The Life of Super-Earths (12 page)

Speaking of chemicals, there is a third important environmental condition—life seems to need environments allowing chemical concentration. Most environments in the Universe are dilute. Relatively complex molecules are observed to form
in molecular clouds, but their concentrations are always extremely dilute. This severely limits the complexity in molecules and their reaction networks. Gas giant planets like Jupiter or Neptune are another example of chemically dilute places. Their cloudy atmospheres and bright display of colors are all there is—no solid or liquid surface. As you plunge deeper below the clouds, the gas gets denser and hotter, and all but the smallest molecules are destroyed. Life on Earth uses enclosures such as cells and vesicles within them to further concentrate chemicals in useful places; maintaining this kind of disequilibrium, of course, requires energy.

The combination of relatively low temperatures, steady sources of energy, and access to chemical concentration leaves us with planets as the best, if not the only, places where the molecules of life and their interrelated reactions can emerge and sustain themselves. No other type of object has the complete set of conditions. And then only some planets—gas giants won't do. Terrestrial planets are unique in providing a range of rich chemical concentrations, energy sources, and sheltered environments. This is a profound realization!

H
alf a day's sail northwest of the Cape of Good Hope lies Cape Town, South Africa. Even today, the city feels like an outpost, a cozy yet uneasy harbor at the edge of the world. After all, there is, to human eyes, nothing south of there but cold and ice, a frigid southern ocean encircling the least hospitable landmass on Earth.

There, in the shadow of exotic Table Mountain, in November 1873 an unusually outfitted British ship, HMS
Challenger,
was being readied for a grueling journey to the icy shores of Antarctica, then on to Australia and around the world. Forty years after the famous trip of the HMS
Beagle
with Charles Darwin onboard, HMS
Challenger
was on a four-year voyage to explore a new frontier—the depths of the world oceans.
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HMS
Challenger
had a complement of six scientists in
its crew and a deck loaded with curious instruments: for measuring oceanic temperatures, depths, and currents; for taking deep-ocean samples; and for capturing living specimens of whatever creatures might call the strange place home.
²

The results astounded laypeople and scientists alike. From the total darkness and tremendous pressure found eight or more kilometers below the surface, HMS
Challenger
collected 600 cases of specimens. The rich harvest proved that the vast underwater landscape of the oceans is not a desert. Life was everywhere on the surface of our planet.

More recently, research in the last twenty years has led to two more astonishing realizations. First, rather than simply providing a home for life, planet Earth has been thoroughly transformed by life, which has accompanied it almost since its formation 4.5 billion years ago. What's more, as fragile as life may appear to be, clinging to the surface of a small planet that is subject to violent cosmic events, what we have learned has led us to conclude that life on Earth is virtually indestructible. The evidence suggests that it has been that way for a very long time, perhaps 3 billion years.
³
Destroying all living things on Earth, including spores and complex biomolecules, would take nothing short of melting and vaporizing the planet in the sterile interior of the Sun. Destroying life would also require annihilating all the spacecraft in Earth orbit, as well as the other places we've sent space probes. Though unlikely, the possibility that we've colonized Mars with some microbes
,
for example, is not that far-fetched; many extremely resilient microbes have been discovered in the past thirty years, and
some—such as
Deinococcus radiodurans
—are hardy enough to survive the trip to Mars.
⁴
Real space travelers!

The secret behind the indestructibility of the Earth biosphere lies in the sheer diversity and inventiveness of the organisms that have always ruled “our” planet—the microbes. The hardiest among them are called extremophiles, meaning that they inhabit extreme environments. Some are able to withstand 250°F (122°C) in hot springs and in ocean floor “black smokers”—hot volcanic vents. Others survive high pressure levels, for example, in the sterilizing high pressure vats for orange juice
⁵
or in the natural habitat of the Marianas trench in the Pacific Ocean at 800 atmospheres.
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Yet others have made their home in the tiny cracks of rocks four kilometers underground, discovered as history has gone through another cycle, and a flotilla of twenty-first-century ships, mimicking the
Challenger
before them, has drilled into the ocean floor.
⁷

Scientists have probably not yet reached the bottom of the Earth's biosphere. Microbes that live deep within the Earth's crust are often dubbed SLiME communities, for subsurface lithoauthotrophic microbial ecosystem. They depend on nothing from the surface. The heat comes from the depths of the planet, the chemicals and water are already there, and sunlight is not needed. Though extreme, their environment is very protected.

