Getting a place to live: where planets come from
Ideas for the origin of the Solar System have a long heritage. In 1755, Immanuel Kant suggested that the Solar System coalesced out of a diffuse cloud in space. Later, in 1796, the mathematician Pierre Simon, the Marquis de Laplace, elaborated the notion. The basic Kant–Laplace concept is known as the ‘nebular hypothesis,’ after nebula, the Greek word for cloud.
The hypothesis starts with the idea that some part of the cloud is slightly denser than others and attracts material by gravity. It is likely that the cloud also has some slight initial rotation, so that the shrinking cloud spins faster, like an ice skater drawing in her arms. The random motion of the gas and dust will oppose material attracted along the axis of rotation but matter converging in the plane of rotation will also be resisted by the spin. As a result, the cloud flattens into a disc. The Sun forms in the centre, while planets coalesce in the plane of the disc from sparse material. This is consistent with the mass of the planets being only 0.1 per cent of the Sun’s mass. Because they form from a disc, planets will all be in the same plane and they will all orbit the Sun in the same direction, as we observe.
In recent decades, it was thought that evidence from isotopes suggested that a shock wave from a nearby supernova triggered the collapse of the nebula. Isotopes are atoms that contain the same number of protons in their nucleus but a different number of neutrons. The Greek root, isos topos, means ‘equal place’, which refers to the same location in the periodic table of elements. Sometimes an isotope has a nucleus that is too big to be stable, and breaks apart by radioactive decay. An unstable aluminium-26 atom (which has a nucleus of 26 particles = 13 protons + 13 neutrons) decays into stable magnesium-26 (containing 12 protons + 14 neutrons). In any sample of aluminium-26 atoms, the time it takes for half of them to change into magnesium-26, the half-life, is 700,000 years. The presence of magnesium-26 in some meteorites, which must have been produced relatively quickly from aluminium-26, was thought to suggest a nearby supernova when the Solar System formed because massive stars make aluminium-26 and their supernovae distribute it.
However, in 2012, new measurements in meteorites showed that the levels of iron-60, which is an isotope only formed in supernovae, are too low for a nearby supernova. To explain abundant aluminum-26, a neighbouring massive star (perhaps more than twenty times the solar mass) of a type known as a Wolf–Rayet star may have shed its outer layers and spread aluminum-26 into the solar nebula. Aluminum-26 was a dominant heat source in the solar nebula for the first few million years. By melting ice in the earliest rocky material, the aluminum-26 caused water to go into hydrated minerals where it was safe, unlike ice that evaporates. If the aluminum-26 had not been present, the Earth might not have gained water-rich minerals and it might have neither oceans nor life.
The nebular hypothesis explains the broad distribution of different types of planet. The planets in the inner Solar System (Mercury, Venus, Earth, and Mars) are relatively small and rocky, while those in the outer Solar System (Jupiter, Saturn, Uranus, and Neptune) are giants. When the disc was forming, the matter drawn inwards gained energy of motion and the centre of the disc, where the Sun formed, became extremely hot. This created a temperature gradient from the hot centre of the disc to a cold exterior. Outside the Sun, pressures were low, which meant that substances existed either as solids or gases but not liquids. In the inner disc, it was far too hot for gases such as water vapour to form ice, but metal and rock vapour could condense nearly anywhere in the disc. Consequently, planets that formed in the inner region, such as the Earth, ended up with iron-rich cores surrounded by rocky mantles. In contrast, water condensed into ice from just inside the orbit of Jupiter (‘the ice line’) and farther out, providing more material to make larger planets. Methane could also condense as an ice from the orbit of Neptune outwards.
