Astrobiology

by David C. Catling

  To fill these holes in human knowledge, astrobiology has emerged as a branch of science concerned with the study of the origin and evolution of life on Earth and the possible variety of life elsewhere. This is my own preferred definition. NASA has defined astrobiology as the study of the origins, evolution, distribution, and future of life in the universe. Other common definitions are the study of life in the universe or the study of life in a cosmic context. Within this purview, astrobiologists pursue the question ‘What’s the history and future of terrestrial life?’ as well as ‘Is there life elsewhere?’

  Four developments coincided with the emergence of astrobiology as a discipline in the late 1990s. In 1996, controversial signs of ancient life were described within a Martian meteorite—a 1.9 kilogram piece of rock that had been blasted off the surface of Mars by an asteroid impact and had eventually landed in Antarctica. Whether the interpretation of fossilized microscopic life was correct or not (see Chapter 6), it set people thinking. Furthermore, over the preceding two decades, biologists had established that some microbes not only tolerated a much larger range of environments than had previously been thought but actually thrived in extremes of temperature, acid, pressure, or salinity. So it became plausible to contemplate extraterrestrial microbes existing in seemingly hostile places. A third finding came in 1996 from pictures taken by NASA’s Galileo spacecraft of the ice-covered surface of Jupiter’s moon, Europa, which revealed pieces of ice that had drifted apart in the past, suggesting an ocean below an icy crust. Then, from the mid 1990s onwards, astronomers found increasing numbers of extrasolar planets or exoplanets, which are planets orbiting not our Sun but other stars. The possibility that life might reside on exoplanets or in the cosmic backyard of our own Solar System provided an impetus for asking whether life might be common in the universe.

  Astrobiology in the history of ideas

  Although astrobiology came to the fore in the 1990s, the question of whether we’re alone in the universe goes back millennia. Thales (c. 600 BC), often regarded as the father of Western philosophy, espoused the idea of a plurality of worlds with life. Subsequently, the Greek atomist school from Leucippus to Democritus and Epicurus, which believed that matter was made of indivisible atoms, favoured such ‘pluralism’. Metrodorus (c. 400 BC), a follower of Democritus, wrote, ‘It is unnatural in a large field to have only one stalk of wheat, and in the infinite universe only one living world’. But it would be incorrect to equate the ancient philosophers’ pluralism with our modern conception of life on Mars or exoplanets. Metrodorus had no clue that stars were Sun-like objects at enormous distances and believed that they formed daily from moisture in the Earth’s atmosphere. The populated worlds of the atomists’ imagination were bodies in an intangible space, similar to modern ideas of parallel universes. In any case, the opposing view of Plato (427–347 BC) and Aristotle (384–322 BC) ultimately dominated. Their belief that the Earth was uniquely inhabited and at the centre of the universe prevailed for over a thousand years.

  Eventually, Renaissance astronomers showed that the Earth orbited the Sun. With the realization that the Earth was merely another planet, speculations soon arose about extraterrestrial life on other planets in the Solar System. Johannes Kepler (1571–1630), the German astronomer responsible for astronomy’s three laws of planetary motion, happily entertained the idea of inhabited planets. Then, by the end of the 17th century, the Dutch astronomer Christiaan Huygens (1629–95) was imagining life beyond the Solar System in his book Cosmotheoros (1698): ‘all those Planets that surround that prodigious number of Suns. They must have their plants and animals, nay and their rational creatures too.’ Extraterrestrial life was so in vogue that in 1755 the philosopher Immanuel Kant (1724–1804) wrote of intellectuals on Jupiter and amorous Venusians.

  In contrast, some scholars with religious views continued to cling to Earth’s uniqueness. An example was Cambridge University’s William Whewell (1794–1866), whose Of the Plurality of Worlds (1853) argued against other inhabited planets in a sort of forerunner of a contemporary debate called the Rare Earth Hypothesis that I discuss in Chapter 8.

