Big Steps in the Story of Big History – Part 2

The evolution of the elements and planets.

Continuing our essay series on Brian Villmoare’s “Big History” book The Evolution of Everything: The Patterns and Causes of Big History, this essay will explore the evolution of the elements and the planets.

As Villmoare writes:

After the Big Bang, the Universe was full of very simple atoms – mostly hydrogen (which has only one proton and one electron, and no neutrons). Astronomers estimate that 75 percent of the mass of the Universe today is composed of hydrogen atoms, but initially hydrogen comprised almost all of the mass. Much of the story of the Universe over the next 13.8 billion years is the story of what happens to these hydrogen atoms. Over time, these hydrogen atoms tended to clump together. Even though these atoms are very tiny, they do have mass, and anything with mass will pull on other objects, simply through gravity. As these clumps got larger and larger, they started to have stronger gravitational forces and pull in more and more hydrogen. Once these clumps of hydrogen became large enough, they started to burn, and these burning hydrogen balls are what we today call stars.

The pressure at the center of stars was so great that their hydrogen atoms fused with other hydrogen atoms to form new elements, with different numbers of protons and electrons:

[I]n stars we have another process entirely. Here, two different atoms are compressed together enough that they form another element. This process is called fusion, and it is a type of nuclear process. In fact, it is the same type of process that is used in most modern nuclear weapons (the so-called hydrogen bombs). The process starts with atoms of the simplest element, hydrogen (which were created in the Big Bang), being forced together to form the next simplest element, helium. The force that pushes these hydrogen atoms together is their own gravity, compressing the atoms. One way to think about this force is to imagine yourself diving to the bottom of the ocean. The deeper you go, the greater the pressure of the water, and because humans have spaces of air in their bodies (especially in their lungs, and in the sinuses of their heads), they cannot dive beyond a certain depth before the pressure crushes them. The force of the water under the effect of gravity is, in effect, greater at greater depth. It is much this way in a ball of gas, in which the center is under greater pressure … [W]hat makes these balls of gas become stars – nuclear reactors generating millions of degrees of heat? It is their own gravitational force. When a ball of gas gets massive enough, the force of gravity at the center compresses the hydrogen, and the nuclear fusion reaction starts … Normally, the nuclei of atoms repel each other, but if the ball of gas is large enough, with enough gravitational force, hydrogen atoms will be forced together, and the nuclei undergo nuclear fusion … [Incidentally,] [h]ave you ever wondered why stars, planets, and many galaxies are in the shape of a sphere? Why are they not saucer shaped, or donut shaped, or cubes for that matter? The answer has to do with something unique about a sphere. In a sphere, all points in the surface are equidistant from the center. For a large object in space, this means that gravity effects the surface equally. Imagine an irregularly shaped object – as gravity pulls on the surface of the object, the surface material might be drawn toward the center. Spots further from the center (high spots) will be pulled closer. But as the object becomes a spherical shape, the gravitational forces start to equal out, so that every part of the surface is being pulled equally. This is a stable state that many celestial bodies ultimately become … [Now back to fusion.] Fusion releases a tremendous amount of energy, and when you look up at the Sun you are witnessing exactly this fusion reaction … [O]nce the atoms are fused into helium the process does not stop. In fact, the fusion process continues, forcing together simpler atoms, and creating more complex, heavier atoms. In most stars, the process generates the elements you are familiar with – lithium, beryllium, boron, carbon, and nitrogen, right on down the periodic table until they get to iron. Iron is so dense that, in most stars, the gravity is not strong enough to squeeze it. So the end point of most stars is a big ball of iron floating in space. However, if the star is large enough, the gravity can be so strong that even a ball of iron can be compressed. When this occurs, the star is said to become a supernova, which is the release of energy from the fusion of iron. This explosion, through fusion, generates force that can fuse iron into the heavier elements, and there are many (zinc, lead, gold, tungsten, and uranium among them). The supernova is an explosion that spreads these elements across the Universe, where they later become other stars, planets, asteroids, or other celestial bodies. The fact that these heavier elements are only produced by supernovas is why they are less common on Earth, whereas iron is very common as it is the end product of most stars. The Universe produces a great deal of iron but not as much gold and silver … The Universe has been seeded with elements from these stars living, then dying, in massive explosions, over billions of years.

