Origins: Fourteen Billion Years of Cosmic Evolution (22 page)

More than a natural
curiosity as to our own beginnings hinges on the resolution of this issue. Different origins for life imply different possibilities for its origin, evolution, and survival both here and elsewhere in the cosmos. For example, Earth’s ocean floors may provide the most stable ecosystem on our planet. If a jumbo asteroid slammed into Earth and rendered all surface life extinct, the oceanic extremophiles would almost certainly continue undaunted in their happy ways. They might even evolve to repopulate Earth’s surface after each extinction episode. And if the Sun were mysteriously plucked from the center of the solar system and Earth drifted through space, this event would hardly merit attention in the extremophile press, as life near deep sea vents might continue relatively undisturbed. But in 5 billion years, the Sun will become a red giant as it expands to fill the inner solar system. Meanwhile, Earth’s oceans will boil away and Earth itself will partially vaporize. Now that would be news for any form of Earthlife.

The ubiquity of extremophiles on Earth leads us to a profound question: Could life exist deep within many of the rogue planets or planetesimals that were ejected from the solar system during its formation? Their “geo”thermal reservoirs could last for billions of years. What about the countless planets that were forcibly ejected by every other solar system that ever formed? Could interstellar space be teeming with life—formed and evolved deep within these starless planets? Before astrophysicists recognized the importance of extremophiles, they envisioned a “habitable zone” surrounding each star, within which water or another substance could maintain itself as a liquid, allowing molecules to float, interact, and produce more complex molecules. Today, we must modify this concept, so that far from being a tidy region around a star that receives just the right amount of sunlight, a habitable zone can be anywhere and everywhere, maintained not by starlight heating but by localized heat sources, often generated by radioactive rocks. So the Three Bears’ cottage was, perhaps, not a special place among fairy tales. Anybody’s residence, even one of the Three Little Pigs’, might contain a bowl of food at a temperature that is just right.

What a hopeful, even prescient fairy tale this may prove to be. Life, far from being rare and precious, may be almost as common as planets themselves. All that remains is for us to go find it.

CHAPTER 16

Searching for Life
in the Solar System

T
he possibility of life beyond Earth has created new job titles, applicable to only a few individuals but potentially capable of sudden growth. “Astrobiologists” or “bioastron- omers” grapple with the issues presented by life beyond Earth, whatever forms that life may take. For now, astrobiologists can only speculate about extraterrestrial life or simulate extraterrestrial conditions, to which they either expose terrestrial life forms, testing how they may survive harsh and unfamiliar situations, or subject mixtures of inanimate molecules, creating a variant on the classic Miller-Urey experiment or a gloss on Wächtershäuser’s research. This combination of speculation and experiment has led them to several generally accepted conclusions, which—to the extent that they describe the real universe—have highly significant implications. Astrobiologists now believe that the existence of life throughout the universe requires:

1. a source of energy;

2. a type of atom that allows complex structures to exist;

3. a liquid solvent in which molecules can float and interact; and

4. sufficient time for life to arise and to evolve.

On this short list, requirements (1) and (4) present only low barriers to the origin of life. Every star in the cosmos provides a source of energy, and all but the most massive 1 percent of these stars last for hundreds of millions or billions of years. Our Sun, for example, has furnished Earth with a steady supply of heat and light during the past 5 billion years, and will continue to do so for another 5 billion. Furthermore, we now see that life can exist entirely without sunlight, relying on geothermal heating and chemical reactions for its energy. Geothermal energy arises in part from the radioactivity of isotopes of elements such as potassium, thorium, and uranium, whose decay occurs over time scales measured in billions of years—a time scale comparable to the lifetime of all Sun-like stars.

On Earth, life
satisfies point (2), the requirement of a structure-building atom, with the element carbon. Carbon atoms can each bind to one, two, three, or four other atoms, which makes them the crucial element in the structure of all the life we know. In contrast, hydrogen atoms can each bind to only one other atom, and oxygen to only one or two. Because carbon atoms can bind with as many as four other atoms, they form the “backbone” for all but the simplest molecules within living organisms, such as proteins and sugars.

Carbon’s ability to create complex molecules has made it one of the four most abundant elements, together with hydrogen, oxygen, and nitrogen, in all forms of life on Earth. We have seen that although the four most abundant elements in Earth’s crust have only one match with these four, the universe’s six most abundant elements include all four of those in Earthlife, along with the inert gases helium and neon. This fact could support the hypothesis that life on Earth began in the stars, or in objects whose composition resembles those of the stars. In any case, the fact that carbon forms a relatively small fraction of Earth’s surface but a large part of any living creature testifies to carbon’s pivotal role in giving structure to life.