  By the late 19th century, the issue of life elsewhere was seen as a purely scientific matter, though science itself soon developed some blind alleys. Telescopic observations by Giovanni Schiaparelli (1835–1910) and Percival Lowell (1855–1916) created a surge of interest in the possibility of intelligent life on Mars. Unfortunately, Lowell’s belief that he saw canals on Mars was an optical illusion created when the mind connects dots in blurry images, and his ideas of Martian civilizations were fantasy. Increasingly, as astronomers employed painstaking techniques such as examining spectra of light from planets, it became apparent that the physical conditions on various Solar System planets might not be so favourable for life after all. The pendulum swung so firmly in the other direction that by the mid 20th century few astronomers were interested in planets. It took the Space Age to rekindle old curiosities.

  While the basic questions of astrobiology are ancient, the term ‘astrobiology’ only surfaced from time to time before becoming common in the 1990s. In 1941, an essay entitled ‘Astrobiology’ by Laurence Lafleur (a philosopher at Brooklyn College, New York) described the word more narrowly than its modern incarnation as the consideration of life other than on Earth. Otto Struve, an astronomer at the University of California in Berkeley, also used the term in 1955 to describe the search for extraterrestrial life. A Russian astrophysicist, Gavriil Tikov (1875–1960), and a German astronomer, Joachim Herrmann, published books entitled Astrobiology in 1953 and 1974, respectively, covering popular ideas of extraterrestrial life.

  The modern use of ‘astrobiology’ was introduced in 1995 by Wes Huntress, then at NASA’s headquarters in Washington DC. At the time, NASA scientists argued that a study of life over scales from the microbial to the cosmic was essential for understanding life in the universe. Huntress liked the word astrobiology for this aspiration and the name stuck.

  In fact, astrobiology was really a reinvention and expansion of exobiology, a field that goes back several decades. In 1960, Joshua Lederberg (1925–2008) coined the word exobiology for ‘the evolution of life beyond our own planet’. Lederberg, a Nobel Prize winner for discoveries in bacterial genetics, argued that an essential part of space exploration should be searching for life. Then, from the 1960s onwards, NASA followed Lederberg’s advice and financed exobiology research. But exobiology soon developed its critics. In 1964, George Gaylord Simpson, a Harvard biologist, quipped that ‘this “science” has yet to demonstrate that its subject matter exists’.

  The advantage of today’s astrobiology is that it cannot be held to Simpson’s charge because it includes the study of the origin and evolution of life on Earth at its core. Astrobiology also emphasizes the origin and evolution of planets as a context for life, and so embraces astronomers more firmly than exobiology.

  Exobiology is not the only term similar to astrobiology. Since 1982, astronomers have officially used bioastronomy for the astronomical aspects of the search for extraterrestrial life, while earlier the word cosmobiology was favoured by J. Desmond Bernal (1901–71), an influential Irish-born British physical chemist. But neither of those terms has become widespread.

  What is life?

  Astrobiology raises the difficult question of how to define life. What is it exactly that we are looking for beyond Earth? A common approach is to list life’s characteristics, which include reproduction, growth, energy utilization through metabolism, response to the environment, evolutionary adaptation, and the ordered structure of cells and anatomy. Unfortunately, this way of defining life is unsatisfactory for a couple of reasons. First, the list describes what life does rather than what life is. Second, most of these aspects of life are not unique. Life has structural order such as cells, but salt crystals are also ordered. Some of my friends have no children but they’re alive, I think, as are mules that cannot reproduce. Growth and development apply to living entities, but also to sprea
ding fires. All life metabolizes but so does my car. Life reacts to its environment, but a mercury thermometer also responds to its surroundings.

  Alternatively, some scientists try to define life using thermodynamics, that is, heat and energy and their relationship to matter, by suggesting that the essence of life is the presence of stable structures, such as cells and genetic material, alongside entropy produced by metabolic waste and heat.

