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 January 12, 2017
Why Most Planets Will Either Be Lush or Dead

 Can a planet be alive? Lynn Margulis, a giant of late 20th-century biology, who had an incandescent intellect that veered toward the unorthodox, thought so. She and chemist James Lovelock together theorized that life must be a planet-altering phenomenon and the distinction between the "living" and "nonliving" parts of Earth is not as clear-cut as we think. Many members of the scientific community derided their theory, called the Gaia hypothesis, as pseudoscience, and questioned their scientific integrity. But now Margulis and Lovelock may have their revenge. Recent scientific discoveries are giving us reason to take this hypothesis more seriously. At its core is an insight about the relationship between planets and life that has changed our understanding of both, and is shaping how we look for life on other worlds.

Studying Earth's global biosphere together, Margulis and Lovelock realized that it has some of the properties of a life form. It seems to display "homeostasis," or self-regulation. Many of Earth's life-sustaining qualities exhibit remarkable stability. The temperature range of the climate; the oxygen content of the atmosphere; the pH, chemistry, and salinity of the ocean---all these are biologically mediated. All have, for hundreds of millions of years, stayed within a range where life can thrive. Lovelock and Margulis surmised that the totality of life is interacting with its environments in ways that regulate these global qualities. They recognized that Earth is, in a sense, a living organism. Lovelock named this creature Gaia.

Margulis and Lovelock showed that the Darwinian picture of biological evolution is incomplete. Darwin identified the mechanism by which life adapts due to changes in the environment, and thus allowed us to see that all life on Earth is a continuum, a proliferation, a genetic diaspora from a common root. In the Darwinian view, Earth was essentially a stage with a series of changing backdrops to which life had to adjust. Yet, what or who was changing the sets? Margulis and Lovelock proposed that the drama of life does not unfold on the stage of a dead Earth, but that, rather, the stage itself is animated, part of a larger living entity, Gaia, composed of the biosphere together with the "nonliving" components that shape, respond to, and cycle through the biota of Earth. Yes, life adapts to environmental change, shaping itself through natural selection. Yet life also pushes back and changes the environment, alters the planet. This is now as obvious as the air you are breathing, which has been oxygenated by life. So evolution is not a series of adaptations to inanimate events, but a system of feedbacks, an exchange. Life has not simply molded itself to the shifting contours of a dynamic Earth. Rather, life and Earth have shaped each other as they've co-evolved. When you start looking at the planet in this way, then you see coral reefs, limestone cliffs, deltas, bogs, and islands of bat guano as parts of this larger animated entity. You realize that the entire skin of Earth, and its depths as well, are indeed alive.

The acceptance of the Gaia hypothesis was, and remains, slow, halting, and incomplete. There are several reasons for this. One is just the usual inertia, the standard conservative reluctance to accept new ways of thinking. Yet Gaia was also accused of being vague and shifting. Some complained that the "Gaians" had failed to present an original, well defined, testable scientific proposition. How can you evaluate, oppose, or embrace an idea that is not clearly stated, or that seems to mean different things to different people? There was certainly some truth to this. Gaia has been stated many different ways. Also, it didn't help that Margulis and Lovelock were more than willing to mix science with philosophy and poetry, and they didn't mind controversy; in fact, I'd say they enjoyed and courted it.

The truth is, despite its widespread moniker, Gaia is not really a hypothesis. It's a perspective, an approach from within which to pursue the science of life on a planet, a living planet, which is not the same as a planet with life on it---that's really the point, simple but profound. Because life is not a minor afterthought on an already functioning Earth, but an integral part of the planet's evolution and behavior. Over the last few decades, the Gaians have pretty much won the battle. The opposition never actually surrendered or admitted defeat, but mainstream earth science has dropped its disciplinary shields and joined forces with chemistry, climatology, theoretical biology, and several other "ologies" and renamed itself "earth system science."

The Gaia approach, prompted by the space-age comparison of Earth with its apparently lifeless neighbors, has led to a deepening realization of how thoroughly altered our planet is by its inhabitants. When we compare the life story of Earth to that of its siblings, we see that very early on in its development, as soon as the sterilizing impact rain subsided so that life could get a toehold, Earth started down a different path. Ever since that juncture, life and Earth have been co-evolving in a continuing dance.

As we've studied Earth with space-age tools, seen her whole from a distance, drilled the depths of the ocean floor, and, with the magic glasses of multispectral imaging, mapped the global biogeochemical cycles of elements, nutrients, and energy, we've learned that life's influence is more profound and pervasive than we ever suspected.

All this oxygen we take for granted is the byproduct of life intervening in our planet's geochemical cycles: harvesting solar energy to split water molecules, keeping the hydrogen atoms and reacting them with CO2 to make organic food and body parts, but spitting the oxygen back out. In Earth's upper atmosphere some of this oxygen, under the influence of ultraviolet light, is transformed into ozone, O3, which shields Earth's surface from deadly ultraviolet, making the land surface habitable. When it appeared, this shield allowed life to leave the ocean and the continents to become green with forests. That's right: It was life that rendered the once deadly continents habitable for life.