These are not anomalous creatures, either. The “bottom” line is that life on Earth appears indestructible today because this subsurface environment hosts a large fraction of the
planet's total life. Some scientists, such as the late Thomas Gold at Cornell University, have argued that indeed most of the biomass on planet Earth is below the surface. Recent estimates are up to 300 billion tons of carbon biomass, which is comparable to the entire continental surface biomass, which is mostly plant life.
⁸
Most of this deep biosphere consists of microbial communities living in rocks and sediments roughly 500 to 1,000 meters below the ocean floor; a case at 1,600 meters below the Atlantic seabed is the current record holder.
⁹
With the ocean floor covering more than 70 percent of our planet, and a measured million cells in every cubic centimeter of subfloor sediment, this would make for more than half of all microbial cells on Earth.
¹⁰

Most recently, deep biosphere hunters discovered the first nonmicrobial life-form from a 1- to 3.5-kilometer depth in South Africa—a tiny worm that feeds on subsurface bacteria.
¹¹
This emphasizes the richness of life in the deep crust of the Earth.

How can microbes survive in miles of rock without sunlight or oxygen, and having scarce nutrients and water? The drill samples show a predominance of microbes that are resilient to stress and especially skillful in conserving energy by growing (doubling) extremely slowly—on timescales of centuries! If they had any cares, they would not be about us, the surface dwellers, and yet it is clear that they descended, albeit hundreds of millions of years ago, from ocean floor and surface dwellers. This is revealed in their genome maps. They are not that extraordinary, after all.

The history of life on Earth shows rapid adaptation and colonization of any place where there is water, regardless of extremes in temperature, pressure, and acidity. The deep water cycle—the water from the surface that reaches deep into the crust and below the oceans—has brought life along with it, probably as soon as life existed on this planet.

What dangers there are to life mostly come from outside Earth. The most dramatic threats are cosmic: asteroid and comet collisions, as well as major atmospheric change, including the loss of the entire atmosphere. Dramatic, yes, but not necessarily a death blow to life on the planet.

Asteroid impacts are a part of life in any planetary system. Asteroids, the mile-size (sometimes many miles) fragments left over from the accumulation of the rocky planets, have orbits that are prone to be influenced by the big planets. Many of them, as a result, have been “swept up” by larger planets in the Solar System, including Jupiter, but many remain, especially in a large belt that exists between Mars and Jupiter. Over long periods of time, the gravitational influence of the planets is enough to put the asteroids on a collision course with a planet or another asteroid.

An impact by a two kilometer asteroid would be a catastrophe for humankind, but most of the microbes in the deep biosphere would not even notice the event. It would take an impacting body almost the size of Mercury to destroy Earth's crust and oceans and perhaps sterilize all colonies of microorganisms that are miles below the surface. However, in our Solar System, at the start of the twenty-first century,
astronomers have a pretty complete census of asteroids crossing Earth's orbit. We know all bodies larger than two kilometers that could hit us. Astronomers know of no such planet threatening to impact the Earth.

Collisions between asteroids and planets would have been very common during the period of planet formation and about 500 million years after. We know this from observing other solar systems. Were large collisions more common, the amount of small particles lingering among the planetary orbits would be more than enough to notice in our remote census of known nearby planetary systems. Such “debris disks,” as they are called, are well-known and easy to detect with modern infrared technology. The Spitzer space telescope, an infrared cousin to the Hubble space telescope, has provided evidence that our Solar System is quite typical in that respect.

What about catastrophic climate change—the total loss of the Earth's atmosphere and the loss or freezing over of any remaining oceans. This could occur due to a massive impact by an asteroid comparable to the Moon or a nearby stellar explosion: a supernova or a gamma ray burst.

