Two factors make super-Earth planets more habitable than a planet the size of our own. First, by being more massive these planets have an easier time keeping their atmospheres and water from evaporating. This is very important for the ones that would orbit closer to their stars than, say, Mars is to the Sun. Second, if they are rocky super-Earths, their tectonic plate activity is as high as the Earth’s or even higher, according to our theoretical models.5 This is important for life and its origins. In our Solar System, Mars never had moving plates, and Venus seems marginally capable of moving its plates. It seems that our Earth barely made it!
Tectonic plate activity is what we observe as continental drift on Earth. Modern GPS technology allows us to measure this motion. But why is it beneficial to life? The answer, in a nutshell, is stability and chemical concentration.6 Over billions of years the Earth has kept its surface temperature stable. The oceans, for example, have generally always been liquid. We know this from geological evidence. We also know that our Sun has brightened up by 30 percent since the Earth formed. The solution to this seeming mystery appears to be a global geochemical cycle. The Earth is a large ball that is very hot and boiling inside. On the surface this energy moves the set of rocky continental and ocean plates around. The essential thing about planetary plate tectonics is exchange. The molten, mixed interior can exchange chemical elements with the surface and atmosphere and vice versa. The elements are not simply recycled in this process, but their chemical transformations and concentrations exchange energy in a rich dynamic equilibrium. The alternative is a much poorer steady-state equilibrium that will set in on the inside and at the surface with no local energy sources. Thus plate tectonics makes a planet dynamic, renewing, and vital. As Ward and Brownlee put it, plate tectonics promotes environmental complexity.7 The exchange occurs through global cycles.
FIGURE 10.2. The CO cycle (a.k.a. carbonate silicate cycle) starts with the volcanoes on the left that provide the supply of CO2 gas, which gets absorbed by water in the atmosphere, rains down, erodes the continents, and ends up recycled back into the hot interior via plate tectonics, only to return again.
The best-known and dominant global planetary cycle is the carbonate silicate cycle (see Figure 10.2 on preceding page). The cycle begins with CO2 released by volcanoes into the atmosphere, where it is easily absorbed by water droplets. Raining down on the surface, the carbonated water helps erode rocks and soil into the oceans, where it gets deposited in carbon-rich rocks such as limestone. Tectonic plate activity brings the carbon back under the Earth’s crust, only to be made molten, mixed, and recycled back into the atmosphere through volcanoes.8 As we saw in the last chapter, this cycle is important to the existence of our planet’s atmosphere, but the cycle does more: it acts like a thermostat, because CO2 is a greenhouse gas and its cycle has a built-in feedback loop that returns Earth’s temperature to a normal average (see Figure 10.3 on page 128). A greenhouse gas lets sunlight heat the surface and then helps keep that heat like a blanket. Water vapor and methane gas are other common greenhouse gases. If the Earth’s surface temperature increased slightly, the CO2 content of the atmosphere would decrease because the gas would dissolve in plentiful water due to increased evaporation, and rain would carry it to the surface to speed up rock erosion and ocean deposition. This reduced quantity of CO2 in the atmosphere would weaken the greenhouse effect and lower the Earth’s surface temperature. As the Earth’s surface temperature decreased, the CO2 content of the atmosphere would rise because there would be less rain. In turn, more CO2 would strengthen greenhouse warming and bring the Earth’s temperature back to normal. A perfect thermostat!
Well, perfect may be too strong, given the cycles of ice ages that the Earth has endured. The CO2 cycle thermostat has a very long time delay (about 400,000 years).9 However, ice ages are minor inconveniences in the history of life. Even humans survived the last one 10,000 years ago, and smaller organisms, like subterranean microbes, wouldn’t have noticed what was going on. The CO2 cycle has protected our planet from much more serious trouble over billions of years. It will continue to do so as the Sun gets brighter in the next 2–3 billion years. If solar heat continues to rise, a tipping point will come (Venus crossed its own sometime in the past), after which the thermostat will break down.
To the skeptical reader, the CO2 cycle thermostat might seem like a very special feature of our planet Earth. Not so. Carbon and oxygen are common elements in the Universe, so planets that formed around most stars in our galaxy will have plenty of CO2. Volcanoes will keep replenishing their atmosphere, even without any plate tectonics. In our own Solar System, Venus and Mars have atmospheres dominated by CO2. Our Earth would too, if not for the limestone and oceans that keep much of the CO2 locked up. And although life (most in the form of seashells) takes an active part in the CO2 cycle today, the cycle would go on happily without it. What the CO2 cycle thermostat needs is liquid water and tectonic plate activity. Mars and Venus seem too small to have kept their water from escaping and the tectonic activity going. Our Earth barely did! Super-Earths would have an easier time keeping both, and therefore provide long-term stable environments.
FIGURE 10.3. The CO cycle functions like a thermostat for the climate on Earth and on Earth-like planets. If the temperature gets too hot (left panel), more greenhouse CO2 gas is removed and the temperature drops; if it drops too low (right panel), CO2 accumulates and warms up the planet.