Is carbon essential to life throughout the cosmos? What about the element silicon, which often appears in science fiction novels as the basic structural atom for exotic forms of life? Like carbon, silicon atoms bond with as many as four other atoms, but the nature of these bonds leaves silicon far less likely than carbon to provide the structural basis for complex molecules. Carbon bonds to other atoms rather weakly, so that carbon-oxygen, carbon-hydrogen, and carbon-carbon bonds, for example, break with relative ease. This allows carbon-based molecules to form new types as they collide and interact, an essential part of any life form’s metabolic activity. In contrast, silicon bonds strongly to many other types of atoms, and in particular to oxygen. Earth’s crust consists largely of silicate rocks made primarily of silicon and oxygen atoms, bound together with sufficient strength to last for millions of years, and therefore unavailable to participate in forming new types of molecules.

The difference between the way that silicon and carbon atoms bond to other atoms argues strongly that we may expect to find most, if not all, extraterrestrial life forms built, as we are, with carbon, not silicon, backbones for their molecules. Other than carbon and silicon, only relatively exotic types of atoms, with cosmic abundances much lower than those of carbon or silicon, can bond to as many as four other atoms. Purely on numerical grounds, the possibility that life uses atoms such as germanium in the same way that Earthlife uses carbon seems highly remote.

Requirement number (3)
specifies that all forms of life need a liquid solvent in which molecules can float and interact. The word “solvent” emphasizes that a liquid allows this float-and-interact situation, in what chemists call a “solution.” Liquids allow relatively high concentrations of molecules but do not place tight restrictions on their motions. In contrast, solids lock atoms and molecules in place. They actually can collide and interact, but they do so far more slowly than in liquids. In gases, molecules will move even more freely than in liquids, and can collide with even less hindrance, but their collisions and interactions occur far less often than they do in liquids, because the density within a liquid typically exceeds that within a gas by a factor of 1,000 or more. “Had we but world enough and time,” as Andrew Marvell wrote, we might find life originating in gases rather than liquids. In the real cosmos, only 14 billion years old, astrobiologists do not expect to find life that began in gas. Instead, they expect all extraterrestrial life, like all life on Earth, to consist of sacs of liquid, within which complex chemical processes occur as different types of molecules collide and form new types.

Must that liquid be water? We live on a watery planet whose oceans cover nearly three quarters of the surface. This makes us unique in our solar system, and possibly a highly unusual planet anywhere in our Milky Way galaxy. Water, which consists of molecules made from two of the most abundant elements in the cosmos, appears at least in modest amounts in comets, in meteoroids, and in most of the Sun’s planets and their moons. On the other hand, liquid water in the solar system exists only on Earth and beneath the icy surface of Jupiter’s large moon Europa, whose worldwide covered ocean remains only a likelihood, not a verified reality. Could other compounds offer better chances for liquid seas or ponds, within which molecules could have found their way to life? The three most abundant compounds that can remain liquid within a significant range of temperatures are ammonia, ethane, and methyl alcohol. Ammonia molecules each consist of three hydrogen atoms and one nitrogen atom, ethane of two hydrogen atoms and two carbon atoms, and methyl alcohol of four hydrogen atoms, one carbon atom, and one oxygen atom. When we consider the possibilities for extraterrestrial life, we may reasonably consider creatures that use ammonia, ethane, or methyl alcohol in the way that Earth life employs water—as the fundamental liquid within which life presumably originated, and which supplies the medium within which molecules can float their way to glory. The Sun’s four giant planets possess enormous amounts of ammonia, along with smaller amounts of methyl alcohol and ethane, and Saturn’s large moon Titan may well have lakes of liquid ethane on its frigid surface.

The choice of a particular type of molecule as life’s basic liquid immediately implies another requirement for life: the substance must remain liquid. We would not expect life to originate in the Antarctic ice cap, or in clouds rich in water vapor, because we need liquids to allow abundant molecular interactions. Under atmospheric pressures like those at Earth’s surface, water remains liquid between 0 and 100 degrees Celsius (32 to 212 degrees Fahrenheit). All three of the alternative types of solvents remain liquid within temperature ranges that extend far below water’s. Ammonia, for example, freezes at
–
78 degrees Celsius and vaporizes at
–
33 degrees. This prevents ammonia from providing a liquid solvent for life on Earth, but on a world with a temperature 75 degrees colder than ours, where water could never serve as a solvent for life, ammonia might well be the charm.

