Death From the Skies!: These Are the Ways the World Will End...

by Philip C. Plait

  The new generation of particle colliders—what used to be called atom smashers—can actually slam subatomic particles into each other so hard that it’s theoretically possible that they will create extremely tiny mini black holes. A few years back, this news made some headlines when it was revealed that the Relativistic Heavy Ion Collider (RHIC) in New York might be able to do just that. Would the Earth get eaten by an artificial black hole?

  Many newspaper articles speculated it might, but there are two reasons why that can’t and won’t happen. One is that, as we saw, tiny black holes will evaporate through Hawking radiation extremely rapidly. A black hole made by the types of collisions done at RHIC would last the tiniest fraction of a second. They’d never get a chance to accrete any mass before evaporating (and their mass would be so small that the explosion would be really tiny too).

  Second, the energies created at RHIC are actually much smaller than what naturally occur at the top of the Earth’s atmosphere billions of times a day! Cosmic rays—subatomic particles accelerated to fiendish energies in supernova explosions—slam into the air all the time at far higher energies than we can hope to create here on Earth. These are more than enough to create extremely tiny black holes, yet here we are. Over the billions of years that these particles have been raining down on us, not once has the Earth been eaten by a subsequently created black hole.

  Newspapers, magazines, and TV like to inflate such stories because they know they will sell. But when you look at the actual science, you see that we’re in no danger of being gobbled up by a black hole, whether by nature’s hand or our own.

  And that is the hole truth.56

  CHAPTER 6

  Alien Attack!

  MINDLESSLY—AT LEAST, LACKING WHAT WE WOULD call a mind as we know it—it examined the bright light of the star ahead of it. Employing a highly sophisticated complex of observational instrumentation, it patiently took data, examining each bit of information as it came in. After weeks of steadily staring at its target, the results were in.

  The star was orbited by several gas giants. Each of these had icy moons with possible water under the surface. The star also had not just one but three smaller planets with the potential for liquid water as well. And on the second one of these out from the star were unmistakable signs of biotic life—free O2 in the atmosphere at large levels of disequilibrium. If it had been equipped with emotions it would have whooped with joy. Instead, it silently and efficiently began preparing for the next phase of its mission.

  Using sophisticated engineering and technology the probe began to slow its approach to the solar system. Its fantastic speed—nearly that of light itself—gradually bled away over the course of nearly a year. Course corrections were made, angling the probe this way and that. All the while, it took observations, scouting for the target it needed. Finally, the target was acquired: a metallic asteroid over a mile wide. As the probe passed the asteroid, aiming carefully, it released a small package just a few meters across.

  The package was a probe in its own right, and it used its onboard rocket to decelerate further and land on the surface of the asteroid. It immediately sent an “all clear” signal to the mothership, which did not respond, but instead sharply accelerated away, heading off to the next star on its list, a star it would not reach for decades.

  On the asteroid’s surface, a hatch opened on the probe, and a small spider emerged . . . then another, and another. In all, a dozen such robots started crawling over the landscape. Composed of a sophisticated amalgam of metal, ceramics, and spun carbon fiber, they went to work: digging, smelting, manufacturing. They worked without fatigue, without emotion, tirelessly day and night (such as there was on the slowly rotating asteroid). After a month they were ready.

  Like a fungus expelling spores, the asteroid erupted in thousands of tiny explosions. Each puff imparted velocity to a ball of metal a meter across, each of which headed toward one of the planets and moons initially targeted by the interstellar probe. Inside each ball were over a hundred of the spiders. The robotic arachnoids were possessed of sophisticated programming, but in the end the goal was simple: convert any and all available materials into more spiders. When enough were manufactured, build more mothership probes. Launch them, and repeat the cycle.

  They needed metal of almost any sort, and what they couldn’t find they could create. Their programming was very sophisticated, honed over millions of years of such missions. And they weren’t picky about materials; almost anything would do. Each spider could create the components needed to replicate itself in just a few hours, and these would then move on to replicate themselves as well. Once the first spiders touched down, they could cover a planet in just a few days, converting everything—everything—into more spiders and more probes.

  Being smaller and closer, the surface of Mars was destroyed first. The rocks were rich in iron, which made things easier. Within hours, that many more spiders went off in search of raw material.

  Food.

  Earth fell within days. The first spiders landed in Australia and consumed everything in their sight. Rock, metal, gas; all could be converted if needed. Water, plants, flesh—these would do as well. Humans never had a chance. Though the intense light of the interstellar probe’s engine had been tracked by Earthbound telescopes for months, it didn’t answer any hails, and there wasn’t enough time for any of the governments to react anyway. By the time the spiders landed it was already far too late. They swept over the planet, and after less than two weeks there were in essence no living creatures left on Earth. The entire surface of the globe had been converted into robotic factories. Within a year, bright flashes blossomed over the planet as more interstellar probes were launched, each an exact replica of that first one—which itself was generations removed from the first probe launched so many eons before. That first probe was long since dead, having expended all its packages. It was no longer useful. But its progeny “lived” on, sweeping across the galaxy.

  And now, several thousand more headed out into deep space. When the original mission started, mankind didn’t exist; only hominids ambled across the African plains. Their descendants ruled the Earth, but their reign was brief. All those billions were now gone, converted into hordes of little metal spiders and more interstellar probes.

  Man’s dream of reaching the stars was finally achieved, but not quite in the manner in which he thought.

  THAT’S LIFE

  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.

  A BRIEF HISTORY OF THE SOLAR SYSTEM

  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 mad
e 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.

  Self-portrait, 4.6 billion years BC. This illustration shows what our solar system looked like when it was young. A disk of rock, gas, ices, and metals revolved around the newly born Sun, just starting to form the planets we know today.

  NASA/JPL-CALTECH/T. PYLE (SSC)

  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.

  MARTIAN CHRONICLES

  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.

  The Mars rover Opportunity took this snapshot of the Red Planet on March 1, 2004. The presence of sulfates and other chemicals in the rocks indicates that water once flowed over the surface of Mars.

  NASA/JPL

  But four billion years ago it was a different story. Did ancient Martian oceans teem with simple life-forms, bacteria, protozoans? We don’t know, and we may never know. Still, future probes may yet find fossils in ancient Martian rocks.