The more we look through a Gaian lens, the more we see that nearly every aspect of our planet has been biologically distorted beyond recognition. Earth's rocks contain more than 4,000 different minerals (the crystalline molecules that make up rocks). This is a much more varied smorgasbord of mineral types than we have seen on any other world. Geochemists studying the mineral history of Earth have concluded that by far the majority of these would not exist without the presence of life on our planet. So, on Earth's life altered surface, the very rocks themselves are biological byproducts. A big leap in this mineral diversity occurred after life oxygenated Earth's atmosphere, leading to a plethora of new oxidized minerals that sprinkled colorful rocks throughout Earth's sediments. Observed on a distant planet, such vast and varied mineral diversity could be a sign of a living world, so this is a potential biosignature (or Gaiasignature) we can add to the more commonly cited Lovelock criterion of searching for atmospheric gases that have been knocked out of equilibrium by life. In fact, minerals and life seem to have fed off each other going all the way back to the beginning. Evidence has increased that minerals were vital catalysts and physical substrates for the origin of life on Earth. Is it really a huge leap, then, to regard the mineral surface of Earth as part of a global living system, part of the body of Gaia?

What about plate tectonics and the dynamics of Earth's deep interior? At first glance this seems like a giant mechanical system---a heat engine---that does not depend upon biology, but rather (lucky for life), supports it. Also, although we're probably still largely ignorant about the deeply buried parts of Earth's biosphere, it's unlikely there are any living organisms deeper than a couple of miles down in the crust, where it gets too hot for organic molecules. Yet, just as we've found that life's sway has extended into the upper atmosphere, creating the ozone layer that allowed the biosphere to envelop the continents, more and more we see that life has also influenced these deeper subterranean realms. Over its long life, Gaia has altered not just the skin but also the guts of Earth, pulling carbon from the mantle and piling it on the surface in sedimentary rocks, and sequestering massive amounts of nitrogen from the air into ammonia stored inside the crystals of mantle rocks.

By controlling the chemical state of the atmosphere, life has also altered the rocks it comes into contact with, and so oxygenated the crust and mantle of Earth. This changes the material properties of the rocks, how they bend and break, squish, fold, and melt under various forces and conditions. All the clay minerals produced by Earth's biosphere soften Earth's crust---the crust of a lifeless planet is harder---helping to lubricate the plate tectonic engine. The wetness of Earth seems to explain why plate tectonics has persisted on Earth and not on its dry twin, Venus. One of the more extreme claims of the Gaia camp, at present neither proven nor refuted, is that the influence of life over the eons has helped Earth hold on to her life giving water, while Venus and Mars, lifeless through most of their existence, lost theirs. If so, then life may indeed be responsible for Earth's plate tectonics. One of the original architects of plate tectonic theory, Norm Sleep from Stanford, has become thoroughly convinced that life is deeply implicated in the overall physical dynamics of Earth, including the "nonliving" interior domain. In describing the cumulative, long-term influence of life on geology, continent building, and plate tectonics, he wrote, "The net effect is Gaian. That is, life has modified Earth to its advantage." The more we study Earth, the more we see this. Life has got Earth in its clutches. Earth is a biologically modulated planet through and through. In a nontrivial way, it is a living planet.

Now, 40 years after Viking landed on Mars, we've learned that planets are common, including those similar in size to Earth and at the right distance from their stars to allow oceans of liquid water. Also, Lovelock's radical idea to pay attention to the atmosphere and look for drastic departures from the expected mixture of gases now forms the cornerstone of our life detection strategies. Gaian thinking has crept into our ideas about evolution and the habitability of exoplanets, revising notions of the "habitable zone." We're realizing that it is not enough to determine basic physical properties of a planet, its size and distance from a star, in order to determine its habitability. Life itself, once it gets started, can make or keep a planet habitable. Perhaps, in some instances, life can also destroy the habitability of a planet, as it almost did on Earth during the Great Oxygenation Event (sometimes called the oxygen catastrophe) of 2.1 billion years ago. As my colleague Colin Goldblatt, a sharp young climate modeler from the University of Victoria, once said, "The defining characteristic of Earth is planetary scale life. Earth teaches us that habitability and inhabitance are inseparable."

In my 2003 book Lonely Planets, I described what I call the "Living Worlds hypothesis," which is Gaian thinking applied to astrobiology. Perhaps life everywhere is intrinsically a planetary scale phenomenon with a cosmological life span---that is, a life expectancy measured in billions of years, the timescale that defines the lives of planets, stars, and the universe.

