Death from the Skies! (24 page)

No matter where you go on the Earth, you find life.

On the plains, on mountaintops, high in the air, down to the deepest ocean depths, life abounds. Even far underground, microscopic life has adapted to conditions we would consider lethal. Life is everywhere.

Earth seems marvelously tuned to support life, but that’s an illusion:
we
are the ones who are in fact tuned by evolution, as are all the other forms of life on, below, and above the Earth’s surface. As the Earth has changed over the eons, so has life. It seems almost inevitable that, once life first got its start on Earth, it would flourish.

We know there are other planets in our solar system, and even orbiting other stars. If life is so plentiful here, it stands to reason it may also be on those other worlds. They may teem with simple microorganisms, and it’s possible that there could also be more complex forms of life in space, things that we would recognize as intelligent.

If that were so, what would they think of us? Would they be a threat to us?

To understand that, we’ll start by taking a short journey backward in time—though my definition of “short” may differ a bit from yours.

Some 4.6 billion years ago, you were spread out over countless of cubic miles of space.

So was I. So was the book you’re holding now, and the clothes you’re wearing, and everyone and everything you’ve ever known, ever seen, ever touched, ever dreamt about. All your atoms were part of a vast disk, tens of billions of miles across and a million miles thick. The disk was almost entirely made up of hydrogen and helium, but it also contained scattered impurities of zinc, iron, calcium, phosphorus, and dozens of other elements. It rotated slowly, held together by its own gravity and the gravity of the lumpy swell at the center.

Over millions of years matter accumulated in the center of the disk, gravity pulling in ever more material. As it compressed it got hotter, and eventually the core temperature reached 27 million degrees Fahrenheit. Hydrogen fusion triggered, and it was at that moment that our Sun became a real star. Light flooded out, followed by a wave of subatomic particles, a nascent solar wind.

In the meantime, the outer parts of the disk were busy accumulating matter as well. At first clumps only stuck together because of chemical processes. Ice crystals formed farther out from the Sun, where temperatures were low. Chunks of silicates smacked into each other and stuck. Over time, as the aggregations grew, so did their mass, and so did their gravity. These
planetesimals
started actively pulling in more matter in a runaway process—more mass, more gravity, more matter, more mass, and so on—that was finally only quenched when there was no more material to accrete. Some of these new planets were small, some large. Some decent-sized ones were ejected out of the system entirely when they passed too close to the larger ones.

 

The ones that survived all had rocky cores and thick atmospheres. Some had atmospheres thousands of miles deep, and no real surface to speak of. Others were smaller, with dense atmospheres to be sure. They also had molten surfaces, heat left over from the formation process.

When the Sun in the center turned on, its fierce light and solar wind hit the new planets. The pressure from the light and the ejected matter slammed into the thinned disk, blowing away the leftover detritus, clearing the space. Eventually, all that was left was a handful of planets, billions of asteroids, and trillions of icy comets, all orbiting a young, hot star.
⁵⁷

Our solar system was born.

It looked different then! Jupiter was farther out from the Sun than it is now, while Saturn, Uranus, and Neptune were closer in. Their mutual dance of gravity would eventually migrate them to their current distances. In the inner solar system, Mercury, Venus, Earth, and Mars all had thick, soupy atmospheres. Over time, as the inner planets solidified, they too would change. Mercury, so close to the Sun and with such low gravity, would have its atmosphere stripped away. Also, the tiny planet’s lack of a magnetic field left it open to the full brunt of the Sun’s solar wind, which aided in tearing Mercury’s atmosphere away atom by atom.

Venus would eventually lose its hydrogen and helium, but chemical processes over billions of years, including a runaway greenhouse effect, would give it a thick atmosphere of carbon dioxide. Trapping the heat from the Sun, the planet would become a forbidding desert of kilnlike heat. The surface rocks are always just a hairbreadth from being molten.

Earth too had a dense atmosphere, nothing at all like today’s—it looked more like Jupiter or Saturn back then, consisting mostly of hydrogen and helium left over from the disk from which it formed. At its distance from the Sun, incoming heat (plus the heat coming from the surface) puffed up the thick air, in a delicate balance with gravity. Over millions of years, the lighter elements were lost, leaving behind an atmosphere of carbon dioxide, water vapor, carbon monoxide, ammonia, methane, and other noxious gases, most of which leaked out from inside the Earth.

Eventually the surface cooled, forming a thick crust over the molten, semiplastic mantle of rock. The heavy elements like iron, iridium, and uranium sank to the center. The radioactive elements decayed, generating heat, adding to the heat trapped inside left over from the formation of the planet. Convection currents began, a magnetic dynamo ensued, and Earth became protected from the ravages of the solar wind.