The giant planets are considered to have formed before the rocky ones. Jupiter and Saturn probably formed when rocky cores reached a size of about ten or so Earth masses that had sufficient gravity to attract more and more hydrogen and helium gas directly from the disc until all that was available in their orbit was sucked up. Because they are huge balls of mostly gas, we call these planets gas giants. This process happened within about 10 million years after the formation of the Sun. Uranus and Neptune are smaller, and drew in a greater proportion of icy solids, so we call these planets ice giants. Unlike the rapid growth of Jupiter, the formation of the inner rocky planets was spread over 100–200 million years. Planetesimals, which are ‘pieces of planet’, coalesced to make larger, rocky objects called planetary embryos with a size between that of the Moon and Mars. In turn, several planetary embryos merged into Venus and Earth and fewer into Mercury and Mars.
Although the inner planets accumulated material in their locality, gravitational nudges from the giant planets, particularly Jupiter, would have sent planetesimals careering into the inner Solar System. Scattered hydrated asteroids that originated from beyond the orbit of Mars were likely responsible for bringing water to the Earth that eventually turned into our oceans and lakes. We drink asteroid water. Computer simulations show that Jupiter ejected more water-rich material than it scattered inward, so that if Jupiter had had a less circular orbit and ejected even more water-rich material, Earth might have ended up without oceans and life.
We also know that planets do not necessarily stay put in their orbits. So-called hot Jupiters are exoplanets similar in mass to Jupiter that orbit at least twice as close to their host stars as the Earth orbits the Sun. Such exoplanets cannot have formed where they currently reside because it would have been too hot. It turns out that planets migrate because of large tails of gas and dust in the nebula that accompany their formation as well as the gravitational influence from other bodies. So the traditional nebula hypothesis is nuanced by planetary migration that could destroy or favour planetary habitability in extrasolar systems, depending on the details. In the next chapter, I’ll discuss the idea that even the giant planets of our own Solar System may have migrated somewhat.
Nonetheless, the essential idea of the nebular hypothesis—that the planets formed from a disc—was confirmed in the 1980s when discs of debris around young stars were seen through telescopes. In fact, there is still some leftover debris in our own Solar System. Comets are icy bodies and asteroids are rocky rubble that were never assimilated. Occasionally, small chunks that have been knocked off asteroids through collisions end up falling onto the Earth’s surface as meteorites. Consequently, meteorites provide us with key data about the early Solar System.
The age of the Earth and the Moon
Perhaps the most profound information gleaned from meteorites is the age of the Earth and Solar System. Ever since the 18th century, the vast layers of sedimentary rock seen by geologists had led them to suspect that the Earth, and hence the Solar System, must be of great age, but proof was lacking.
The first person to attempt to measure the age of the Earth was an English geologist, Arthur Holmes, who had the idea of examining lead isotopes formed from radioactive uranium. Uranium-238 decays into a cascade of further unstable isotopes of other elements until reaching a stable lead isotope, lead-206. The half-life for uranium-238 to change into lead-206 is 4.47 billion years. Another radioactive isotope, uranium-235, decays into lead-207 with a half-life of 704 million years. So, by measuring the amounts of lead-207 and lead-206 in different mineral grains of a rock, you can determine the age of the rock, because although there may have been different amounts of uranium in each grain, the fixed decay rates add the same ratio of lead-206 to lead-207 in a given time. In 1947, Holmes applied his method to a piece of lead ore from Greenland, and estimated that the Earth formed by 3.4 Ga (where Ga means ‘Giga anna’, equivalent to 1,000 million years or ‘billions of years ago’).
Holmes had two problems. First, he could never be sure that even the oldest rock he could find was as old as the Earth itself. Second, for a precise age, Holmes needed to know t
he small, original ratio of lead isotopes present when the Earth formed, so-called ‘primeval lead’, before subsequent uranium decay added further lead atoms. Clair Patterson, an American geochemist, realized that the first problem could be avoided by looking at meteorites because these are leftover building materials that formed around the same time as the Earth. He also recognized that certain types of meteorite, the iron meteorites, contain negligible uranium and so their ratio of lead isotopes provides a measure of primeval lead. With this approach, in 1953, Patterson accurately aged the Earth at 4.5 Ga. He was so excited that he feared a heart attack, and his mother had to take him to hospital.
