In this module on habitability, the first lecture is cosmic chemistry. We've seen how the sites for life, planets, are widespread in the universe. Astronomers had a pad enormous success in finding exoplanets including hundreds that are Earth-like. Now, we move to the ingredients of life, the prerequisites we think are needed for biology to operate, starting with cosmic chemistry. Carl Sagan once said, "If you want to make an apple pie from scratch, first, create a universe". What he meant was that the chemical ingredients for apple pies and other mundane things, but also more profound things like human beings, start from cosmology. They start from the ingredients that the universe has provided that go into these living organisms. Let's look at three very different parts of the universe and consider the frequency of abundance of their most common atoms. Let's start with the sun. These are the percentages of atoms in a typical sample of the sun. We can see that the sun is almost entirely made of hydrogen and helium, and the sun is currently converting its hydrogen into- And the three biogenic elements, or life important elements of carbon, nitrogen, and oxygen highlighted in red, are far rarer than the abundant ingredients of the sun. So we see less than a tenth of a percent of oxygen by number of atoms, only a third of a tenth of a percent of carbon, and even less of nitrogen. Strikingly, small numbers of the atoms are believed to be essential for life. As a counter point, look at the ingredients of a human being or, really, any biological specimen. We see a large amount of hydrogen, second most common ingredient is oxygen, and the 2:1 ratio reflects the fact that humans and most mammals are about two-thirds water. Other creatures may be larger or lesser components of water, but this 2:1 ratio of hydrogen and oxygen is going to be typical. The third most common ingredient is carbon. We are carbon-based life forms. Carbon provides the many bonds that are required to make extremely long molecules and generate biological complexity. But you can see that the carbon, oxygen, and nitrogen that are essential for life are far more abundant by at least an order of magnitude in humans or in biological tissue than they are in the sun. Let's look at the planet we're standing on. The Earth's crust is composed primarily of oxygen and silicon with the next most abundant ingredients, the two metals, aluminum and iron. That's because the rocks we stand on are mostly silicates or carbonates involving a fairly large amount of oxygen, silicon, calcium, carbon, and sodium. Again, very different composition of course from the sun, which is a gas ball made of hydrogen and helium, or human beings that are primarily made of water. The basic point here is that carbon, nitrogen, and oxygen are enriched in living organisms relative to a typical sample of the universe, which is a star, by a factor of hundreds. We have to know where this came from and why did this happen. Let's start with the lightest elements, the ones that the sun is made of. Hydrogen, helium, and a small amount of deuterium and lithium, deuterium is a heavy hydrogen isotope, were from the big bang itself produced by fusion and the first few minutes when the universe itself, and as a whole, was as hot as the center of the sun is now, which is to say 10 million Kelvin or larger. This process, cosmic nucleosynthesis, was a profound process. Then in literally a couple of minutes, created one-quarter of the universe by mass into the form of helium from the primordial ingredient of the universe, hydrogen, which is the simplest atom on proton and on an orbiting electron. If no stars had formed in the expanding universe, and they of course were caused by the collapse of gravity in pockets of the universe as it got cooler and older, then we would not be having this discussion. There would be no astrobiology course, no humans, nothing to talk about because life would not exist. A universe consisting of only hydrogen and helium is unable to make any monic molecule. Hydrogen can form its own molecule, H2, and helium is inert and does not bond with any other element. So a universe composed of only hydrogen and helium is a boring universe as far as biology is concerned. In other words, although these raw materials were available early in the universe, they alone are not sufficient to make biology. For biology, we need stars. Stars are nuclear reactors which by the fusion process, are building heavier elements from lighter elements. The most familiar version of this process is what happens in the sun. Where four hydrogen nuclei or protons are combined to make a helium nucleus, which consist of two protons and two neutrons. That is the process of fusion that the sun has been doing for five billion years and will do for another roughly five billion years. All the heavy elements, however, beyond helium, were made in the cores of stars. Either stars more advanced in their evolution than the sun or particularly stars more massive than the sun. To make heavier elements requires a temperature in the gas or plasma. They can overcome the electrical repulsion between nuclei. Two protons will resist being close to each other by their electrical force. Two hydrogen nuclei have two protons and so the electrical force is four times larger of repulsion, requiring correspondingly larger temperature to force them to fuse and so on up the periodic table. So the principle is that, inside stars, higher and higher temperatures are required to fuse heavier and heavier