It turns out that perovskite—specifically as it behaves under high pressure—is very important to the study of super-Earths, and indeed forms the link between our planet and other planets. The breakthrough occurred in 2004, when a long-standing mystery about the Earth’s interior was finally explained. Seismologists and other scientists had noticed a thin layer at the bottom of the mantle, just above the core-mantle boundary, that had an unpredicted effect on the behavior of earthquake waves propagating through it. This layer, known as D”, is only 150 kilometers thick, and no one knew how to model it or why it affected seismic waves as it did. A combination of high-pressure experiments by Murukami and team and theoretical calculations by Oganov and Ono uncovered the culprit—perovskite under very high pressure.6 At that depth—where the pressure reaches 1.3 million atmospheres—the closely packed structure of perovskite deforms into a layered structure of nanoscale sheets—a new phase the discoverers called post-perovskite.7
As already noted, this layer is very small on our planet—just 150 kilometers—but turns out to be the main component of a super-Earth! For any super-Earth of about 2 ME and above, the pressure inside most of the mantle exceeds 1.3 million atmospheres; therefore, super-Earths, whether rocky or oceanic, are post-perovskite planets.
You might have noticed the unremarkable naming of the new mineral. No naming rights here. This is because post-perovskite cannot be present in a rock on the surface of the Earth, so no one has held it in their hands yet, or ever will. (This is apparently what it takes to name a mineral according to the medieval rules of the geological societies.) This is because the structure of post-perovskite makes it act like a loaded spring; if it were ever brought nearer to the surface by convection, it would revert back to perovskite.
Discovering post-perovskite was timely for the upcoming study of super-Earths, but we might not be finished with the project yet. The problem is that pressure throughout the mantle of the most massive super-Earths reaches 10 million atmospheres, much higher than what is needed to turn perovskite into a post-perovskite.8 It is possible that other mineral phases might be inside them—they could even be a post-post-perovskite?
Theory and experiment have already given some hints to the fate of post-perovskite under higher pressure. The main player in this high-pressure drama is the oxygen atom. After submitting to two dense crystalline arrangements, called packing, the further belt tightening leads the oxygen atoms to rearrange themselves again. Their efficient packing seems to have one more possible crystalline structure—a 12-point phase.
The hints come from the study of a little-known crystal called gadolinium gallium garnet—GGG (Gd3Ga5O12). GGG is transparent and very hard under pressure—useful properties to the scientists who try to compress hydrogen to a million atmospheres in the hope of making thermonuclear energy commercially viable.9 The compression of hydrogen (e.g., very small pellets of it) is done by powerful lasers, happens quickly, lasts briefly, and has to be observed through a tiny window that does not break under high pressure. In 2005 scientists in Japan and the United States used strong laser impulses to squeeze GGG to 1.2 million atmospheres and found, somewhat to their surprise, that the crystal deformed into a state that was harder (less compressible) than diamond.10 This was a world record. So, GGG emerged as a better material for the high-pressure experiments with hydrogen, replacing the sapphire and diamond that had provided the tiny windows until then.
The GGG experiment shows that an even denser packing of the oxygen atoms is possible in a post-perovskite material. It may be the last such deformation we need to consider inside a super-Earth planet. Fortunately we can do the experiment in a lab, although it is expensive, so the wait for the answer should not be long. In the meantime, it is already clear that super-Earths are made of the hardest materials known in the Universe: post-perovskite in the rocky and water ones, and diamond in the rare carbon planets. Indeed, this captures part of the beauty of studying super-Earths: the richness of new exotic materials they bring to the table, or lab bench as it may be. Hot water ices VII, X, and XI are as unusual to our world and the rest of the Universe, as are post-perovskite and GGG. Under pressure the solid form of water attains the more compact cubic crystal structure. Ice VII is analogous to crystal rock salt in that way and happens to have similar mean density. What a strange world!