Since then, improved techniques using radioactive isotopes have given us a timeline for the events surrounding the Earth’s formation. The very oldest grains in meteorites suggest that the Solar System formed at 4.57 Ga. The Earth formed slightly later at 4.54 Ga. Then, around 4.5 Ga, the Earth was apparently hit by a Mars-sized object, which is named Theia after the Greek goddess who gave birth to the Moon goddess Selene. According to this giant impact hypothesis, debris that was blasted out from the impact went into orbit around the Earth and coalesced to form the Moon.
The Moon is important for astrobiology because its gravity stabilizes the tilt of Earth’s axis to the plane of its orbit, which helps the Earth maintain a relatively steady climate. Currently, the Earth’s axis is tilted 23.5 degrees, but if it were able to vary widely, great climatic swings would occur. For example, at 90 degrees tilt, the Earth would be tipped on its side and ice would form seasonally at the equator. Computer simulations show that if we had no Moon, the Earth’s axial tilt would vary chaotically over periods of millions of years and have a large range, potentially from 0 to more than 50 degrees. Microbial life could probably withstand large climate swings. But advanced animal life and civilizations such as ours would be challenged.
In following our astronomical trail, we have now arrived at the point of the creation of an abode for life, our own planet. There were contingencies in whether the water necessary for life was delivered to the Earth and controls on where an Earth-like planet might occur. Then, after Earth formed, how and when did life arise?
Chapter 3
Origins of life and environment
The early Earth
So little is known about the very earliest aeon of Earth history that it doesn’t even have an official name. Informally, it is called the Hadean, which started around 4.5 Ga when the Moon formed, and ended at a date that hasn’t been agreed upon but is usually taken as either 3.8 or 4.0 Ga. It is probable that life originated in the Hadean, but so far we haven’t found evidence because there are no sedimentary rocks from this time. Such rocks consist of layers of sediment grains laid down in water or from airfall, and so best preserve traces of biology or the environment. Consequently, our ideas of what happened during the Hadean have to be constructed from sparse data aided by the theoretical constraints.
Theory suggests that heat from the giant impact that formed the Moon would have turned rock into gas. The atmosphere of vaporized rock would have lasted for a few thousand years and then condensed and rained down on a molten magma surface, which eventually solidified into a crust. Subsequently, the atmosphere would have consisted mainly of extremely dense steam for a few million years before it condensed to form oceans.
When did continents begin to form? We get some clues from tiny mineral grains less than 0.5 mm across that are left behind from the first half billion years or so of Earth history. These are zircons, crystals of zirconium silicate with chemical formula ZrSiO4, where Zr is zirconium, Si is silicon, and O is oxygen. Zircons are so tough that they remain even after the rock in which they were once hosted has eroded away. Some zircons as old as 4.4 to 4.0 Ga have been found in fossilized gravel in the Jack Hills, which are about a thousand kilometres north of Perth in western Australia. The zircons contain flecks of quartz, which is a crystalline form of silica, SiO2. The quartz may have been derived from granites, which is the silica-rich type of igneous rock that makes up much of the continents. In this way, zircons suggest that Earth’s continental crust existed as long ago as 4.3 Ga. Isotopes support this inference. Some ancient zircons are enriched in stable oxygen-18 atoms relative to stable oxygen-16 ones. Such enrichment occurs when surface waters make clays and the mud is then buried and melts underground, passing the isotopic signature to igneous rocks.
In the Hadean, the Earth was probably hit by a few huge pieces of debris left over from Solar System formation, but none as big as the Moon-forming impactor. The energy of a small number of very large impacts could have vaporized the entire ocean or its upper few hundred metres. If so, life would have had to restart or it might have been trimmed back to only those microbes sheltered underground that were able to survive the heat. In fact, partially sterilizing impacts may explain the nature of the last common ancestor for all life on Earth. Genetics traces the common ancestor to a thermophile (see Chapter 5)—a microbe that lives in hot environments. Essentially, DNA analysis implies that your ‘great-great-great- … grandmother’ was a thermophile, if you insert enough ‘greats’. This may be because thermophiles were the only survivors of huge impacts.