Water’s most significant
distinguishing feature does not consist of its well-earned badge of “universal solvent,” about which we learned in chemistry class, nor of the wide temperature range over which water remains liquid. Water’s most remarkable attribute resides in the fact that while most things—water included—shrink and become denser as they cool, water that cools below 4 degrees Celsius expands, becoming progressively less dense as the temperature falls toward zero. And then, when water freezes at 0 degrees Celsius, it turns into an even less dense substance than liquid water. Ice floats, which is very good news for fish. During the winter, as the temperature of the outside air drops below freezing, 4-degree water sinks to the bottom and stays there, because it is denser than the colder water above, while a floating layer of ice builds extremely slowly on the surface, insulating the warmer water below.

Without this density inversion below 4 degrees, ponds and lakes would freeze from the bottom up, not from the top down. Whenever the outside air temperature fell below freezing, a pond’s upper surface would cool and sink to the bottom as warmer water rose from below. This forced convection would rapidly drop the water’s temperature to zero degrees as the surface began to freeze. Then denser, solid ice would sink to the bottom. If the entire body of water did not freeze from the bottom upward in a single season, the accumulation of ice at the bottom would allow full freezing to occur over the course of many years. In such a world, the sport of ice fishing would yield even fewer results than it does now, because all the fish would be dead—fresh-frozen. Ice anglers would find themselves on a layer of ice that was either submerged below all remaining liquid water or atop a completely frozen body of water. No longer would you need icebreakers to traverse the frozen Arctic—either the entire Arctic Ocean would be frozen solid, or the frozen parts would all have sunk to the bottom and you could just sail your ship without incident. You could slip and slide on lakes and ponds without fear of falling through. In this altered world, ice cubes and icebergs would sink, so that in April 1912, the
Titanic
would have steamed safely into the port of New York City, unsinkable (and unsunken) as advertised.

On the other hand, our mid-latitude prejudice may be showing here. Most of Earth’s oceans are in no danger of freezing, whether from the top down or the bottom up. If ice sank, the Arctic Ocean might become solid, and the same might happen to the Great Lakes and the Baltic Sea. This effect could have made Brazil and India greater world powers, at the expense of Europe and the United States, but life on Earth could have persisted and flourished just as well.

Let us, for the time being, adopt the hypothesis that water has such significant advantages over its chief rivals, ammonia and methyl alcohol, that most, if not all, forms of extraterrestrial life must rely on the same solvent that Earthlife does. Armed with this supposition, along with the general abundance of the raw materials for life, the prevalence of carbon atoms, and the long stretches of time available for life to appear and to evolve, let us take a tour of our neighbors, recasting the age-old question, Where’s the life? into the more modern one, Where’s the water?

If you were
to judge matters by the appearance of some dry and unfriendly-looking places in our solar system, you might conclude that water, while plentiful on Earth, ranks as a rare commodity elsewhere in our galaxy. But of all the molecules that can be formed with three atoms, water is by far the most abundant, largely because water’s two constituents, hydrogen and oxygen, occupy positions one and three on the abundance list. This suggests that rather than asking why some objects have water, we should ask why they don’t all possess large amounts of this simple molecule.

How did Earth acquire its oceans of water? The Moon’s near-pristine record of craters tells us that impacting objects have struck the Moon throughout its history. We may reasonably expect that Earth has likewise undergone many collisions. Indeed, Earth’s larger size and stronger gravity imply that we should have been struck many more times, and by larger objects, than the Moon. So it has been, from its birth all the way to the present. After all, Earth didn’t hatch from an interstellar void, springing into existence as a preformed spherical blob. Instead, our planet grew within the condensing gas cloud that formed the Sun and its other planets. In this process, Earth grew by accreting enormous numbers of small solid particles, and eventually through incessant impacts from mineral-rich asteroids and water-rich comets. How incessant? The early impact rate of comets may have been sufficiently large to have brought us the water in all our oceans. Uncertainties (and controversies) continue to surround this hypothesis. The water that we observed in comet Halley has far greater amounts than Earth does of deuterium, an isotope of hydrogen that packs an extra neutron into its nucleus. If Earth’s oceans arrived in comets, then those that hit Earth soon after the solar system formed must have had a chemical composition notably different from today’s comets, or at least different from the class of comet from which Halley is drawn.

In any case, when we add their contribution to the water vapor spewn into the atmosphere by volcanic eruptions, we have no shortage of pathways by which Earth could have acquired its supply of surface water.

If you seek
a waterless, airless place to visit, you need look no farther than Earth’s Moon. The Moon’s near-zero atmospheric pressure, combined with its two-week-long days when the temperature rises to 200
degrees Fahrenheit, causes any water to evaporate swiftly. During the two-week lunar night, the temperature can drop to 250 degrees below zero, sufficient to freeze practically anything. The
Apollo
astronauts who visited the Moon therefore brought all the water and air (and the air conditioning) that they needed for their round-trip journey.