PART II
ORIGINS OF LIFE
CHAPTER SEVEN
THE SCALE OF LIFE
Shakespeare once said that all the world’s a stage, and all the men and women merely players.1 To paraphrase Shakespeare, scientists are acting in a play not knowing the script (or if one exists) and not knowing if there is anyone in the audience. And the stage is certainly not built to our size. It is way too big, even by the standards of fairy tales. Yet we humans are funny little creatures—we possess reason and a spirit capable of matching the vastness of the world.
When I was in high school, I had a small telescope. My father had shown me how to use it, mostly during the day; I was on my own at night, spending hours out in the backyard. The place where I grew up was small, no telescope or observatory at school, no planetarium to visit nearby. So I had almost no idea what I would see through my small telescope. Sure, I had read books, some with pictures, but the view through the telescope was something else. The experience was truly visceral—shivers would run up and down my spine every time I pointed the telescope at a patch of stars. Eventually the feeling went away, but I still remember it. As I looked at the darkness between the stars, I felt as if I could fall into it. Like a fear of heights in reverse. Like an upside-down vertigo.
Perhaps I had read one book too many, and I was imagining things too vividly. After all, I had read about the vastness of the empty space between the stars in my books. But if we had a feel for how big the Universe is, we would have permanent shivers up our spines. To stay sane, astronomers use math, lots of it, and this can ruin even the best party.c Seriously, though, this is a way to deal with the problem. Since the time of Eratosthenes in ancient Greece, who measured the size of the Earth, humans have used math (and geometry) to take the measure of the Universe. Almost always the new knowledge increases our feeling of wonder. Many scientists (and I am one of them) will tell you that this is why they do what they do.
Ironically, this vastness—as unfit as it may be to our tiny scaled—may be essential for life to emerge and survive. So let us explore it some. I will use the word “scale” often from now on; it means the extent or relative size of something, whether space or time. When it applies to time, I use “timescale.” Let’s deal with space first.
We live in a galaxy—the Milky Way—an “island” of stars and gas swirling in spiral arms around a center. The Universe, as seen through telescopes, is filled with galaxies. The current estimate is that there are at least 200 billion of them.
My night sky exploits as a high school youth included viewing different galaxies. It was a challenge, since most galaxies require dark, clear skies and a sizable telescope. However, there is one galaxy that can be seen with an unaided eye, and, unlike most galaxies, it even has a name: Andromeda. If you live north of the equator, you can try to see it—a faint nebulous smudge in the constellation of Andromeda—rising in the east during the late summer nights, and overhead during the fall and winter evenings. I recommend you try hard because this is—by far—the most distant object a human can ever see unaided, with no help from telescopes or any technology. The Andromeda galaxy is 2.5 million light-years away, about 10,000 times farther than the average stars you see at night.
The Andromeda galaxy is very similar to our own Milky Way. Andromeda is a similar flattened disk of stars and gas, most of them bunched in spiral arms, and it’s roughly the same size. If you are blessed with a dark, clear sky, you will notice that the “smudge” of the Andromeda galaxy appears to be an elongated ellipse. This is because its disk is oriented sideways to us.
Galaxies are mostly close to each other; you could build a scale model of our Milky Wa
y Galaxy neighborhood in your living room. If we were to take our Galaxy to be a dinner plate, then the Andromeda galaxy would be a dinner plate about twelve feet away, and the Triangulum galaxy (another neighboring galaxy, known as M33) would be a salad plate about ten feet away from the Milky Way and a bit to the side of Andromeda. A dozen M&Ms could stand in for the multitude of dwarf satellite galaxies. This is common for our Universe. The galaxies that fill it wall to wall are separated from each other by distances that are comparable to their sizes. You can visualize how this would go in all directions. They are just far enough apart not to bother each other too much.
The picture changes dramatically in the world of the stars, and then again in the world of the planets. You can’t build a scaled model of the Sun’s stellar neighborhood in your living room—the stars are minuscule compared to the distances between them. While for the galaxies, the ratio between size and distance would be about 1:50 to 1:10 (like comparing M&Ms to dinner plates), that same ratio for stars would be 1:100,000,000 and more (like comparing humans to atoms). With planets, it is similarly huge, albeit less so than for stars. Such is the world!
