Читать книгу Astrobiology - Charles S. Cockell - Страница 15

Having explored the main questions in astrobiology and summarized them, we might ask ourselves when all this scientific interest began. Sometimes a historical perspective on any science can be useful to understand how we got here and why certain questions have become prominent lines of enquiry. A thorough review of the history of space sciences and where astrobiology fits within it could (and indeed does) consume entire books. This textbook is more about the science than the history of astrobiology, but at this point it is worth making some observations about its historical origins to provide some introductory context.

The questions that astrobiology asks are, from a philosophical standpoint, ancient. This is particularly the case for the question of whether we are alone in the Universe. Although this is just one part of astrobiology, let us briefly explore this history. Greek philosopher, Metrodorus of Chios (fourth century BCE; Figure 1.8), a student of Democritus (c. 460–370 BCE) (Democritus proposed an early atomic theory of matter) stated: “It would be strange if a single ear of wheat grew in a large plain, or there were only one world in the infinite.” In other words, if you walk into a field, you rarely see one ear of wheat in a large field that is otherwise completely dead. Where there is one ear of wheat, there are usually lots. Metrodorus was essentially saying: Surely the existence of planet Earth implies many planet Earth's in the Universe? The Greeks had a very different view of the Universe than the one we have today, so we shouldn't draw too many conclusions about what was going through his mind. The Greeks had no real concept of the planets as rocky bodies or the vast distances to the stars. They thought that all the stars were held on the surface of a huge sphere. Metrodorus's statement was, nevertheless, a remarkable line of thought, because even today we still ask the question: Does life on Earth imply life elsewhere? We ask this with the benefit of modern technology and astronomical observations, but the basic conceptual question remains the same: Do the origin and evolution of life on our planet allow us to say anything about the universality of biology?

Figure 1.8 Metrodorus of Chios, ancient Greek philosopher. He wondered about the existence of other worlds like our own.

Source: Reproduced with permission of Keith Schengili-Roberts, https://commons.wikimedia.org/wiki/File:Metrodorus-PergamonMuseum.png.

Metrodorus's view of the world was very different from Aristotle's (384–322 BCE), who asserted the uniqueness of Earth in the cosmos. The idea that Earth was the center of the Universe was based on the observation that the stars never moved with respect to one another, which the ancients interpreted to be a result of the fixed position of Earth rather than the great distances to the stars. Aristotle's view would dominate for many centuries. Until the Enlightenment (during the seventeenth and eighteen centuries), the idea that Earth was the sole inhabited world in the cosmos held its grip on the public view, bolstered by religious doctrine.

In the sixteenth century, the geocentric view of the Universe, which firmly placed Earth as the center of the action, was overturned by Nicolaus Copernicus (1473–1543) (Figure 1.9). He was not the first person to consider that the Sun was at the center of the Solar System and Earth orbited around it. Greek philosopher Aristarchus proposed a heliocentric model, but Copernicus's treatise, De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), caused a landmark shift away from the geocentric view. The Copernican outlook paved the way for a view of the cosmos that would allow for the idea that stars may be other suns.

Figure 1.9 Nicolaus Copernicus, renaissance mathematician and astronomer. He advanced a heliocentric model of the Solar System.

Source: Reproduced from the “Torun portrait,” anonymous, c. 1580.

In the sixteenth century, more enquiring and inquisitive minds appeared and, with them, new speculations about Earth's place in the larger order of things. One of the most astonishing speculations about worlds beyond Earth was made by the Italian Dominican friar, mathematician, and philosopher Giordano Bruno (1548–1600; Figure 1.10), who stated in his book On the Infinite Universe and Worlds: “In space there are countless constellations, suns and planets; we see only the suns because they give light; the planets remain invisible, for they are small and dark. There are also numberless earths circling around their suns, no worse and no less than this globe of ours. For no reasonable mind can assume that heavenly bodies that may be far more magnificent than ours would not bear upon them creatures similar or even superior to those upon our human earth.”

Figure 1.10 Giordano Bruno, Dominican friar, mathematician, and philosopher. He speculated about other Earth-like worlds (A bronze statue by Ettore Ferrari [1845–1929], Campo de' Fiori, Rome).

Source: Reproduced with permission of Jastrow, https://commons.wikimedia.org/wiki/File:Giordano_Bruno_Campo_dei_Fiori.jpg.

