The Life of Super-Earths (14 page)

The best-known and dominant global planetary cycle is the carbonate silicate cycle (see
Figure 10.2
on preceding page). The cycle begins with CO
₂
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 CO
₂
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 CO
₂
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 CO
₂
in the atmosphere would weaken the greenhouse effect and lower the Earth's surface temperature. As the Earth's surface temperature decreased, the CO
₂
content of the atmosphere would rise because there would be less rain. In turn, more CO
₂
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 CO
₂
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 CO
₂
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 CO
₂
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 CO
₂
. Volcanoes will keep replenishing their atmosphere, even without any plate tectonics. In our own Solar System, Venus and Mars have atmospheres dominated by CO
₂
. Our Earth would too, if not for the limestone and oceans that keep much of the CO
₂
locked up. And although life (most in the form of seashells) takes an active part in the CO
₂
cycle today, the cycle would go on happily without it. What the CO
₂
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 CO
₂
gas is removed and the temperature drops; if it drops too low (right panel), CO
₂
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 isotopes
^g
of helium (
⁴
He/
³
He) 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 notwithstanding, 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?

 

 

 

 

I
n 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.