The Earth today is not entirely free from potentially catastrophic impacts. For example, Chiron is a comet-like object about 230 km across in the outer Solar System that crosses the orbit of Saturn. One day within the next 10 million years or so, a nudge from Saturn’s gravity will fling Chiron towards the Sun or away from it. In the former case, Chiron’s chances of impacting the Earth would be less than one in a million. But if Chiron did hit, the heat would turn the upper few hundred metres of the ocean into steam, and land would be sterilized down to about fifty metres’ depth. Thermophiles might become the ancestors of subsequent life in a sort of evolutionary déjà vu, if the impact explanation for a thermophile ancestor is correct.
The last hurrah of the big Hadean impacts is known as the Late Heavy Bombardment, which happened about 4 to 3.8 Ga. Craters on the Moon bear witness to this massive bombardment. Rocks brought back by Apollo astronauts have been dated using radioisotopes and indicate that many craters were produced within the same 200-million-year interval.
The leading hypothesis for explaining the Late Heavy Bombardment was devised by astronomers in Nice, France, and is therefore called the Nice model. It relies on the astonishing idea that the orbits of Jupiter and the other giant planets shifted at the end of the Hadean. Calculations suggest that after the Solar System formed, mutual gravitational effects caused Saturn and Jupiter to reach a state called a ‘resonance’ in which Saturn orbited the Sun once for every two Jupiter orbits. Regular alignment created periodic gravitational prodding that made the orbits of Saturn and Jupiter less circular. In turn, the orbits of Neptune and Uranus were perturbed and moved outwards, also becoming more elliptical. Neptune could have even begun inside the orbit of Uranus and then overtaken it in a migration outward, and both planets, particularly the farther one, would have scattered small icy bodies, some towards the inner Solar System. Meanwhile, the gravity and movement of Jupiter would fling some asteroids into the inner Solar System and push others away. During this time, the Earth must have suffered even more impacts than the Moon because of Earth’s greater size and gravity. Eventually, after wreaking havoc, the orbits of the giant planets would have settled down.
The origin of life
Quite how life arose is unknown. It may have originated on Earth or it was carried here by space dust or meteorites. The latter idea is called panspermia, and doesn’t solve how life originated, but pushes the problem elsewhere. Also, there may be difficulties with survival over long transit times if life came from around other stars. For these reasons, we’ll concentrate on a terrestrial origin of life.
There is wide agreement that the origin of life would have been preceded by a period of chemical evolution, or prebiotic chemistry, during which more complex organic molecules were produced from simpler ones. The idea goes back to the 19th century.
In 1871, Charles Darwin imagined that such chemistry might have occurred in a ‘warm little pond’, according to a letter to the botanist Joseph Hooker:
It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present.—But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts,—light, heat, electricity etc. present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter would be instantly devoured, or absorbed, which would not have been the case before living creatures were formed.
Thus, Darwin concluded that life is unlikely to originate today because organisms are continually eating the chemical compounds that are needed. On the other hand, before life existed, chemical conditions for the origin of life would have been prevalent.
In the 1920s, the Russian biochemist Alexander Oparin and the British biologist J. B. S. Haldane both recognized that the environment under which life arose would have lacked oxygen. Earth’s oxygen-rich atmosphere is a product of photosynthesis by plants, algae, and bacteria. An oxygen-free atmosphere would have been better suited to prebiotic chemistry because oxygen converts organic matter into carbon dioxide, which prevents the build-up of complex molecules. Indeed, a contemporary geologist, Alexander MacGregor, reported in 1927 that he had found sedimentary rocks dating from within the Archaean Aeon (3.8–2.5 Ga) that showed that the ancient atmosphere lacked oxygen. In particular, MacGregor observed that iron minerals in what is now Zimbabwe were unoxidized, unlike the rust-coloured iron oxides in modern sediments that are produced when atmospheric oxygen reacts with iron-containing minerals. Today, MacGregor’s deduction is supported by many other data (Chapter 4).
Astrobiology_A Very Short Introduction Page 4
