by David Toomey
Over the next several years, data from Cassini and Huygens were analyzed. In several attempts to identify ethane and acetylene, the mission scientists found none of either. In 2010, a computer simulation of Titan’s atmosphere suggested that hydrogen was not accumulating near the surface, and that there was the strong possibility of a flux of hydrogen into that surface.11 All evidence implied that Titan’s atmosphere was in disequilibrium. Of course, the simulation of disappearing hydrogen might be wrong, and in an interview with New Scientist, McKay was properly cautious, allowing that what they had was at least a “very unusual and unexplained chemistry.”12
Lovelock had said that he did not expect life to survive in pockets on otherwise barren bodies, except for the brief moments when it was gaining a foothold or just before being extinguished completely. For by far the greater part of its existence, he argued, life on a planet or moon would be widespread. Therefore, we would expect to find only two categories of extraterrestrial worlds: those that were barren and those that were rich with life. Earth, of course, is in the latter category; as we’ve noted, life is very nearly everywhere there is liquid water. McKay and Smith suggested an analogy on Titan, with the role of water played by methane. Since the chemical is everywhere on Titan—the landing site of Huygens was moist with methane, its atmosphere has methane-nitrogen clouds, methane rains down on its surface, and the northern hemisphere is dappled with methane lakes—they predicted that if there were any methanogens, there would be a great many, and they would be widespread.
Cassini has since completed its initial four-year mission, as well as an “extended” mission. It is now in its second extended mission to explore the Saturnian system, expected to last through September 2017.* At present there are ideas for exploring Titan remotely, using some unusual and original designs. One idea is for a mission that would be more or less a repeat of Huygens, but targeted at one of the northern lakes or a shoreline. Titan, though, is a world that invites exploration with vehicles utterly unlike the rovers of yore. Another idea is for an orbiting spacecraft, teamed with a surface probe and—most ambitious—a balloon equipped with a helicopter rotor that would fly wherever the mission scientists directed it.13 Those who imagine exploring Titan are especially intrigued by its lakes. Still another idea is for a boat—specifically, a saucer-shaped probe that might parachute and splash down in one of those lakes, where it would drift with wind and currents, taking measurements of temperature and methane humidity, as well as images of the shoreline.14
HOT JUPITERS AND OCEAN PLANETS
In 1992, astronomers confirmed the first discovery of two planet-sized bodies outside our Solar System. They were orbiting the remnant of a star that had exploded as a supernova, leaving a compact sphere of neutrons spinning furiously at thousands of times a second, and sending pulses of electromagnetic radiation streaming into space. No one expected planets to have survived the unimaginable violence of a supernova explosion, and no one knew—and in fact, no one knows still—exactly what those two bodies (and a third confirmed later) are. They may be the cores of extrasolar Jupiters with their atmospheres ripped away, the remnants of a companion star, or something else.
Soon enough, planetary scientists would identify stellar phenomena that were rather more conventional—but perhaps more exciting. In 1995, astronomers discovered evidence of a body orbiting a star like our Sun. It was a planet by anyone’s definition, and it was found by identifying the tiny but measurable wobble a planet produces in its parent star as it orbits that star. The “wobble detection” method proved reliable and quickly led to the discovery of many more planets, but it had a built-in bias, as it was best suited to detecting large wobbles produced by large planets. These were enormous worlds that astronomers call “hot Jupiters”—“hot” because they are nearer their parent star than the planet Mercury is to our Sun, “Jupiters” because they are at least as massive as that world.
In the 1990s, as instruments and techniques were refined, astronomers found smaller planets in wider orbits, and the rate of discovery increased so quickly that by the first years of the twenty-first century, a new world was swimming into our ken every two weeks. There was a steady stream of planetary firsts: the first planet discovered orbiting two stars, the first orbiting a red dwarf star, the first Neptune-sized planet, and so on. As this book goes to press, scientists have identified nearly 800 planets and imaged several in the infrared. A handful of these planets have orbits that take them on a direct line of sight between their parent star and us, such that, in the field of view of a hypothetical telescope far more powerful than any in existence, the atmosphere of any of them would appear as a bright ring around an otherwise darkened sphere. Even with existing telescopes, astronomers have been able to conduct spectrographic analyses of such atmospheres, and have found them to contain (among other gases) sodium, water vapor, and methane.
