However, a stable medium is necessary for biochemical processes such as metabolism or genetic replication; on Earth this medium is liquid water. For extraterrestrial life, the medium could be another liquid or a dense gas as long as it doesn’t easily become a solid in the prevailing environment. Nonetheless, water (H2O) has some unique properties. Unlike its smelly twin, hydrogen sulphide (H2S), which condenses to a nasty liquid only at –61°C, water turns to liquid below 100°C at normal pressures. Liquid water’s stability occurs because the oxygen atom in water molecules is slightly negatively charged and allows a relatively strong ‘hydrogen bond’ to the slightly positive hydrogen atoms of other water molecules. Sulphur provides weaker hydrogen bonds between hydrogen sulphide molecules. Water also forms stronger hydrogen bonds to other water molecules than to molecules of oily substances. As a consequence, oils separate from water, which allows cell membranes to form and provide homes for genes and metabolic processes.
Another unusual property of water is that ice is less dense than liquid water. When water freezes, the molecules align into ring-like structures containing open holes on the atomic scale. If ice were denser than liquid, the cold bottom of lakes and seas would collect ice, which would be insulated and remain frozen. Seas would freeze from the bottom up and become uninhabitable. This would arise because sunlight would be reflected back in the areas where the ice reached the surface, causing cooling and more ice to accumulate. Gradual freezing might be the unfortunate fate of seas of other liquids such as ammonia. Ammonia is a liquid from about –78°C to about –33°C at a pressure of one atmosphere. But any seas of ammonia would tend to solidify from the bottom up, unlike seas of water that remain liquid even if cold temperatures cause an ice cover.
On Earth, we find microbes wherever there’s liquid water (excluding sterilized apparatus), so ‘life as we know it’ is as much ‘water based’ as ‘carbon based.’ Consequently, for Solar System exploration, the detection of liquid water or its past presence provides an objective for planetary probes, such as those visiting Mars. Nonetheless, it is possible to conceive of organic solvents as alternatives to water, which is potentially important for Titan, Saturn’s largest moon (Chapter 6).
Another observation from terrestrial life is that just six non-metallic elements—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur—make up 99 per cent of living material by mass. These elements are often abbreviated as ‘CHNOPS’, but I prefer ‘SPONCH’, which is easier to say. We find H and O in water, which makes up most living tissue, and C, H, and O in the nucleic acids of genetic material and in carbohydrates. C, H, N, and S exist in proteins, while P is essential for nucleic acids and energy-storage molecules. Consequently, detecting the SPONCH elements in chemical forms that life could use is another practical goal for planetary space probes searching for Earth-like life.
The significance of life elsewhere
You might wonder whether the discovery of simple, microbial-like extraterrestrials would really matter. But if we could find a single instance of life that originated elsewhere, it would prove that life is not a miracle confined to Earth. We wouldn’t be alone. Even the simplest microbes native to Mars or to Europa’s oceans would change the balance of probabilities that life exists elsewhere in the galaxy for they would demonstrate that life can originate twice within one solar system. At the moment, we have no convincing evidence of life beyond Earth. However, in Chapter 6, I will argue that at least nine other bodies in our Solar System might be habitable today, if we keep an open mind. Solar System astrobiology is far from settled.
A second significant factor in finding life harks back to the difficulties of defining life. The planetary scientist Carl Sagan (1934–96) commented in his book Cosmic Connection (1973) that ‘the science that has by far the most to gain from planetary exploration is biology’. An examination of extraterrestrial life would be of profound significance not only in identifying those elusive characteristics common to all life but also in shedding light on how life originated, which remains unsolved.
Chapter 2
From stardust to planets, the abodes for life
To make an apple pie from scratch, you must first invent the universe.
Carl Sagan (1980)
When the universe began, temperatures everywhere were far too hot for atoms to be stable, let alone join up into complex biological molecules. Life exists because following a Big Bang 13.8 billion years ago, a hot, dense cosmos expanded and cooled. As it did so, atoms, galaxies, stars, planets, and life arose. Here, we examine how this process produced an abode for life—the Earth.