Organisms and species do not have cosmological life spans. Gaia does, and this is perhaps a general property of living worlds. Influenced greatly by Lovelock and Margulis, I've argued that we are unlikely to find surface life on a planet that has not severely and flagrantly altered its own atmosphere. According to this idea, a planet cannot be "slightly alive" any more than a person can (at least not for long), and an aged planet such as Mars, if it is not obviously, conspicuously alive like Earth, is probably completely dead. If the little whiffs of methane recently reported by the Curiosity rover turn out to be the signs of pockets of Martian life on an otherwise generally dead world, this would prove that my Living Worlds hypothesis is wrong, and that life can take on very non-Gaia-like forms elsewhere. But a living world may require more than temporary little pockets of water and energy as surely exist underground on Mars. It may require continuous and vigorous internally driven geological activity. I believe that only a planet that is "alive" in the geological sense is likely to be "alive" in the biological sense. Without plate tectonics, without deep, robust global biogeochemical cycles which life could feed off and, eventually, entrain itself within, life may never have been able to establish itself as a permanent feature of Mars, as it did on Earth.

As far as we can tell, around the time when life was starting on Earth, both Venus and Mars shared the same characteristics that enabled life to get going here: They were wet, they were rocky, they had thick atmospheres and vigorous geologic activity. Comparative planetology seems to be telling us that the conditions needed for the origin of life might be the norm for rocky worlds. One real possibility is that Mars or Venus also had an origin of life, but that life did not stick, couldn't persist, on either of these worlds. It was not able to take root and become embedded as a permanent planetary feature, as it did on Earth. This may be a common outcome: planets that have an origin of life, perhaps even several, but that never develop a robust and self sustaining global biosphere. What is really rare and unusual about Earth is that beneficial conditions for life have persisted over billions of years. This may have been more than luck.

When we stop thinking of planets as merely objects or places where living beings may or may not be present, but rather as themselves living or nonliving entities, it can color the way we think about the origin of life. Perhaps life is something that happens not on a planet but to a planet: It is something that a planet becomes.

Think of life as analogous to a fire. If you've ever tried to start a campfire, you know it's easy to ignite some sparks and a little flicker of flame, but then it's hard to keep these initial flames going. At first you have to tend to the fire, blowing until you're faint, to supply more oxygen, or it will just die out. That's always the tricky part: keeping it burning before it has really caught on. Then it reaches a critical point, where the fire is really roaring. It's got a bed of hot coals and its heat is generating its own circulation pattern, sucking in oxygen, fanning its own flames. At that point it becomes self-sustaining, and you can go grab a beer and watch for shooting stars.

I wonder if the first life on a planet isn't like those first sparks and those unsteady little flames. The earliest stages of life may be extremely vulnerable, and there may be a point where, once life becomes a planetary phenomenon, enmeshed in the global flows that support and fuel it, it feeds back on itself and becomes more like a self sustaining fire, one that not only draws in its own air supply, but turns itself over and replenishes its own fuel. A mature biosphere seems to create the conditions for life to continue and flourish.

A "living worlds" perspective implies that after billions of years, life will either be absent from a planet or, as on Earth, have thoroughly taken over and become an integral part of all global processes. Signs of life will be everywhere. Once life has taken hold of a planet, once it has become a planetary scale entity (a global organism, if you will), it may be very hard to kill. Certainly life has seen Earth through many huge changes, some quite traumatic. Life here is remarkably robust and persistent. It seems to have a kind of immortality. Call it quasi immortality, because the planet won't be around forever, and it may not be habitable for its entire lifetime. Individuals are here for but an instant. Whole species come and go, usually in timescales barely long enough to get the planet's attention. Yet life as a whole persists. This gives us a different way to think about ourselves. The scientific revolution has revealed us, as individuals, to be incredibly tiny and ephemeral, and our entire existence, not just as individuals but even as a species, to be brief and insubstantial against the larger temporal backdrop of cosmic evolution. If, however, we choose to identify with the biosphere, then we, Gaia, have been here for quite some time, for perhaps 3 billion years in a universe that seems to be about 13 billion years old. We've been alive for a quarter of all time. That's something.

The origin of life on Earth was not just the beginning of the evolution of species, the fount of diversity that eventually begat algae blooms, aspen groves, barrier reefs, walrus huddles, and gorilla troops. From a planetary evolution perspective, this development was a major branching point that opened up a gateway to a fundamentally different future. Then, when life went global, and went deep, planet Earth headed irreversibly down the path not taken by its siblings.

Now, very recently, out of this biologically altered Earth, another kind of change has suddenly emerged and is rewriting the rules of planetary evolution. On the nightside of Earth, the lights are switching on, indicating that something new is happening and someone new is home. Has another gateway opened? Could the planet be at a new branching point?

The view from space sheds light on the multitude of rapid changes inscribed on our planet by our industrial society. The orbital technology enabling this observation is itself one of the strange and striking aspects of the transition now gripping Earth. If up to now the defining characteristic of Earth has been planetary scale life, then what about these planetary scale lights? Might this spreading, luminous net be part of a new defining characteristic?