Most planets, though, are not so cooperative as to pass between their star and astronomers or astronomers’ instruments. To understand the chemistry of their atmospheres, or much of anything else about them, scientists call upon elaborate models of planetary formation. One such model implies that planets like Neptune, whose core is surrounded by a thick shell of water ice, can be pulled into orbits so near their parent star that the ice melts, making for planetwide oceans tens or hundreds of kilometers deep, giving way at the greatest depths not to a floor of sediment, basaltic rocks, and hardened magma as on Earth, but to a peculiar form of ice that forms under great pressures. Although such a planet, like most planets, would receive a continual peppering of meteorites and so a steady ration of metals and silicates, many planetary scientists suspect it would not be enough to sustain complex chemistry, let alone biochemistry. Those vast oceans would likely be pure—and sterile.
Other models of planetary formation imply more Earthlike worlds, which would have ample water near their surfaces or on them, as well as mantles of silicates and metals—ingredients necessary to any biochemistry. The search for such worlds is the purview of NASA’s Kepler space observatory.
THE NEW WORLDS OF KEPLER
Planetary scientists now suspect that Earthlike planets may be quite common in our galaxy (although, according to one hypothesis, limited in distribution to a “galactic habitable zone”); in fact there is a part of space where a great many such worlds have already been identified. It is a patch of sky between the constellations Cygnus and Lyra, visible on a clear summer night from anywhere in the Northern Hemisphere. This is the star field being observed by Kepler, whose photometer can detect the slight darkening and brightening of a given star, the darkening and brightening that may mean a planet has passed in front of it—or, as an astronomer would say, made a “transit.” Kepler’s mission rules require three transits to confirm a discovery, and as of February 2012, Kepler’s scientists had made provisional identifications of 2,321 new worlds, with forty-six orbiting inside their parent star’s traditional habitable zone.
There are millions of stars in Kepler’s field of view. Given practical limitations of time and budget, the mission scientists cannot look for transits of all of them, but they can look for about 145,000. For various reasons, they are concentrating their efforts on stars belonging to a class of which our Sun is a member, stars that burn more or less steadily for roughly 10 billion years. The selection pleases astrobiologists, who suspect that life requires a source of energy that is stable for long periods. Although the first life arose on Earth 3.5–3.8 billion years ago (almost as soon as it was possible), complex cells with nuclei have been here for only 2 billion years, and multicellular life—complex life—for only 1 billion. It seems that complex life on Earth needed nearly 3 billion years to establish itself, and, owing to a warming sun, it could not have taken much longer. The Sun’s temperature is increasing such that in a billion years it will have become about 10 percent warmer than it is at present, boiling away Earth’s oceans, baking its surface, and making for an environment in which life as we know it could not survive.15
Complex life on Earth, then, had a range of some 5 billion years in which to establish itself, and it hit a mark inside that range, with roughly 2 billion years to spare. But suppose, as theoretical physicist Brandon Carter suspects, that we are a special case, and that on average, complex life takes longer than 5 billion years to gain a foothold.16 If Carter is right, planets orbiting stars like our Sun would be poor bets for life.
OTHER TITANS
Planetary scientist Jonathan Lunine observes that the smaller, cooler sorts of stars that astronomers call red dwarfs are also stable—much more so. Because their nuclear reactions are far slower than those of larger stars, red dwarf stars are expected to burn for trillions of years. Life on a planet orbiting a red dwarf, Lunine notes, would have much more time to arise, and much more time to survive. Nonetheless, such life—if it were life like that we know—would face challenges. Red dwarfs burn at such low temperatures that a planet orbiting a red dwarf star, to be warm enough to have liquid water on its surface, would have to be ten times nearer that star than Earth is to the Sun. Such proximity would expose the planet to intense flares and stellar winds and, perhaps worse, ensure that it was “tidally locked,” with the sunlit side forever baking, the darkened side eternally frozen.