Not that the young Earth was safe from threats from space. A lot of the material in the solar disk was swept up by the planets, but not
all
of it. A huge repository of asteroids still roamed the system, and their paths would sometimes intersect those of the planets. Shortly after the planets formed, they were all bombarded mercilessly. Nearly every solid surface in the solar system bears witness to this devastation; a quick glance at the heavily battered and cratered surface of the Moon will confirm it.

The Earth bore the brunt of its share of collisions too. It was hit significantly more than the Moon was, being bigger and having stronger gravity—in fact, the most commonly accepted theory on the formation of the Moon itself is that it coalesced from material ejected when the Earth was hit by a very large object, perhaps as big as Mars, an apocalyptic collision that is terrifying to imagine. But over billions of years plate tectonics and erosion have wiped out all the evidence of this early bombardment. Only the most recent craters are still around; even ones older than a few million years are nearly invisible. Still, this early pelting from space created an immensely hostile environment. Anytime things began to settle down, some fifty-mile-wide rock would come crashing down, resetting the geological clock.

Eventually, though, the rain of iron and rock ceased. As the Earth cooled, more complex molecules could form. The methane in Earth’s atmosphere was a source of hydrogen, as was ammonia, which also had nitrogen. Carbon dioxide provided the carbon, which, when liberated from the oxygen, could combine to form ever more complex chains of atoms. Amino acids—the building blocks of proteins—probably formed quite early, and started combining in new and interesting ways. Lightning from the atmosphere and ultraviolet light from the Sun may have provided energy needed to break up and reform the molecules. At some point—no one knows exactly when or how, but it was virtually simultaneous with the cessation of the bombardment from asteroids—the molecules formed into a pattern that had a fantastic property: it could reproduce itself. As things stand today, this molecule was probably incredibly simple, but it still possessed the amazing ability to gather up raw materials and construct them in such a way as to reproduce a copy. These then went forth and multiplied.

It was hardly more than a simple chemical reaction, or really a long series of them. These reactions needed materials—the elements found in the air and surface—and emitted waste products. One of these waste products was oxygen. As oxygen built up in the air, the chemical processes started to change. Oxygen, to many of these very simple microbes, was toxic, as waste products sometimes are to the organisms that produce them. As the gas built up, it poisoned them. Some species of microbes adapted to the new environment (and their descendants still live today as blue-green algae and other forms of life), but those that couldn’t perished; they had thrived on Earth for millions of years, but their own waste killed them.
⁵⁸
However, a slightly different complex molecule was around at the same time. It was able to use the waste. Oxygen, when combined with other chemicals, can release a lot of energy, which in turn can be useful for reproduction and increased metabolism. This different microbe fed off the waste of the others, and when the oxygen levels got high enough that the first life-forms started dying, the oxygen-users were ready for the coup. They took over. Some oxygen-producers survived, mutating and adapting all the time as some fit into the environment better than others. Oxygen-users that were more adept at using the fuel flourished; others died off.
⁵⁹

Asteroid impacts, vast solar flares, and the random nearby supernova may have wiped out this process or culled it back to near-extinction levels many times over the millions of years the scenario played out, but eventually a toehold (or a pseudopod hold) too firm to shake off was established.

Earth became alive.

Now, this tale is but one way life may have arisen on Earth. We don’t know for sure how it happened. We’re not even sure where it happened: land, sea, air, in the deep ocean . . . or even on Earth. Our planet is only one of several where, in the early life of the solar system, conditions were ripe for the development of life.

Mars, though, was smaller than the Earth, and farther from the Sun. It cooled more rapidly than the Earth did, and may have gone through the same series of events, but on a shorter time scale. We don’t know this for a fact, of course, but it’s certainly possible that Mars had a thriving microbial ecosystem long before the Earth did. Unfortunately, it was doomed. Once Mars cooled down enough, it lost any magnetic field it might have had, so it became victim to the Sun’s solar wind. Its weaker gravity allowed almost its entire atmosphere to leak away to space, and now the pressure on the surface is a miserable 1 percent of Earth’s, thinner than at the top of Mount Everest.

Robotic explorers sent to Mars have shown us a clear history of a watery surface, however. Chemicals in the ground and rippled wave patterns indicate that in the past there were vast floods, perhaps as frozen water underground was heated by volcanism or impacts. There are also indications of ancient lakes, now desiccated, as big as the Great Lakes in the United States. Even today there are (still controversial) hints of transient, short-lived events where liquid water flows on the surface . . . only to quickly evaporate into the thin air.