And those elemental atoms ebb and flow throughout the universe over vast periods of time.  When I was a substitute teacher in that sixth-grade science class I mentioned in the previous essay, I used the following as an example as a typical “life of an atom”: Once in a star; an atom exploded out of it; landed on an asteroid; came to Earth; landed in a pool of water for a billion years; merged with other atoms to form a protein; was eaten by a worm; then eaten by a bird; then frozen into a glacier for 400,000 years; was released when it melted; was absorbed by a tree leaf; made its way to one of the rings of the tree trunk; was chopped off by a lumberjack; was accidentally eaten by another bird; and when that bird was killed and eaten by a hunter, the hunter burped it out. Indeed, it’s interesting to think that some of the atoms in our body are over 13 billion years old.

When atoms combine or break apart, they gain or lose mass.  But because mass is also energy, breaking atoms apart or combining them together can release energy as well. As Villmoare writes:

You have probably seen the famous equation: e =mc2. The variables of that equation are pretty simple: energy = mass × speed of light squared. This means, of course, that there is a certain amount of energy stored in mass, and the greater the mass (of an object) the more energy it has in it. This is the reason why nuclear reactions, even of relatively small amounts of mass, release a tremendous amount of energy. Mass is lost, and converted into energy. But you can, algebraically, rework this equation in a way that is also important: m = e/c2. This says that mass = energy divided by the speed of light squared. The implication of this overall equation is that, much as mass can be converted into energy, energy can be converted into mass. In an important sense, matter and energy are interchangeable. So when we talk about the Big Bang, we argue that, at the origin, there was nothing but raw energy. Part of the process of the Big Bang was the conversion of this energy into matter. Much of the work of physicists over the last fifty years has been to learn just how energy is converted into matter. One important implication of this equivalency is that you can think of matter, in the form of subatomic particles, as lumps of “condensed” energy. We perceive of it as something different than energy, since it takes a quite distinct form from other forms of energy (light, heat, etc.), but the underlying physics tells us that everything in the Universe composed of matter is ultimately composed of energy.

The relatively slow burn of our own Sun (compared to other stars) allowed the slow process of evolution on Earth enough time for humankind to appear:

The lifetime of a star is also largely determined by size. A truly big star has so much gravitational force that the fusion reactions will happen very rapidly, and the star may only last several hundred years. A smaller star, such as our Sun, has a slower burn rate, and the steady process of fusion in such a smaller star’s furnace can last billions of years. This is lucky for us, because it took almost 4 billion years of life on Earth before humans arrived.

Fusion, the energy production process at work in stars, creates far more energy than fission, the energy production process used in nuclear power plants and in the atomic bomb that ended World War II.  As Villmoare explains:

[A]ll nuclear reactions are not the same. In fact, there are two basic types – fission and fusion – and they are essentially the opposites of each other. The earliest nuclear weapons, like those used in World War II, as well as all nuclear reactors used to generate power, are fission devices. They generate energy by splitting one atom into two atoms (many times over and over again). This division of the atom releases a great deal of energy. Splitting atoms [so they lose mass and release it in the form of energy] is not that easy, so we tend to use uranium, which is among the easiest elements to split. The reaction can be slow and controlled, as in a nuclear reactor, or fast, as in a nuclear explosion, depending on how fast the atoms are split. In a nuclear fission explosion there is a very rapid chain reaction, as one atom being split causes enough energy for others to split, and so on, in a quick chain reaction that causes a rapid release of energy. In a nuclear reactor, atoms are split just a few at a time, preventing a rapid chain reaction, so that it can be controlled … [When] two smaller atoms are fused into a large one, essentially the reverse of fission [occurs]. This process actually releases much more energy than fission and, for us on Earth, it is much less controllable. When you hear about the “hydrogen bomb,” that means a fusion bomb in which hydrogen atoms are fused to make helium. This is precisely the process that stars use to generate their energy. But it requires an enormous amount of pressure to get those atoms to fuse, however, so it is very difficult to do. In fact, in a fusion bomb, a fission explosion is used to generate the force necessary to fuse the hydrogen atoms into helium. So a hydrogen bomb is essentially two bombs – a fission bomb and a fusion bomb … [A] way to generate enough force [like the gravitational forces at work in the center of stars] for a fusion reaction without a fission explosion has never been found, so currently this method is obviously too risky.