elements because a temperature and pressure is required to force those nuclei to merge. The fusion of two nuclei, helium nuclei, doesn't work because we're talking now about how does carbon form. If carbon is they ingredient for life, then carbon is an interesting part of this process because helium fusion must combine three helium nuclei to make carbon. This turns out to be an interesting process. It requires the coincidence of a triple fusion to generate a carbon nucleus. That's because the product of the helium fusion of two nuclei has a very short radioactive decay time and so essentially disintegrates before the third nucleus can be added. This alone, plus a nuclear resonance that was discovered in the 1950s and 60s that facilitates that fusion, means that carbon is far less abundant than helium in the universe by a factor of hundreds. So carbon is actually a rare element cosmetically speaking. But sufficient carbon is made inside stars obviously to create biology on this planet to allow rocky material to exist and we think for life to potentially be abundant throughout the universe. Now, there are other ingredients for life that involve heavier elements than carbon; nitrogen, oxygen, and other trace elements that the human body and other biological tissues depend on. Where do these elements come from? The same process. Except in this case, the stars like the sun are insufficient, we need more massive stars which go on to make heavy elements by simple combination. Think of the nuclei as little Lego blocks combining to make heavier nuclei in situations of enormous temperature and pressure. The most massive stars can go up the periodic table to make heavier elements than carbon, all the way up to iron. Naturally, the process stops at this point because iron is the most stable atomic nucleus and requires an energy input to go beyond this nucleus. That energy input only happens in a dying star or a supernova. Another thing that's going on here is that helium capture reactions that is building atomic nuclei by the addition of a helium nucleus rather than a single proton, add protons two at a time. Since protons are being added one at a time and two at a time, elements with even atomic numbers in the periodic table are slightly more abundant than with odd atomic numbers. When we look at the abundance of elements in the periodic table cosmically, it has a sawtooth pattern representing this fact. So, massive stars are required to generate the heaviest elements in the universe. By the time a massive star has evolved, it has this structure shown in cartoon form. Obviously, the materials are mixed and the demarcations are not as sharp as in a cartoon. Basically, it's an onion skin model where the more central regions have higher temperatures and so are able to create heavier elements by fusion. You see that the outer part of a massive star may still consist of some hydrogen that has not even been fused into helium. Within that, there will be a region where hydrogen fusion is taking place and helium is produced. Within that, a region where helium fusion is occurring, and by that triple combination process carbon is being created. Within that, carbon can fuse with other smaller components producing oxygen and so on. You see the center of this onion skin there's actually iron, and the core of a very massive star light in its life consists of iron. Now, this is not any iron that we've been familiar with. This is not solid iron or even liquid iron. This is iron in the state of a plasma, at a temperature of hundreds of millions of Kelvin and a pressure and a density that's far beyond the density of normal iron. A truly bizarre state of matter. So, there's show burning occurring that creates all these elements, and they're trapped in the star at this point. So, if all the stars did was to make these heavier elements and keep them inside their cores, once again, there would be no life in the universe. Something else has to happen. That something else is cycling of materials, stellar cycling. This lifecycle of massive stars is represented schematically in this diagram, where we can see that the material between stars and very low density gas also including some dust particles collapses to form stars and as we know planets. Those stars go into the main sequence, which is where they turn their hydrogen into helium. If they're massive enough, they go on to create other heavy elements. Late in their lives, stars become unstable and massive stars can eject envelopes consisting of 20 percent, 30, even a half of their mass out into interstellar space. Because the stars have convection which is to say, big convective cells of plasma circulating inside and outside through their core, the heavy elements they already contain or it created are mixed. So, the outer envelopes are not in a simple onion skin but actually contain some of the heavy elements when they move out into space. So in that one way, heavy elements get seated into space. For a massive star, there's a second even more dramatic version of this. When the core of the star collapses because no fusion is possible, there's no new energy source and gravity must win, the rebound from that collapse causes a dying star supernova. A fantastic brightening of the star for a very short period of time, the expulsion of a gaseous envelope at large fraction of the velocity of light, and the spontaneous creation of heavy elements by a process called explosive nucleosynthesis. These heavy elements are then injected directly and quite far into the