We start with the structure of the present universe, which provides the clues about its history. Imagine a journey that goes to the edge of the observable universe. At the speed of light—300,000 km per second—it would take only 1.3 seconds to get to the Moon at its distance of 384,000 km. A diagram of the Earth represented as a spot of 2.7 mm diameter and the Moon looks as follows:
This scale is fairly easy to grasp. But it challenges our imagination when we realize that the Sun on the same scale would be 30 cm in diameter and we would have to place it just over 30 m away from this book. The nearest star, Proxima Centauri, which is small compared to the Sun, would be about 4 cm in diameter. To keep to scale, we would have to place it 8,650 km away, which is roughly the flight path from San Francisco to London.
Continuing our voyage at the speed of light, it would take 8.3 minutes to fly from the Earth to the Sun and a little over four hours to then travel to the average orbital distance of Neptune, the outermost of the eight planets. After 4.2 years, we would reach Proxima Centauri. With the huge distances involved, we define the distance travelled in a year at the speed of light, some 9,500 billion km, as a light year, so that Proxima Centauri is 4.2 light years away.
The Sun and Proxima Centauri are two of about 300 billion stars in the Milky Way galaxy, which is a disc some 100,000 light years across with stars concentrated in spiral arms. Galaxies contain millions to trillions of stars, so the Milky Way is moderately large. The Solar System sits in the Orion Arm, two-thirds out from the Galactic Centre. This arm appears unremarkable compared to some others that are more richly populated with stars. But it’s possible that the Solar System’s location in the galactic sticks was actually vital for terrestrial life. The Earth may have avoided certain catastrophes, such as proximity to exploding stars. If so, there may be a particular region in galaxies favourable for life, called the ‘Galactic Habitable Zone’, discussed in Chapter 7.
On a larger scale, there are more than 100 billion galaxies in the observable universe arranged into groups and superclusters. Within a diameter of about 10 million light years centred between the Milky Way and our nearest spiral galaxy, the Andromeda galaxy some 2.5 million light years away, there are roughly fifty galaxies, forming the Local Group. In turn, this group is one of a hundred or so within a sphere of 110 million light years’ diameter, comprising the Virgo Supercluster. Maps that cover billions of light years show filaments of tiny scattered points where each point is a galaxy. In three dimensions, galactic filaments, which are the largest structures known to humankind, join up into a web separated by vast voids. The whole fantastic structure looks as if it were spun by a crazy intergalactic spider.
How big is the observable universe? If space had not expanded, the farthest distance would be 13.8 billion light years, which is that traversed by a photon—a particle of light—since the Big Bang happened 13.8 billion years ago. But space has expanded. So the actual size of the observable universe is now about 47 billion light years across. Such vastness is a consideration for astrobiology because surveys of planets around stars just within the Milky Way suggest that each star hosts at least one planet on average. Some fraction of planets ought be habitable, perhaps at least 1 per cent, so the number of potential abodes for life might exceed a trillion billion.
The structure of the universe traces back to the Big Bang. In the 1920s, telescopic observations by the American astr
onomer Edwin Hubble showed that galaxies are moving away from each other on the large scale as space expands between them. Thus, going back far in time, everything must have been scrunched up and very hot. This consideration led to the recognition of strong evidence for the Big Bang. Its afterglow, the Cosmic Microwave Background, permeates the entire universe. If the Big Bang is true, physics dictates that before the early universe cooled, it must have been an opaque fireball made of electrons and protons (the electrically negative and positive elementary particles that make up atoms), photons (particles of light), and some small groups of fused protons. Some 380,000 years after the Big Bang, it became cool enough that electrons were able to join with protons, or groups of protons, to form the two smallest atoms, hydrogen and helium. At that point, the universe became transparent to light because previously photons had been scattered by the free-floating electrons. This prior situation was analogous to the way that light bounces around off tiny droplets in a fog so that you can’t see through. Since losing its opaqueness, the universe has expanded by about a factor of 1,000, and the wavelength of the relic photons from the Big Bang has been stretched the same amount, changing the photons from red light to microwave. Amazingly, when you tune an old analogue television or radio between channels, the leftover radiation from the Big Bang contributes to the static hiss, albeit at a level of about 1 per cent or less. In fact, in 1964, this noise in a large radio horn was how the microwave background was discovered. Pigeon droppings in the horn were blamed initially; but after cleaning up and shooting the unfortunate pigeons, the real culprit was identified as the beginning of the universe.