Corroborating the view that Earth is a planet that just barely supports active plate tectonics is research that shows long periods of slowdown or downright stagnation during its geological history. Certain elements have been depleted from Earth’s mantle, which can be taken as a proxy for plate tectonic activity.10 As the Earth ages and cools, its interior gradually loses certain elements through the cycles of outgasing and subduction. Silver and Behn compared ratios of niobium to thorium and of the two isotopesg of helium (4He/3He) in Earth’s interior, and found a cyclic (instead of a gradual) change since the formation of the Earth. Their interpretation is that plate tectonic activity on our planet slows down and even ceases occasionally, then picks up again. The cycle seems to have occurred at least a couple of times in the past 3 to 4 billion years.
In my work with Diana Valencia and Rick O’Connell we investigated whether plate tectonic activity on super-Earths would be higher, lower, or the same as that on Earth. The answer was not obvious—nothing seems to be simple when it comes to plate tectonics. On the one hand, we reasoned that a super-Earth is hotter inside (over many billions of years) because it is bigger. Higher temperatures inside mean more “boiling,” more motion and energy in the mantle, which would lead to more pressure, pushing, and stress applied to the crust from below. Naturally, that leads to breakage as the pieces are pushed around, up and down, leading to the subduction of heavier ones under lighter and less dense ones. Unfortunately, there is another effect we need to consider. The viscosity of the flowing mantle depends strongly on the temperature—just as honey, when you heat it, flows and slips more easily. The problem is this: though super-Earths are hotter and the mantle boils faster, the hotter mantle is also less viscous and slips by the solid crust more easily, without pushing it along. As scientists would say, the two effects seem to cancel each other out. When we looked more carefully at what was going on, we found a third effect that made all the difference. The higher temperature was keeping the super-Earths from growing a thick crust. Finally we had a very robust result for all super-Earths—decreasing crust thickness and increasing mantle flow pressure collaborate to produce a vigorous and “healthy” plate tectonic activity.11
Comparing Earth to the theoretical models of super-Earths of different sizes, we find a rich diversity of stable Earth-like planetary conditions. In fact, we find a family of planets that barely includes the Earth, which just qualifies by means of mass, tectonic activity, or long-term temperature stability. Being smaller, Earth is more vulnerable to any number of cosmic accidents. Our biases for our home planet not
withstanding, in this family, Earth is certainly not the preferred child! This is bad for Earthly chauvinism, I suppose, but it’s great news for life in the Universe—there seem to be plentiful good places for it.
The question before us now is, What is the current planet census? How big could the family of life be?
CHAPTER ELEVEN
THE PASSAGE OF TIME
The Universe Is Young, Life Is Younger
In our story, understanding the passage of time is essential to understanding life in the context of the cosmos.
Next to the building in which I work stands the Harvard College Observatory, built in 1839. It sits atop a hill above Harvard Yard, although you can barely notice the hill. Tall trees and buildings have grown up all around. Much has changed over the years due to population growth, building, landscaping, and so on. And yet something, it seems, has not: the ancient granite rocks of the hill.
Except we know that they have. Tectonic activity, which we on this planet are lucky to have, has actually moved the rocks west by 3.2 meters (the length of my car) since the observatory was built, along with the rest of Massachusetts and the entire North American landmass.
At that rate, the continents would be rearranged completely in about 200 million years. Furthermore, we can confirm this movement with independent evidence from the study of layers of rocks around the world. It paints an ever changing geographical map of planet Earth with cycles of continental rearrangements.
When I am faced with a seemingly unchanging fact—for example, the geology of the Earth’s mountains—I try to transform myself mentally into a being that lives to be a billion years old. One heartbeat is now 1,000 years, and 80,000 years would seem like a minute. I am hovering above the Earth among the weather satellites in a geostationary orbit. What does the Earth look like to me? As I watch, the continents are completely rearranged—large ones breaking apart, flowing away from each other, colliding gently with each other, merging again; mountains forming, folding, and being eroded in the process. Through my new eyes the home planet is no longer solid and unchanging but as flowing and dynamic as a pot boiling on a stove.
Such a perspective on the passage of time is less fantastic than it might seem to you. Just think of the moth that lives for one day. In its eyes the green meadow and the lush forest must be eternal. When it comes to the cycles of planet Earth, we humans, like that moth, can perceive the wind rustling in the leaves but not the passing of the seasons. Life on Earth has existed for about 4 billion years and has followed the geological “seasons” and planetary transformations.
If we think about the history of life, not on our timescale but on the timescale of life itself, important facts become obvious. For example, life has existed and developed on Earth for a time—about 4 billion years—that is comparable to the age of the Universe—about 14 billion years. This is a very significant fact, what scientists call a nontrivial fact. It tells us that the emergence and development of life is a process on a par with processes of formation and development of planets, stars, and even galaxies. In a certain sense, it makes life appear more like a “normal” cosmic process.