This was a prescient statement about the possibility of extrasolar planets, and a person couldn't do much better today in writing a clear summary of why Earth-like exoplanets are hard to find – because they are small and dark. Bruno was eventually burned at the stake for a variety of charges, most of which related to him holding beliefs contrary to the Catholic Church concerning the Trinity, Jesus, and indiscretions about his views on church ministers. However, one of these charges was explicitly for claiming the so-called “plurality of worlds,” the idea that there are other Earth-like planets providing homes for creatures in the Universe.

During the early Enlightenment in the seventeenth century, a major technological step forward was taken with the invention of the telescope. This allowed scientists to see new moons and planetary bodies in our Solar System and to develop a more accurate view of how the Solar System was structured. One might be convinced that this should have reduced speculation, since with much more data available about what was in the Solar System, thoughts about conditions on other worlds could be more constrained. However, the effect was the opposite. With solid evidence for the presence of other worlds in the Solar System that were planetary bodies like the Earth and Moon, but with little information about their surfaces and whether they were appropriate for life, speculation went wild.

Christiaan Huygens (1629–1695), who discovered Saturn's moon Titan, and who invented the pendulum clock, wrote extensively on extraterrestrial life and the habitability of other planets in his book Cosmotheoros, published posthumously in 1698. As well as speculating about astronomers on Venus, he also suggested that other intelligences would understand geometry. About music, he said: “This is a very bold assertion, but it may be true for aught we know, and the inhabitants of the planets may possibly have a greater insight into the theory of music than has yet been discovered among us.”

His book followed on the heels of Conversations on the Plurality of Worlds, published by Bernard le Bovier de Fontenelle (1657–1757) in 1686, an enormously popular book at the time about the inhabitants of the Moon and other planetary bodies. The book is set up as a conversation in a moonlit garden with an intrigued marquise keen to know about the Solar System and how the cosmos works. It is a timelessly delightful and short book. These popular works did much to ignite the public imagination.

William Herschel (1738–1822), discoverer of Uranus and infrared radiation, after observing the strangely circular craters of the Moon speculated about them, in an age when their impact origin was completely unknown: “By reflecting a little on this subject I am almost convinced that those numberless small Circuses we see on the moon are the works of the Lunarians and may be called their Towns.”

Further enthusiasm for the possibility of extraterrestrial life was advanced by Camille Flammarion (1842–1925) in a series of books including La Pluralité Des Mondes Habités (The Plurality of Habitable Worlds) in which he emphasized that life elsewhere should adapt to its environment and would be channeled by the environmental characteristics of different planets, although he stressed that we could probably not predict exactly how it might evolve.

As late as 1909, Percival Lowell (1855–1916), observer of the infamous Martian “canals” (Figure 1.11) said of Mars in his book Mars as the Abode of Life: “Every opposition has added to the assurance that the canals are artificial; both by disclosing their peculiarities better and better and by removing generic doubts as to the planet's habitability.”

Figure 1.11 The “canals” of Mars as depicted by astronomer Percival Lowell. He was convinced he could see artificial canals, built by Martians, on the surface of the planet.

Source: Reproduced with permission of Perelman, https://commons.wikimedia.org/wiki/File:Lowell_Mars_channels.jpg.

We could continue with many such quotes (and many other eminent scientists and philosophers were convinced of alien life), but these examples are adequate to make two points. First, we would have to wait for the space age and the direct and close-up observation of planetary bodies to truly force astrobiology into an empirical era. Second, these quotes are a warning from the past. The desire to believe in alien life should not trump empirical observation. Life should always be the last explanation after all non-biological explanations have been exhausted.

Herschel's observations on lunar craters being the fortresses of lunar towns are a particularly interesting lesson. At the time when he was writing, asteroid and comet impacts were not understood to be an important geological phenomenon. It would be much later that we would understand that these events do occur and that they make craters. Even more interesting is the knowledge that most impacts, unless they occur at a very oblique angle, tend to create circular craters so that a planetary surface, after being pummeled by impacts for billions of years, will record many of these beautifully round features (Figure 1.12).

Figure 1.12 Lunar craters. In the eighteenth century, the almost perfectly circular craters on the Moon, prior to any understanding of impact processes, looked suspiciously artificial. This view shows the Capuanus crater, which is 60 km in diameter, in the lower left.

Source: Reproduced with permission of NASA.

In hindsight, the perfectly circular structures Herschel observed on the Moon seemed unnatural, the products of an advanced civilization. However, assuming one's geological knowledge is incomplete is always a safer and more parsimonious way to interpret data than explaining observations using biology. This is an application of Ockham's razor, a philosophical principle that the explanation that requires the fewest assumptions or speculations is the better one. This principle was propounded by William of Ockham, a Franciscan friar who studied logic in the fourteenth century. It's a very useful principle when you are searching for evidence of life anywhere, in ancient Earth rocks or on other planets.