Is this relevant to life?
One answer could be that it is not. These different scales just happened to be what they are, and that’s all. Or perhaps not. Life is a system—a chemical system—that, at least as we know it, seems to work only on small scales. We do not know what life is, but we do know what some of its basic functions are. There is something special about the scale occupied by life to ensure a stable environment that allows such functions to develop. Let us try to understand this by returning to the big picture.
Galaxies in the Universe move with respect to each other with speeds of about 500 kilometers per second. Stars in the galaxy move with similar speeds on their orbits and slightly slower (say, 50 to 200 kilometers per second) with respect to each other. Such speeds are mind-boggling for our everyday experience; for example, a bullet is about 100 times slower.
Here is the problem: such speeds are still minuscule for the distances between galaxies. The Andromeda galaxy is approaching ours at 400 kilometers per second but will require 3 billion years to come close (and may in fact collide with us).2 Not so for stars! At such speeds, if stars had sizes comparable to the distances between them, they would be running into each other all the time—not to speak of the fate of any orbiting planets. Fortunately, stars don’t exist on such scales, so collisions between them are exceedingly rare. Even if the Andromeda galaxy smashes into the Milky Way in 3 billion years, the stars will not collide. Andromeda stars will just glide past Milky Way stars, and then all will mix and merge their orbits around a common new galaxy.
So, on a galactic scale, there is a relative stability, which is important for life. But how much stability is enough? After all, what is stable enough for a microbe might be chaos and doom for a dinosaur.
This issue is similar to the famous question Erwin Schroedinger asked in 1944, Why is life so big compared with an atom?3 I ask the question in reverse: Why is life so tiny compared to a planet? To answer his question Schroedinger first pointed out how the basic units of life are large chemical complexes of atoms—large molecules. Large molecules and chemical reactions between them are at the heart of every process associated with life. They store and release energy, carry information that can be inherited, and assemble into filaments, walls, structures, and more.
Schroedinger also pointed out that the small scale of the atoms—a world described by the rules of quantum mechanics—is ever changing and not strictly predictable (quite chaotic, indeed).4
He should know, being one of the giants of science who helped develop quantum mechanics, demonstrating how very different it is from the classical mechanics developed three centuries earlier by Isaac Newton. Classical mechanics provides the rules for the large scale and large objects—stars and planets, their orbits, bridges, car engines, and so on. Life is large enough to fit in the realm of classical mechanics, and so too are its essential basic units, the large molecules—but only just so.
In answering his question, Schroedinger suggested that the molecules of life and the cells they build are just large enough to avoid the unpredictable and destructive vagaries of the scale of the atoms—the world of quantum physics. At the same time, life benefits from the richness of chemical bonds that is the hallmark of the atomic scale. From my point of view, the complex molecules and chemical networks of life avoid the violent destructiveness of the very large Universe by inhabiting a scale small enough to allow for many stable environments.
So, it seems that the scale inhabited by life has some special qualities. We can observe curious things if we take a swift tour through the space scales of the Universe. The galaxies move slowly like giant turtles, the stars inside them buzz around like bees, the planets orbiting the stars move faster still, and so on until we reach the scale of the microworld—the quantum world of atoms and electrons. The smaller the scale, the crazier the world seems to appear. In fact, it is crazy, and modern physics has a good explanation for it. To oversimplify it a bit, big things move slowly, small things move faster. Just think of the truck and the motorcycle at the stoplight when the light turns green. Remember that mass and speed combine to give you energy, and energy is conserved. If mass goes up, speed must go down. There is order to all this, after all.
There is more to the scale of life, though, than simply the profusion of stable environments. To understand the special qualities that the scale of life has, one needs to know nonliving matter first.