Galaxies appeared a few hundred million years after the Big Bang. Some places had very slightly more material than average and so had higher gravity. Clumping produced galaxies, and within the galaxies, on a smaller scale, gas clouds collapsed under their own gravity. The interior of each shrinking cloud heated up as gas particles collided, eventually making a hot, glowing ball of gas—a star.
Part of the ‘astro’ in astrobiology comes from the fact that all of the atoms used by life except for hydrogen were created inside stars. The first stars would have been made only of the elements synthesized in the Big Bang: hydrogen, comprising three-quarters of the mass, and helium, which made up the rest except for a trace of lithium. The oxygen in water, the nitrogen in proteins, or the carbon in every organic molecule—none of these elements was present at first. But eventually stars made them.
To understand how stars make elements, consider how the Sun shines. Inside the Sun, immense heat strips each atom down into its constituents: a positively charged nucleus and negatively charged electrons. The temperature at the centre of the Sun, some 16 million degrees Celsius, is enough to fuse the nuclei of four hydrogen atoms into a helium nucleus, which is a nuclear reaction that releases photons. Each photon then endures a one-million-year journey from the interior of the Sun to space. It takes so long because each photon is continually absorbed and emitted as it encounters material. The photon also loses energy. It starts out as a high-energy gamma ray and on average turns into a lower-energy photon of visible light by the time it escapes from the Sun. In the 1950s, physicists reproduced the kind of nuclear reactions that occur inside the Sun with hydrogen bombs. Cores of stars like our Sun are akin to hydrogen bombs that, in effect, can’t explode because they are contained by the weight of material above them.
The Sun will not fuse hydrogen in its core forever and the repercussions will destroy life on Earth (partly answering astrobiology’s question of ‘What’s the future of life?’). In most stellar cores, the accumulation of helium ‘ash’ causes temperatures to drop too low to support further hydrogen fusion. At this point, the star shrinks under its own weight, which causes the temperature to rise until it ignites hydrogen fusion in a shell surrounding the core. The energy release causes outer layers of the star to expand, cool, and redden. This is how a red giant forms, such as Aldebaran, the brightest star in the constellation of Taurus, the Bull. The Sun will eventually become a red giant and swell two-hundredfold by 7.5 billion years’ time, probably engulfing the Earth. The weight of further helium ash eventually squeezes a red giant’s core to a temperature of 100–200 million degrees Celsius, which is enough to fuse helium nuclei and make carbon and oxygen. In turn, a helium-burning shell can eventually surround a core of carbon and oxygen ‘ash’. Stars with four to eight times the mass of the Sun even end up fusing the carbon and oxygen into heavier elements, including neon and magnesium.
Generally, the death throes of a Sun-like star involve shedding outer layers into space. These shells of glowing gas are called planetary nebulae because they look somewhat like planets through low-magnification telescopes, but they have nothing to do with planets. The remnants of the star cool down into a white dwarf with a radius comparable to that of the Earth but an enormous density. In theory, a white dwarf stops shining after tens to hundreds of billions of years, producing a black dwarf. But the universe is not yet that old.