It was only at the beginning of the space age (Figure 1.13) that the photographic study of planetary surfaces yielded new and more empirically constrained views of the surfaces of other planets. In general, they showed other planets to be devoid of life, and this led to a strong retreat from previous optimism. Nevertheless, astrobiology entered into the realms of experimental testing with a range of pioneering experiments and discoveries that would take it from its previous philosophical underpinnings to its present-day status as a branch of science.

Figure 1.13 One of the first orbital photographs of Mars, taken by the Mariner 4 craft on July 15, 1965. This and other photographs suggested a dead, desiccated environment unfit for life. The area shown is 262 × 310 km and is a heavily cratered region south of Amazonis Planitia, Mars.

Source: Reproduced with permission of NASA.

Laboratory experiments from the 1950s onwards brought studies of the origin of life into mainstream scientific investigations. Scientists simulated conditions on early Earth and demonstrated the production of amino acids and other building blocks of life from simple gaseous precursors. The publication of evidence, in the 1980s and onwards, of fossil microbial life on Earth, preserved for more than three billion years, turned the search for ancient life on Earth and the timing of the emergence of life into a scientific quest.

The first experimental search for life on another world was undertaken by the robotic Viking biology experiments, which landed on Mars in 1976. The consensus is that their observations are explained by reactive chemical compounds in the Martian soil, not life. However, the landers demonstrated that we can go to other planets and implement the scientific method in the search for life.

Attempts were made in the 1970s to transmit radio messages to other civilizations with all of its social and ethical implications. Despite the lack of response, the efforts to search for, and communicate with, extraterrestrial intelligence triggered a vigorous discussion about the intersection of astrobiology with social sciences.

The discovery of liquid water oceans in the planetary bodies orbiting in the frigid wastes beyond Mars, such as the moons of Jupiter (Europa, Ganymede) and Saturn (Enceladus) and the discovery of a complex hydrocarbon cycle on Saturn's moon Titan, has shown us that we can learn about the habitability of planetary bodies and organic chemistry in surprising places (Figure 1.14). In recent years, the discovery of planets, particularly rocky planets, around other stars (exoplanets) has led to a flourishing of astrobiology and our ability to assess the statistical chances of habitable worlds elsewhere in the Universe. These experiments and discoveries, from the mid-twentieth century and onwards, set the stage for astrobiology as the truly experimental science that we know today.

Figure 1.14 Plumes of water emanating from the south polar region of Saturn's moon Enceladus. These are just one of the many discoveries that have provided an empirical basis with which to test the hypothesis that habitable conditions exist beyond Earth.

Source: Reproduced with permission of NASA.

Throughout this history, different terms have been used to describe what we now call astrobiology, which can, if you don't take care, cause much confusion. In the mid-twentieth century, although not the first time the word was used, Astrobiology was the title of a 1953 book by Gavriil Tikhov (1875–1960; Figure 1.15). His book explored the possibility of life on other worlds. Tikhov was an interesting and in many ways, a pioneering character. He was particularly fascinated by the idea of using the absorption spectroscopy of vegetation to seek vegetation on other planets and even founded a Sector of Astrobotany allied to the Science Academy of Kazakhstan. His notions of using spectroscopy to search for life elsewhere are today at the forefront of methods to search for biosignatures on exoplanets.

Figure 1.15 Gavriil Tikhov, who wrote an early book called Astrobiology. He took a great deal of interest in spectroscopy as a means of looking for signatures of extraterrestrial life. Here he observes the spectroscopic signatures of vegetation.

In 1960, Joshua Lederberg (1925–2008; Figure 1.16), a pioneer in bacteriology and molecular biology who won the Nobel Prize for his work on bacterial genetics, used the term exobiology to describe the search for life beyond Earth. This term later became associated specifically with the search for life beyond Earth, which now constitutes just one word in the wider term: astrobiology. Other terms that appear in the popular and scientific literature have included cosmobiology, xenobiology, and bioastronomy, the latter used mainly by astronomers.

Figure 1.16 Nobel Laureate Joshua Lederberg. He was at the forefront of the United States' efforts in exobiology in the twentieth century, at his laboratory in the University of Wisconsin, October 1958.

Today, the word astrobiology is used in a wide sense to mean not just the search for life beyond Earth, but also the study of life in its cosmic context in general, including the history of life on Earth.