Gravity also formed galaxies and solar systems:

Once a star has enough gravitational mass it will find itself attracting, and being attracted to, other stars. As these stars are tugged together, they will combine their forces of attraction and their inertia, to form an orbit. Over time, as more and more stars are pulled together, they will make an enormous rotating mass called a galaxy. Many of the “stars” you see in the night sky are actually these groups of stars, burning brightly but distantly … [A]bout 5 billion years ago … [c]lumps of matter near our star, the Sun, were far enough away not to be pulled into the star but not far enough away to resist its gravitational pull. This matter orbited the Sun, clumping under its own gravitational force into medium-sized bodies that we know today as the planets. One of those clumps, forming roughly 4.5 billion years ago, is our planet Earth … One product of these collisions is our Moon. Roughly 4.5 billion years ago, a meteor struck the new planet and blasted a chunk of it off into space. This chunk started orbiting its parent, and we now call it the Moon. Over time, the forces of gravity inside the Moon pulled it into a sphere (see Box 2.5). Likewise, the Earth’s gravity forced it to return to a sphere. This means that the Moon is composed of the same material as is the Earth, a fact that we discovered when astronauts landed on the Moon almost fifty years ago.

The Earth itself is composed of types of materials that made it particularly suitable for the development of life.  Let’s start with how the elements in the Earth form a protective shield around it:

[E]ven long before the evolution of mammals, the Earth was uniquely suited to life, and this is because of its geological structure. The Earth is large enough to retain a fluid inner core, which generates the magnetosphere that helps protect the Earth from the Sun’s radiation. This protective barrier helps retain liquid water on the Earth’s surface, which is essential for life … The cyclical motion of iron in the fluid metal outer core has the effect of generating a powerful magnetic field that extends from the core of the Earth out tens of thousands of miles into space. For most people, the visible effect of this powerful magnetic field is apparent when you pick up a magnetic compass. The needle points toward the north because that is the orientation of the magnetic field generated by the motion of liquid metal inside the Earth. The polarity of this magnetic field is effected by the Earth’s rotation, so the poles of the magnetic field are at the poles of the Earth. [For further explanation, see this video, starting at the 3:00-minute mark.] The magnetic field is convenient for us, because we can use compasses to identify the north and south poles of the Earth, but more importantly, it shields the Earth from the Sun’s powerful radiation. Without this protection, the Sun would have stripped away Earth’s atmosphere and liquid water. Mars has no fluid inner core and mantle, and generates no magnetosphere, which is why it has hardly any atmosphere and no liquid water … Since the presence of water is a critical precondition for the evolution of life, the magnetosphere is one of the most important factors that a planet must have if life is to evolve … Mars has no liquid center, and no magnetosphere. This is due to its small size. Mars is about the same age as Earth, but it is roughly the half the mass of the Earth. Since smaller objects cool faster, its liquid core cooled enough to solidify more than 3 billion years ago. This means the planet can generate no magnetosphere, and without a magnetosphere Mars has no atmosphere and very little water.

Next, while the iron in the center of the Earth was essential to the creation of a protective magnetic field, fortunately there was not too much iron at the surface of the Earth to prevent the buildup of oxygen, which is essential for life.  As Villmoare explains:

The Earth has several layers – the outer crust, which is about 25 miles thick in the continents, is all we ever see, but it actually represents less than 1 percent of the Earth’s mass. Far below that is the inner core. The core is made up of the heavier elements on Earth – iron and nickel, which have settled through the other molten materials to the center of the Earth under the forces of gravity. The inner core is actually thought to be in solid form, even though it is still hot enough to be liquid. The pressure of the outer layers are so great that the molecules in the core are forced together, and once packed that tightly they become a solid. The outer core, under slightly less compressive pressure, is a metallic liquid, that plays a critical role for life on Earth … The period of the formation of the surface lasted for about 700 million years, and is known as the Hadean Eon. The Earth during most of this time was uninhabitable, as it was far too hot for life to survive … Toward the very end of the Hadean, the Earth’s surface had cooled enough for water to form … [O]ne thing to keep in mind is that water is probably absolutely necessary for life to form. And it is during this time, after about 3.8 billion years ago, that we see the first evidence of life. This period of the Earth is called the Archaean Eon, and it lasted more than a billion years, until 2.1 billion years ago. This is when we see the first continents form … The life on Earth at this time was very simple – probably just bacteria [cyanobacteria] – and it survived through photosynthesis, using the energy from the Sun to live and reproduce … During the Archaean [period] primitive single-celled life expanded, and photosynthetic bacteria spread around the world. Photosynthesis produces oxygen, and for over a billion years oxygen was being generated, but it could not readily build up in the atmosphere because of the presence of elements on the surface of the Earth that readily bind with oxygen, notably iron. In fact, this is the period of time in which we see large bands of oxidized iron, evidence that oxygen was present, but also evidence that it was not available for the atmosphere. Without the large amounts of free atmospheric oxygen generated by the cyanobacteria, no later complex life could have evolved. It took more than 2 billion years of photosynthesis to build up enough atmospheric oxygen for this later complex life to evolve … Oxygen is necessary for more complex life, so this build-up was a critical step in the evolution of multicellular life … One of the incidental results of 2 billion years of photosynthesis by single-celled cyanobacteria was the accumulation of oxygen in the atmosphere. Oxygen is the byproduct of photosynthesis, and, from the perspective of plants and cyanobacteria, is a waste product. Conversely, animals use this oxygen and the glucose created in part by plants to fuel themselves, releasing carbon dioxide and water as byproducts. Of course, this accumulation has had profound consequences for us, and all other complex multicellular organisms. Oxygen has some important chemical properties – it makes bonds readily with carbon, so it can form some of the complex organic molecules that exist in all organisms, such as lipids, sugars, and proteins. Since oxygen normally exists as a molecule of two atoms (O2), it can form chains by bonding with two separate molecules. Oxygen can also readily transfer energy by donating an electron, which is essential for the transfer of energy to cells during cellular respiration. Complex organisms have large energy requirements and the chemical properties of oxygen make the element essential, which is why we do not see the appearance of complex life until large amounts of atmospheric oxygen had accumulated. Some animals, such as arthropods, are constrained in their ability to absorb oxygen because they do not possess lungs. Instead of using a vascular system, they rely on the diffusion of oxygen through their bodies. This is one of the reasons why modern arthropods are fairly small, and why there is a fossil record of large terrestrial arthropods around 340–280 million years ago, when oxygen levels spiked in the atmosphere.

We also owe the existence of any atmosphere at all on Earth to two factors: mass and distance from the Sun.  Fortunately for us, Earth’s gravity is strong enough to hold onto its atmosphere and keep the atoms in the atmosphere close.  Mars, on the other hand, is around one-tenth Earth’s mass. Its lower gravity allows gas atoms on the surface to escape into space.  And regarding distance from the Sun, fortunately the Earth is not so close to the Sun that the Sun’s heat heats up the gas molecules on the surface so much that they vibrate enough to escape Earth’s gravity.  Mercury, for example, the closest planet to the sun, not only has a much smaller mass, but it’s so close to the Sun that any atoms in the atmosphere absorb so much heat they vibrate right off the planet.  Earth is also close enough to the sun to have both water and solid ground.  As Villmoare writes:

One thing about the planets that you might have noticed is that the inner planets (Mercury, Venus, Earth, and Mars) are all rocky planets, whereas the outer planets (Jupiter, Saturn, Uranus, Neptune) are all gas planets. That is not an accident. The same powerful solar winds that stripped away the atmosphere from Mars pushed the gas molecules in the inner solar system out past all the rocky planets. Once out at that greater distance from the Sun, the solar winds are not as strong, so the gas could accumulate into planets. In general, there is so much hydrogen in the solar system that the mass of the gas planets far exceeds the mass of the rocky planets; Jupiter alone is many times more massive than all the rocky planets combined … [W]hen astronomers, using powerful telescopes, look out at more distant solar systems, that pattern appears to hold, with rocky planets close to stars and gas planets further out, suggesting that there is nothing particularly unique about our solar system … For living things, Earth is just the right temperature – warm enough for water to be liquid, yet not so warm that water simply vaporizes. And since life cannot exist, as far as we know, without water, Earth is the perfect place for life to evolve … But where did water come from? [One hypothesis] is that the chemical elements that form water were present in the rocks below the crust; they formed water deep underground and percolated up over millions of years. Water is simply hydrogen and oxygen, so the elements are simple, and more importantly, common throughout the Universe. They could have been present deep under the crust as separate elements that combined in the mantle. Evidence for this idea is water-worked diamonds found near the surface of the Earth. Diamonds only form under extreme heat, and so are billions of years old. Some diamonds, rounded and eroded, show evidence of having traveled with water as water rose to the surface.

In the next essay in this series, we’ll explore the origins of life on Earth.