reputation as one of the best observational astronomers of his age. Although Huygens took care to avoid mixing seeing and imagining (dismissing Johannes Kepler’s claim to have observed artificial constructions on the Moon as a “pretty story”), he was open-minded on the matter of life elsewhere. In a 1698 work elegantly entitled Cosmotheoros, or, Conjectures concerning the Celestial Earths and Their Adornments, Huygens asserted that given one example of a life-bearing world, we have no reason to conclude that others are barren. In fact, he made a caveat that might allow us to claim him as an early proponent of weird life. He maintained that given the Moon’s evident lack of atmosphere and water, life there would necessarily be quite different from that we know.
In a 1656 treatise, Huygens announced his discovery of a moon orbiting the planet Saturn. Three years later he published Systema Saturnium, in which he described the moon’s period of revolution as slightly less than sixteen days, very near the value derived by later observations. Huygens began to call it “my moon,” and other astronomers obligingly followed suit, calling it “Huygens’s moon.” The practice continued until the late nineteenth century, when, following the suggestion of English astronomer William Herschel, most astronomers began to call it Titan.
Titan was large for a moon, roughly the size of the planet Mercury. For the next three centuries little else was learned about it. Even in the most powerful telescopes, astronomers could see only a featureless reddish orb, its surface hidden beneath a dense, opaque atmosphere. In the 1950s astronomers detected methane in that atmosphere; some thought it evidence of an ocean of hydrocarbons. In the 1990s, radar signals bounced off Titan’s surface suggested land terrain in some parts, seas or large lakes in others. In the mid-1990s the Hubble Space Telescope imaged the ground in near-infrared light and revealed light and dark features. But exactly what those features represented was an open question, and Titan’s surface remained a mystery.
In 1997, NASA, in conjunction with the European Space Agency (ESA) and the Italian Space Agency, launched the probe Cassini (named after the discoverer of Saturn’s rings), carrying a Titan lander named Huygens, a spacecraft that, for all its technical sophistication, had an appearance that was particularly unprepossessing, looking like a large, inverted pie tin. In fact, the design was a frank admission by the engineers that their reach had exceeded their grasp. Because no one had ever seen the surface of Titan, no one knew whether the lander would come to rest on solid—that is to say, frozen—ground, or in a lake or sea, or in a methane slush. So Huygens was designed to withstand a hard impact on rock or ice. It was also designed to float.
On Christmas Eve, 2004, Cassini was in orbit around Saturn and it released Huygens. Three weeks later, the lander fired its small vernier engine and began a long arcing fall. For an hour and forty minutes, its camera made 3,500 images. They have since been assembled into mosaics and sequenced into a video that represents the view from the lander through its entire descent, making for a record as dramatic as any in the history of human exploration. For anyone who cares to look, the video is posted online, accompanied by the sound of pinging instruments.* It begins as Huygens nears the fringes of Titan’s atmosphere, and the moon appears as a reddish sphere, with no distinguishable features. Soon the lander ejects its heat shield and deploys a parachute, slowing its descent. Forty minutes in, Titan seems to develop dark stains over large areas. The rate of exposures increases, the pings come faster, and the stains resolve themselves into badlands, rough hills, and arroyos that might be part of a mountainous desert on Earth. What seems a canyon appears in the left of the frame, and for a moment we see rugged mountain ranges like outspread fingers, cut through with complex networks of channels. Then, abruptly, we are seeing a still image: roundish, dust-covered ice pebbles in the foreground, giving way to a flat plain that stretches to the horizon—all beneath a hazy reddish sky.