In contrast, we could not say the same about humankind because the timescales are not similar. The oldest known human remains (genus Homo) are about 2 million years old. What makes humans different—the appearance of language and technology—is much more recent, both having probably emerged just 40,000 years ago.
The disparity between those two timescales—the human and the cosmic—is great. The ratio between the history of life on Earth and the history of modern humans, for example, is 100,000:1. This is a very significant fact as well. It can tell us one of two things: (1) Either the very brief process (development of human society) that emerged from the very long process (development of Earth life) is not comparable to the process of planet development; or (2) we are very lucky to be at the very beginning of a new but short-lived process. The latter case also means that we have little predictive ability.
Predictive ability is important in science because in most cases it means a solid understanding of what is going on. Because we understand the laws of gravity, we are able to predict the future position of the Moon in its orbit and then launch a rocket and land on it precisely as planned. Option 2 is an unfortunate caveat—comparing timescales clearly has its limits.
On the other hand, option 1 is tantalizing! It tells us that a planetary process (life itself) was necessary to develop a life-form (humans) capable of transcending the planetary timescale. It tells us that life might be a cosmic phenomenon that develops on planets over planetary timescales but leads to forms that are no longer coevolving with the planets—that become independent of the timescale and create their own, along which they evolve, or at least change their environments. However, it takes the process a long time to build up to that level. Option 2 suggests that humanlike life is part of the planetary timescale and consequently represents its final phase but may not last long enough to fulfill its cosmic potential.
At least the potential does exist—humans demonstrated it by going as far as the Moon. A colleague of mine at the California Institute of Technology likes to say that if a process is allowed by the laws of physics, then somewhere in the Universe that process is happening. All kidding aside, there is a deep truth in how we judge potential and plausibility. To me, the fact of our existence on planet Earth today, even with merely the potential to transcend the planet’s existence, already implies that this can happen somewhere (and sometimes!) in the Universe, even if we fail to survive. Whether that transcendence can happen depends on whether our Universe has the potential for future life. Are life’s origins at their peak rate, or in their decline? Or, perhaps, just getting started?
Scientists do not know how to answer this question yet, but now they know something about the development of environments hospitable to life. Considering life as a planetary process enables them to make new predictions about its future.
The hierarchy of timescales that involve life is very interesting, and they intertwine with the spatial scale of life. The large molecule scale, as we saw, is on average 10—7 meters, and the chemical reaction of replicating one unit of DNA lasts about 10—3 seconds.1 This is slow compared to what can be accomplished by atoms in the tiny volume defined by 10—7 meters. However, it is extremely fast compared to any planetary process. This means that life as a process (or sum of processes) will have time to adapt to, coopt, or simply survive whatever it is that happens on the longer, geological timescales, such as the ups and downs of global temperature or the rearrangement of continents. But a chemical reaction is a chemical reaction, and a planetary geochemical process is most often simply the sum of the myriad chemical reactions underlying it. So all those individual geochemical reactions would appear to have similar (short) timescales to the individual biochemical ones that make up the processes of life.
How, then, could processes of life rise above the destructive chemistry of the planetary environment? If it takes the same time to do them, then ordinary chemistry wins out. For life and its biochemistry to prevail, their timescales should be shorter. Life on Earth is competitive that way by using two tricks. One is getting help from special molecules (catalysts, typically called enzymes) to speed up reactions; the other is by keeping tabs on how it does that. In other words, biochemistry has neatly ordered sequences of reactions that do something very well and fast (like storing and releasing energy, forming an enclosure, etc.). In addition, a special molecule keeps tabs on the ordered sequences so they don’t have to be reinvented each time. We know such molecules—we all have them and we call them DNA and RNA.
The genetic molecules of Earth life, DNA and RNA, are unique and common to all of Earth biochemistry. Their complexity is the result of a long process of evolution. Interestingly, a molecule can have a structure that encodes a sequence, and that code can be copied and inherited. Equally interesting is that RNA—as has been shown on multiple occasions in laboratories—is capable of catalyzi
ng its own replication. The result is a dramatic shortening of the biochemical timescales—fast rates that can rise above the destructive chemistry of the planetary environment.
Most objects in the Universe that retain their identity over long periods of time are either very big (such as galaxies) or very stable (such as stars and planets). Our Sun will be a star to a venerable age of 10 billion years by being very thrifty in how it spends its energy; it is very stable indeed. But life, thanks to these two tricks it has, presents a third way. It endures thanks to its individual, short-lived, and localized units—organisms that have the flexibility to adapt by doing chemistry faster than the changing environment—that are nonetheless balanced by longer-lived and global entities such as entire populations or species. These larger groups are flexible and allow various members to try different things to survive. That is an extraordinarily smart invention! For all we know, thanks to this, life may be a cosmic phenomenon that, once it has emerged, can continue for an indefinite time.
The Life of Super-Earths Page 10