interstellar medium. So this cycling process is gradually seeding the space between stars, out of which a next generation of stars will form with heavy elements including those necessary for life. The interstellar medium is being enriched in this process and so, the relative abundance of carbon, nitrogen and oxygen to take those important biogenic elements is steadily increasing over cosmic time. Soon after the Big Bang, before stars and galaxies had formed, there was literally just the hydrogen, the helium, a little bit of lithium, a little bit of deuterium. But over time as stars and galaxies form, as they cycle our elements and in the 13 billion years of the universe since then, the amount of heavy elements including carbon, nitrogen, and oxygen has been steadily and smoothly increasing. In the galaxies with active star formation, spiral galaxies, this process is more vigorous and the fraction of heavy elements in those galaxies is larger than in a galaxy with mostly old stars like elliptical galaxies. So, we can look at the common elements for life now in the context of this story of stellar nucleosynthesis. We can see that while the Sun consists mostly of primordial material, the hydrogen and helium that the universe was born with or created in the first few minutes, it still includes a small amount of heavy elements. Those heavy elements pre-existed the sun's formation. So, since the sun is not making carbon or oxygen or nitrogen, the relatively small amount that it contains was produced by previous generations of stars and was swept up into the material that collapsed to form the sun. Similarly, the heavy elements that we are made of we're obviously pre-existing and so those elements including those carbon, nitrogen and oxygen atoms in all of our bodies are at least 4.5 billion years old, because they were created in generations of stars before the Earth and the solar system and the sun formed. In fact, we cannot say how old those atoms are. Some of them could have been generated by fusion in a distant star billions of years before the Earth formed. We can take another couple of examples in the solar system and see that composition of the atmospheres of the giant planets contains hydrogen and helium as we've seen before, similar to sun material and then molecules which have carbon and nitrogen in them. Once again, those heavy elements were pre-existing the formation of the solar system. In a situation like Mars, we have carbon dioxide, so once again carbon is involved. A life producing substance but not involved in life in this particular situation. The second most abundant gradient in the Mars atmosphere is actually the inert ingredient argon, which was once again produced by stars. So, we have a picture where a massive star initially in the early universe is actually made of primordial, hydrogen and helium, and so there must have been a time at which the first heavy elements In the Universe were produced. Astronomers or cosmologists call this first light. This is the time when the first stars and galaxies formed and the first heavy elements were produced. We don't exactly know when first light was, but it was probably about a 100 million years after the Big Bang. So, really in the first few percent of the age of the universe, and elements have been produced steadily since then. So, this process of star formation and the creation of heavy elements in massive stars is a universal process. We see it playing out in distant galaxies. We know it's played out over cosmic time and therefore the production of the biogenic or life essential elements, is also a universal process. Similarly, we've seen that the process by which stars form and surrounding planets form, that we also believed to be a universal process playing out in distant galaxies throughout the universe and throughout cosmic time. So, two of these very major pieces of the story of biology on our planet are parts of a much larger story, a cosmic story of life and the formation of the sites for life and the ingredients for life. To summarize this lecture, we've seen that there are ingredients for life that are essential. They're considered to be carbon, nitrogen, and oxygen. However, the universe did not originally contain these ingredients. The universe was made of hydrogen and then within the first few minutes after the Big Bang, helium was created. In the mirror of the process occurring in the sun right now, only in a situation where the entire universe was hot and dense and able to make fusion turn hydrogen into helium. All the heavy elements beyond those and a smattering of other light elements were created inside stars. Most of the heavy elements were created in the cores of stars much more massive than the sun, able to create high enough temperatures through the fusion process to generate elements up to iron. To go beyond iron, extra energy is needed and that typically comes from a supernova. The collapse of a massive star, and then an explosive blast wave that creates heavy elements all the way through the periodic table. As stars age, they eject envelopes of gas into space and when they die explosively, they eject even more material. In this way, the heavy elements are seeded through space where they can become part of the next generation of stars. So, the existence of life in the universe given its dependence on heavy elements, has been a process that's played out for over 13 billion years as the fraction of heavy elements in the universe, from a still very low-level has steadily increased over cosmic time.