In the hour and ten minutes available before its batteries died, Huygens performed a preliminary reconnaissance. Its instruments detected methane, presumably sublimed into the local atmosphere when the lander heated the ice beneath it. Meanwhile, Cassini began to map the moon’s surface with both radar and near-infrared imaging, and it would continue to do so for years. Much of that surface was water ice, as hard as granite. Near the equator were fields of dunes made not of silica, but of a mysterious organic material having the texture of coffee grinds. The dune fields stretched for hundreds of kilometers, and radar showed some dunes to be 150 meters high. Elsewhere were great shield volcanoes built by discharges of “cryomagma,” a slushy mix of melted water and ammonia pushed upward from a subsurface ocean 480 kilometers below the ice crust. In the far south, river channels had carved great canyon systems; and in the northern latitudes were more channels—some dry, some perhaps not. In the same latitudes were lakes of liquid methane and ethane—one as large as Lake Ontario. These were—and are—the only bodies of liquid known to exist on the surface of any world other than Earth.
It is precisely because liquids have shaped and still shape its surface that, despite the frigid temperatures, Titan is the most Earthlike of any planet or moon in the Solar System. If we could enter a Titanian landscape and, say, walk along the shore of a northern lake, we would find the scene at once strange and familiar. We would see foaming waves lapping gently at a pebbled shore, and that shore curving around a large bay framed on either side by low, weathered hills—all lying uneasily under a reddish twilit sky. We might guess we were on a particularly inhospitable and rocky coast somewhere on Earth, following an evening storm. If we stayed very long, of course, we would learn that the daylight sky always seems twilit, and if we looked closer at the beach we would find that the pebbles were water ice and the surf was methane and ethane.
LIFE ON TITAN
The authors of the NRC report were particularly intrigued by Titan. Its atmosphere was in thermodynamic disequilibrium. Its surface was cold by Earth or Mars standards but not too cold for chemical bonds; in fact, ground near Huygens had many molecular compounds that contain carbon. And the surface had not one but two mediums and solvents in which life might arise: liquid methane and a slush of water and ammonia. All these findings led the report’s authors to realize that Titan might help them understand the relation of life to chemistry. Envisioning Titan as a “control” in what might well be the most profound life sciences test possible, the authors of the NRC report put forth a startlingly simple hypothesis—one whose resolution would explain that relation decisively. “If life is an intrinsic property of chemical reactivity,” they wrote, “life should exist on Titan.”7 But what sort of life?
A year before Cassini entered Saturn’s orbit, Steve Benner and several colleagues suggested that on Titan, liquid hydrocarbons like methane might fill the
role of biosolvent, a liquid medium that allows and facilitates chemical reactions conducive to life.8 Soon after, two teams of scientists took up the idea. One team was made up of Chris McKay of NASA’s Ames Research Center and Heather Smith of the International Space University in Strasbourg, France. The other team consisted of astrobiologists Dirk Schulze-Makuch (who, recall, had hypothesized hydrogen peroxide–drinking Martians) and David Grinspoon. Both teams calculated the energy available for methanogenic life in Titan’s atmosphere, and both teams drew the same conclusion—that on Titan, hydrogen might play the role that oxygen plays in the biology of Earth.9 Organisms on Earth metabolize with energy derived from the chemical reaction of oxygen and organic material and produce carbon dioxide and water as waste. Titanian organisms, so these scientists thought, might metabolize with energy derived from the chemical reaction of hydrogen and organic material, and produce methane as waste.
McKay and Smith recommended that their colleagues, by way of testing the hypothesis, look for chemical disequilibrium. It was exactly the sort of test that James Lovelock had advocated to detect life on Mars some half a century earlier. If Titan’s atmosphere was in equilibrium, scientists should expect to find significant amounts of certain chemicals—three in particular. There should be quite a lot of ethane, enough to submerge the moon’s entire surface to a depth of several meters. There should be enough acetylene (produced through reactions triggered by ultraviolet sunlight) to be detected by Cassini’s instruments. And there should be a good deal of hydrogen—produced constantly through reactions triggered by ultraviolet light, with some rising through the atmosphere and leaking into space, and some sinking toward the surface. McKay and Smith posited that if Cassini/Huygens mission scientists found an absence of these chemicals—or in the words of the paper, “anomalous depletions of acetylene and ethane as well as hydrogen at the surface”10—they might also have found evidence of life. It would, of course, be a very specific sort of life—organisms that would absorb ethane, acetylene, and hydrogen, and produce methane as waste.