  The term entropy requires some clarification. Desperate teachers searching for a quick and dirty explanation have often called it ‘disorder’. Entropy is not disorder but an exact measure of energy dispersal amongst particles, be they atoms or molecules. Energy disperses spatially and so the energy of groups of particles that move together, which is said to be coherent energy, can dissipate. Thus, a bouncing ball comes to rest because its coherent energy of motion is converted into incoherent thermal motion of molecules and atoms through friction. In contrast, a stationary ball never spontaneously begins to bounce (as if it were alive) because even though sufficient thermal energy exists in the floor below, that energy is unavailable and dispersed in random jiggling of the atoms of the floor. The Second Law of Thermodynamics governs such phenomena and states that entropy in the universe never decreases. The entropy increase (or energy dispersal) conserves energy but ruins its quality. High-quality energy is not distributed but concentrated, such as in a barrel of oil, the nucleus of an atom, or in photons (particles of light) possessing high frequency and short wavelength. Such photons include ultraviolet and visible ones that cause sunburn and that power plant life, respectively. In physics, such high-quality energy has low entropy.

  The physicist who most prominently linked entropy to life was the Nobel Laureate Erwin Schrödinger (1887–1961). In What is Life? (1944), Schrödinger commented that an organism ‘tends to approach the dangerous state of maximum entropy, which is death. It can only keep aloof from it, i.e. alive, by continually drawing from its environment negative entropy … Indeed, in the case of higher animals we know the kind of orderliness they feed upon well enough, viz. the extremely well-ordered state of matter in more or less complicated organic compounds, which serve them as foodstuffs. After utilizing it they return it in a very much degraded form.’ Regrettably, Schrödinger introduced the concept of ‘negative entropy’, which does not exist in science, to describe the ordered structure of food. Also, in the growth of some organisms, the increase in entropy primarily comes from heat generation rather than the degraded form of metabolic waste products compared to food. Linus Pauling (1901–94), who was arguably the greatest chemist of the 20th century, bluntly remarked that Schrödinger ‘did not make any contribution whatever [to our understanding of life] … perhaps, by his discussion of “negative entropy” in relation to life, he made a negative contribution’.

  Nonetheless a curious by-product of the ever-increasing entropy in the universe is that ordered, low-entropy structures, such as organisms, spring into existence. In fact, efficient production of entropy is best achieved by so-called dissipative structures involving a coherent structure of an immense number of molecules that dissipates energy. A simple example is a convection cell in boiling water. Warm water rises and is balanced by sinking water on its periphery. This circulating cell helps to disperse energy and so increases entropy more efficiently than if the cell were absent. All living organisms are complicated dissipative structures. However, attempts to define life with thermodynamics have so far failed to distinguish clearly between the living and non-living. For example, the writer Eric Schneider defines life as a ‘far from equilibrium dissipative structure that maintains its local level of organization at the expense of producing environmental entropy’. A fire also fits this definition.

  Pauling’s criticism aside, Schrödinger argued correctly that organisms must run a sort of computer program, which is what we now call the genome. Indeed, life anywhere probably has to possess a genome. By a genome, we mean a heritable blueprint subject to small copying errors, which allows an organism to have evolved from an ancestor and provides a recipe for life’s other characteristics such as metabolism. Evolution—the changes in populations over successive generations caused by selection of individuals’ characteristics—is important because it is the only process that can explain the diversity of life and how the features of life that were listed previously were configured. In Darwin’s natural selection mechanism, the genetic variation in populations of individuals means that some are better adapted for greater reproductive success than others. Natural selection favours genes that leave more descendants, so that lineages accumulate genetic adaptations.