It is true that liquid water might exist and life might survive in the twilit regions. It is also true that scientists have models suggesting that a sufficiently dense atmosphere might work to moderate temperatures, and that some of these planets might have such an atmosphere. But Lunine suspects that most are lifeless. He also believes, however, that inhospitable planets in Earthlike orbits are not a reason for astrobiologists to take a pass on red dwarf systems. If we want to find a planet with temperate conditions in such a system, he advises that we look to an orbit at about the same distance from the star as Earth is from the Sun—about 150 million kilometers out. From that distance the effect of most flares and stellar winds would be greatly diminished. Moreover, any planet orbiting at that distance would not be tidally locked, so temperatures across its surface would moderate.
Of course, any water on that surface would be frozen, as would any ammonia. In fact, such a planet would be as cold as Titan. But as we’ve seen, an environment like Titan’s could be home to certain kinds of life. And because red dwarfs greatly outnumber Sun-like stars by a ratio at least ten to one and perhaps as much as a hundred to one, the galaxy may have far more Titans than it has Earths. If such worlds are congenial to life, then life like that hypothesized by McKay and Smith may be far more common than familiar life. In which case, statistically speaking anyway, life on Earth might be the life that’s weird.
AND COLDER STILL
Suppose we return to our own Solar System, but venture now more than 4 billion kilometers out to the orbit of Neptune, the outermost of the planets, from where the Sun appears as only a bright star. Our interest is in Neptune’s largest moon, Triton. The most detailed images we have of Triton were made by Voyager 2 during its 1989 flyby, and since the moon’s atmosphere is the thinnest wisp of nitrogen, they showed a clearly visible geography. There were craters, impact basins, and vast snow-covered plains. A third of the moon’s surface was an area, long since melted and refrozen into thousands of circular indentations, that looked like nothing so much as the ice-rimed skin of a cantaloupe. There was some water ice, but most was of the type chemists might call “exotic”—carbon dioxide, methane, and nitrogen—and you and I would call very, very cold. A typical midday temperature on Triton might be –235°C, a number whose chill is better appreciated as 35 degrees above absolute zero, the temperature at which frigid turns decidedly rigid, and all molecular motion ceases. Yet evidence suggests that there’s liquid there, and it is near the surface.
Voyager imaged several long, parallel dark streaks on Triton’s surface. They are something of a mystery, and many planetary geologists suspect that they were produced by geysers. The thinking is as follows. Parts of the moon’s surface are clear nitrogen ice; beneath it is more nitrogen ice mixed with organic material. In the weak light of the distant Sun, the clear ice on the surface acts like greenhouse glass, making for, one must note, a very chilly greenhouse: at a pressure of 1 atmosphere, nitrogen melts at 63 degrees above absolute zero and boils at 77 degrees above absolute zero. Nonetheless, the frozen nitrogen beneath the clear ice melts, gurgles, and trickles, and mixes with nearby organic material. Sooner or later the nitrogen boils and erupts violently through the surface ice, carrying the organics with it. In Triton’s low gravity, the mixture gushes upward for several kilometers until it meets prevailing winds that carry the organics great distances before they fall to the surface. Where they fall, they leave the dark streaks.
Building on what little is known and can be surmised of Triton’s geology and subsurface chemistry, William Bains has conjectured a way that a complex chemistry involving silanols and silanes, reacting in liquid nitrogen and driven by heat from Triton’s core, might set the stage for a biochemistry—a system that organizes chemistry and, through feedback mechanisms, sustains itself.17
SINKERS, FLOATERS, AND HUNTERS
The mediums used by the hypothetical life we’ve discussed so far are liquids—one of the three states of matter that, recall, are termed “classical.” There are several nonclassical states of matter, and a number of scientists have imagined organisms that might use them. To find their habitats, we’ll need to reverse course and head Sunward, to the warmer regions of the inner Solar System. We begin with the planet Venus.