Stars larger than about eight times the mass of the Sun eventually explode as supernovae. The Sun is about half way through its ten-billion-year phase of hydrogen fusion in its core, but these massive stars spend less than 60 million years in the same phase before becoming red supergiants, of which Betelgeuse, in the constellation of Orion, is an example. In such stars, nuclear fusion in shells around the core produces the elements neon, magnesium, silicon, and iron. Iron is generally the heaviest element made, although some heavier elements are also produced when free neutrons are added to existing nuclei. (Neutrons are particles that have no electrical charge and are commonly found in atomic nuclei.) When the fuel in such a star runs out, the core is so compressed that negatively charged electrons amalgamate with the positively charged protons of the iron nuclei. The electrical charges cancel out and uncharged neutron particles are created. As a result, the stellar core shrivels to about 12 km diameter, forming something like a gigantic atomic nucleus made only of neutrons. Because of the shrinkage, the rest of the star collapses onto the dense neutron core, and a violent rebound creates the incredibly bright supernova.
Elements heavier than iron are generated and distributed by supernovae. A few seconds after a supernova, the outer layers of the star are heated to an incredible 10 billion degrees Celsius while the break-up of nuclei from deeper layers supplies abundant neutrons to fuel reactions that make heavy elements. This cosmic alchemy creates the heavy, precious elements such as gold, silver and platinum. Importantly, the material blown out in a supernova forms the basis of new generations of stars and abodes for life, their planets. In the case of stars reaching tens of solar masses, although a supernova still occurs, the collapse at the centre produces a black hole—an object so massive that nothing escapes its gravity, including light.
1. The Hertzsprung–Russell diagram for the Sun and nearby stars. Temperatures are in Kelvin, which is 273 plus the temperature in degrees Celsius; the Kelvin scale is defined so that 0 K or ‘absolute zero’ is where all molecular movement stops
The phase when a star converts hydrogen to helium in its core is the main sequence lifetime, which is generally the interval we consider optimal for life to thrive on planets around the star. The ‘main sequence’ refers to a diagonal swathe from upper left to lower right on the most famous graph in astronomy, the Hertzsprung–Russell (H–R) diagram (Fig. 1), named after its two originators. The graph plots a star’s luminosity versus its surface temperature. By a ‘surface’, astronomers don’t mean a hard surface but the level in a star’s atmosphere where most light emerges, which is as deep as we can see.
Oddly, the temperature axis runs backwards from high to low in the H–R diagram. The purpose is to match the colour coding of stars from hot blue-white stars to cooler red ones with the letters O, B, A, F, G, K, and M. The letter gives a star’s spectral class. Generations of astronomy students have remembered spectral types with the mnemonic ‘Oh Be A Fine Girl/Guy Kiss Me!’ (the letters don’t stand for anything
and have origins in 19th-century astronomy, which need not concern us).
On the main sequence, the most massive stars plot at the upper left and the lightest at the lower right. Whatever its mass, a main sequence star is called a dwarf—such as the Sun, a G-type dwarf. The lifetime on the main sequence can be over 50 billion years for cool red dwarfs.
The importance to astrobiology of the way stars ‘live’ and ‘die’ is wide ranging. Our Sun is a middle-aged, main sequence star with stable sunlight that fuels most life on Earth through photosynthesis. As mentioned at the outset, the atoms of life were generated in red giants and supergiants. Also, oxygen, silicon, magnesium, and iron are made from integral numbers of helium nuclei and so are particularly abundant products of nuclear fusion, which is significant because these atoms are the ones that make rocks. Rocky planets, like the one we inhabit, are a natural consequence of the physics of starlight. We also know that the Sun is at least a second-generation star because we have supernova elements on Earth such as gold. But since the Sun is only 4.6 billion years old in a 13.2 billion-year-old Milky Way, many stars came and went before life arose on Earth. Did earlier stars support planets and life, or even intelligent life, and what happened to them? This leads us to the question of how our own planet formed.
Getting a place to live: where planets come from
Astrobiology Page 3