  Mindful of the centrality of evolution, astrobiologists often define life as ‘a self-sustaining chemical system capable of Darwinian evolution’. Unfortunately, this definition is not helpful if we want to design an experiment to find life. Do we have to wait for evolution to happen for a positive detection? A better definition uses the past tense: ‘life is a self-sustaining, genome-containing chemical system that has developed its characteristics through evolution’. So far, space-borne life detection experiments have not tried to measure a genetic make-up. For example, NASA’s Viking Lander probes, which looked for life on Mars in the 1970s, were designed to recognize the Earth-like metabolism of microbes in the soil (Chapter 6).

  The philosopher Carol Cleland and scientist Christopher Chyba have suggested that attempts to define life are like those of 17th-century scientists trying to define water. At that time, water was considered a colourless and odourless liquid that boils and freezes at certain temperatures. Without atomic theory, no one knew that water is a collection of molecules, each consisting of two atoms of hydrogen joined to an oxygen atom. By analogy, perhaps we lack the theory of living systems needed to define life.

  Many of the problems in defining life boil down to the fact that we have only one example—life on Earth. All Earth-based organisms use nucleic acids for hereditary information, proteins to control biochemical reaction rates, and identical phosphorus-containing molecules to store energy. It’s the same basic biochemistry in a bacterium or a blue whale. So it is difficult to distinguish which properties of life on Earth are unique and which are needed generally to qualify as ‘life’. Astrobiology could help solve this conundrum if we found life beyond Earth.

  The bare necessities of life

  While there’s no perfect definition of life, there are reasonable grounds to think that certain atoms common in terrestrial biochemistry are likely to be used by extraterrestrial organisms and might help us recognize life elsewhere. On Earth, the chief structural elements in biology are carbon, nitrogen, and hydrogen, while chemical interactions take place in liquid water. In astrobiology, there’s wide agreement that life elsewhere is likely to be carbon based and that a planet with liquid water would, at least, favour ‘life as we know it.’ These deductions arise from realizing that life is constructed from a limited toolkit, the periodic table of chemical elements, which is the same throughout the universe.

  In fact, carbon is the only element capable of forming long compounds of billions of atoms such as DNA (deoxyribonucleic acid). Consequently, only carbon-based extraterrestrial life seems able to have a genome of comparable complexity to terrestrial life. Carbon also has a variety of other properties that allow a unique chemistry of its compounds, sufficient to spawn the discipline of organic chemistry. Carbon’s special properties include the ability to form single, double, and triple bonds with itself as well as bonds with many other elements. Carbon can also build three-dimensional complexity by forming hexagonal rings that join together.

  Because life has to get started and propagate, it’s probable that the main atoms of life are abundant ones. Carbon is fourth in cosmic abundance after hydrogen, helium, and oxygen. Indeed, astronomers have found that many non-biological organic molecules already exist in space. These freebies might serve as precursors to life getting started (see Chapter 3). For example, around 30 per cent by mass of the dust between the stars is o
rganic material. So-called carbonaceous chondrite meteorites and interplanetary dust particles in our own Solar System contain up to 2 per cent and 35 per cent organic carbon by mass, respectively.

  Because silicon has chemical properties similar to carbon, it is sometimes asserted that silicon might allow an alternative extraterrestrial biochemistry to carbon-based molecules, despite being about ten times less cosmically abundant than carbon. But in water, at least, silicon compounds tend to be unstable and silicon easily gets locked into solid silicon oxides. Carbon dioxide is a gas at common planetary temperatures and dissolves in water to concentrations sufficient for organisms to use carbon dioxide as a carbon source. Silicon dioxide, in contrast, is an insoluble solid, such as quartz. Silicon’s bonds with oxygen and hydrogen are strong, whereas carbon–oxygen and carbon–hydrogen bonds are similar in strength to the carbon–carbon bond, which allows carbon-based compounds to undergo reactions of exchange and modification. Silicon–hydrogen bonds also tend to be easily attacked in water. The stability of silicon-based molecules requires low temperatures to slow down reactions that would otherwise destroy them. Appropriately cold solvents include oceans of liquid nitrogen on icy planets far from their stars. At present, such silicon-based life remains purely speculative.