The Venusian surface bakes at blast-furnace temperatures in excess of 460°C, and beneath a dense atmosphere of carbon dioxide that weighs on it with a pressure ninety times that exerted by Earth’s atmosphere at sea level. In the 1960s and 1970s, the Soviet Union parachuted ten probes to that surface. They were built like bathyspheres, but only a few survived more than an hour. It was hard to conceive of an organism that might do better. But some 40–70 kilometers above that surface, things are rather different. Venus’s atmospheric pressure is only half again as great as Earth’s at sea level, and temperatures average 37°C—what we might expect on a warm day in the tropics. At such altitudes there happen to be large volumes of droplets of liquid suspended in a gas—a nonclassical state of matter that chemists call aerosols, and you and I call clouds.
If you think ideas of organisms living in Venusian clouds might seem to have left reason somewhere in the lower troposphere, consider that the cumulus and stratus in our own atmosphere are well populated with bacteria, algae, and fungi. Bacteria survive in clouds for very long periods, and they can do so because there is water (of course), but also because most clouds hold significant quantities of nitrogen, sulfur, and various organic acids—chemicals a bacterium regards as food. It seems that quite a few are at dinner. A cloud sample taken by a French meteorological station was found to be home to no fewer than seventy-one bacterial strains, many individuals of which came from oceans, presumably pushed into the air by a breaking wave or bursting bubble and lofted further by wind.18
Microbes of various sorts can get higher still. In 1978, researchers using meteorological rockets fitted with samplers found bacteria in the mesosphere, that layer of atmosphere above the stratosphere, at altitudes of 50–100 kilometers. (By way of comparison, commercial aircraft fly at altitudes of 10 or 12 kilometers. The Fédération Aéronautique Internationale, the international standard-setting and record-keeping body for aeronautics and astronautics, puts the boundary between Earth’s atmosphere and outer space at an altitude of 100 kilometers, about 62 miles.) At least as surprising as the microbes’ sheer presence was the discovery that some, otherwise identical to their Earthbound brethren, had evolved the ability to synthesize pigments as a resistance to ultraviolet radiation.19
But back to Venus. Earth’s clouds are essentially water droplets suspended in the gases of our atmosphere—mostly nitrogen and oxygen. Venusian clouds, on the other hand, are a fine mist of droplets of sulfuric acid suspended in carbon dioxide. Along w
ith the temperatures and pressures, they would seem to complete a picture of Venus as hell—would, that is, if sulfuric acid were as harsh as its reputation. In fact, though, that reputation is only partly deserved. When the chemical is used as an industrial solvent, it is mixed with water, and then it’s the water that does the corroding and dissolving; the sulfuric acid is only a catalyst. Pure sulfuric acid—the sort in Venus’s clouds—is a mild solvent at best. And a mild solvent, just strong enough to pull apart some molecules and free up carbon for chemical reactions, is the kind a metabolism prefers.
The metabolisms of known acidophiles, by various mechanisms, keep water in and acid out. Astrobiologists Dirk Schulze-Makuch and Louis Irwin suggested that the metabolisms of Venusian cloud dwellers would need to reverse the exchange, keeping the acid in and the water out. Strange as such a metabolism may seem, scientists know of nothing to prohibit it. Certain plants are known to use acids to synthesize molecules,20 and there are several hypotheses for whole metabolisms that might use sulfuric acid. In 2002, Schulze-Makuch and Irwin argued that the possibility of acidophilic, cloud-dwelling Venusians was sufficient to justify a mission that would scoop up a wisp of Venusian atmosphere and return it to Earth for study.21
Ideas of cloud-borne Venusians are not new. As far back as 1967, when there was evidence for substantial amounts of water vapor in the planet’s clouds, Carl Sagan and Yale biophysicist Harold Morowitz hypothesized organisms the size of Ping-Pong balls, with skins a single molecule thick.22 Such organisms, so their thinking went, might have originated on the surface sometime in a distant past when conditions there were more temperate and, as the surface heated up, migrated to the skies. Like jellyfish in terrestrial oceans, they would maintain buoyancy with float bladders—filled, in the Venusian case, with hydrogen. The idea was criticized because although Morowitz and Sagan had suggested ways the cloud-borne might be cloud-born (sexual and asexual reproduction), they had not shown how they might have evolved, and no one could imagine an evolutionary path by which a Venusian surface-dwelling organism might develop a float bladder. Sagan, though, was undeterred. He didn’t abandon the cloud-borne life; he just suggested another place to look for it.