The atomic scale and the atoms are the basic building blocks of ordinary matter, as the ancient Greeks surmised. We still think of this as true, at least for the pure elements of the chemical table, such as carbon, iron, or gold, although we recognize most ordinary matter as being made of compounds of atoms. What the twentieth century revealed, however, is that ordinary matter is really composed of smaller particles. These are called fundamental or elementary particles and fall into three families of four particles (and four antiparticles) each. Most common and familiar among them to us are the light particles of the first family: the electron, the up quark and down quark, and the tiny electron neutrino.
You and I, our planet, our star, are all made up entirely of them, particularly electrons, up quarks, and down quarks. The two types of quarks make the protons (two up plus one down) and neutrons (two down and one up) that combine to form the nucleus of an atom and thus the chemical identity of a given element. The lighter electrons orbit the nucleus and give atoms the ability to bond together into molecules. The nucleus of the atom is where the mass is; the electrons around it are insignificantly light, but they make chemistry possible. When you cook in your kitchen, you are playing with the electrons—breaking and reforming chemical bonds. If eating results in your gaining weight, it is because you have added more quarks to your body.
This is not yet the whole story. We can’t forget about the fundamental forces. One particle can affect another; for example, the positive proton of a hydrogen atom keeps a negative electron in orbit. One piece of matter can influence another piece of matter by means of these forces. There are four fundamental forces—the gravitational force, the electromagnetic force, the strong force, and the weak force. Our daily lives are, for almost all purposes, exclusively affected by the first two forces. Gravity keeps us on the ground (which we experience as weight) and electromagnetism allows us to move around (via friction), among other things. A common feature to all forces is that they can be represented by an associated particle (usually of no mass at rest) which contains the smallest quantum (or “packet”) of force. The electromagnetic force particle is called a photon. We experience photons as light or radiant heat, for example, or we use them to broadcast our cell phone conversations. The gravitational force particle is called a graviton. All these particles have wave properties, as do all elementary particles, including electrons, protons, and neutrons.
Now consider the scales that are much larger—galaxies,
stars, and planetary systems. This is a world governed solely by the force of gravity. There is no friction or any other manifestation of electromagnetic force strong enough to deflect or slow down stars and planets from their paths, and the chemical bonding essential to life doesn’t happen. On the other extreme is the scale that is much smaller than our own—the quantum scale. The force of gravity has no influence here; the individual particles have such little mass that only electromagnetic forces rule. On both the very large and the very small scales, there is no shelter for life. The cosmic scale is awash with radiation and beyond freezing cold, pummeling anything that depends on electromagnetism for cohesion. On the smallest scale, things move so fast and unpredictably that nothing as ordered as life stands a chance. Life on Earth exists at a comfortable scale in between those two—let’s call it the large-molecule scale. Its range starts a few steps above the quantum scale (10—9 m) and ends closer to home (10—5 m).
The large-molecule scale is the true scale of life as we know it. All essential life processes, information-carrying molecules, and most organisms (the microbes) fit nicely in it (see Figure 7.1). The scale of life itself fits nicely on a planet. And never mind the big plants and animals that have outgrown the large-molecule scale—they are a recent development.
What is special about the large-molecule scale? Gravity is still weak, but the mass of large molecules is no longer negligible, so they respond to the force in measurable ways. An important effect is that the water solutions in which such molecules and their structures exist and function are likely to be affected by gravity. Gravity acts as a stabilizing agent at these scales—counteracting electromagnetic forces and providing equilibrium. Planet Earth is a good example of such balance: it is massive enough to shrink and compress under its own gravity. Its rocks, as we’ve seen, are squeezed under immense pressure, packing their oxygen, silicon, and iron atoms close together. The electromagnetic forces between these atoms, being repulsive, put a limit on how compressed gravity can make the matter; thus Earth has been stable in this state for billions of years and will continue to be for a long time to come. The same balance keeps our Sun stable and shining over billions of years too.
The Life of Super-Earths Page 7