Gamma rays are the most energetic electromagnetic waves. Gamma ray bursts are among the most violent events we know and they occur infrequently in any given galaxy. Nevertheless, astronomers detect them often—once a day. This is because of their sheer brightness and the penetration of gamma rays. We are able to see a burst across the entire visible Universe. They emanate from the final explosions of
very weighty stars (a special case of a supernova explosion) and the spiraling-in of two neutron stars. Nothing rivals the power of the explosion that brings about a gamma ray burst. However, such explosions are both rare and far apart. At the typical rate and average distance, the only effect we should worry about on Earth is ozone layer loss. In the unlikely event that one occurred within fifty light-years of the Solar System, however, we would be in trouble. The Earth's atmosphere would be completely lost and all life on the surface would become extinct.
¹²
But not life inside the crust.

A sudden loss of the entire atmosphere would likely deprive the Earth of an atmosphere just temporarily, on geological timescales. Because the internal structure of the Earth is not going to change much at all, the basic plate tectonics and the release of gas from the planet's interior through volcanic activity would continue. Carbon dioxide from volcanoes would replenish the atmosphere, which, because carbon dioxide is a greenhouse gas, would melt the frozen oceans (or whatever was left of them). Even if they melted partially, the evaporation of water into the atmosphere, followed by rain and erosion, would restart the carbonate silicate cycle.

The carbonate silicate cycle is almost identical to what is commonly called the inorganic carbon cycle, or the carbon dioxide cycle. It is a fundamental planetary cycle of the abundant gas carbon dioxide as it rises from the Earth's interior, undergoes transformations in the atmosphere, on land, and in the oceans, and returns back inside at the end of it. The carbonate silicate cycle has a typical timescale—a typical time
for changes to take hold. For planet Earth this timescale is about 400,000 years. So, should a gamma ray burst destroy Earth's atmosphere, it might take “just” a few million years for it to return and stabilize, perhaps less. Any microbes that survived deep in the crust—and there should be many—would have ample opportunity to recolonize the Earth's surface. For example, microbial communities discovered in deep drilling in Texas appear to have been cut off from the surface 80 million years ago, much longer than the million years needed to recolonize.

Of course, if such a calamity were to happen, the Earth's atmosphere would be changed dramatically: its two main constituent gases today, nitrogen and oxygen, would be gone and could not be replenished by volcanoes and evaporation from the oceans. Of course, this wouldn't be a big problem for any subterranean microorganisms remigrating to the surface; they have no need for oxygen gas in their present location, and would do just fine on the “new old Earth,” as long as some access to sources of nitrogen remain in the crust. They might even put those gases back, as they are byproducts of microbial life, if given another billion years—as they already did on Earth about 2 billion years ago.
¹³

It is humans and complex life forms that live a precarious existence subject to the vagaries of cosmic change. Earth life, as represented by its most numerous and ancient forms—the microbes—is permanently entrenched on our little planet, at least until the Sun retires and engulfs it. Think about what would happen if the Earth's orbit became unstable and a near
collision with Jupiter were to fling Earth out of the Solar System.
¹⁴
Sounds like the end of days, literally, as darkness and deep freeze would cover the surface. However, hydrothermal activity—those same black smokers in the middle of the Atlantic Ocean—would continue without interruption. Much of life that calls black smokers home would survive, and for quite some time—the crust makes an excellent blanket, trapping the remnant heat from Earth's formation, as well as the heat emitted by radioactive decay of elements like uranium, potassium 40, and thorium. In fact, the rate of loss of internal heat on Earth today is measured to be 87 milliwatts per square meter.
¹⁵
This is nearly a thousand times weaker than the rate at which a household lightbulb uses energy, and you would have to collect all the internal heat from an area larger than a college classroom to light up just a feeble 25-watt lightbulb. This seems like a drop in the ocean for our energy-hungry twenty-first-century human society, but is entirely sufficient for microbes that live deep in the crust and near hydrothermal vents at the bottom of the ocean. At its present rate of cooling, Earth, even lost in space, could keep its hydrothermal habitats alive for at least 5 billion years.
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