Death By Black Hole Part 9

THE CONCEPT OF a habitable zone, when broadened, simply requires an energy source of any variety to liquefy water. One of Jupiter's moons, icy Europa, is heated by the tidal forces of Jupiter's gravitational field. Like a racquetball that heats up after the continuous stress of getting hit, Europa is heated from the varying stress induced by Jupiter pulling more strongly on one side of the moon compared with the other. The consequence? Current observational and theoretical evidence suggest that below the kilometer-thick surface ice there is an ocean of liquid water, possibly slush. Given the fecundity of life within Earth's oceans, Europa remains the most tantalizing place in the solar system for the possibility of life outside Earth. a habitable zone, when broadened, simply requires an energy source of any variety to liquefy water. One of Jupiter's moons, icy Europa, is heated by the tidal forces of Jupiter's gravitational field. Like a racquetball that heats up after the continuous stress of getting hit, Europa is heated from the varying stress induced by Jupiter pulling more strongly on one side of the moon compared with the other. The consequence? Current observational and theoretical evidence suggest that below the kilometer-thick surface ice there is an ocean of liquid water, possibly slush. Given the fecundity of life within Earth's oceans, Europa remains the most tantalizing place in the solar system for the possibility of life outside Earth.

Another recent breakthrough in our concept of a habitable zone are the newly cla.s.sified extremophiles, which are life-forms that not only exist but thrive in climactic extremes of hot and cold. If there were biologists among the extremophiles, they would surely cla.s.sify themselves as normal and any life that thrived in room temperature as an extremophile. Among the extremophiles are the heat-loving thermophiles, commonly found at the midocean ridges, where pressurized water, superheated to well beyond its normal boiling point, spews out from below Earth's crust into the cold ocean basin. The conditions are not unlike those within a household pressure cooker, where high pressures are supplied by a heavy-duty pot with a lockable lid and the water is heated beyond ordinary boiling temperatures, without actually coming to a boil.

On the cold ocean floor, dissolved minerals instantly precipitate out from the hot water vents and form giant porous chimneys up to a dozen stories tall that are hot in their cores and cooler on their edges, where they make direct contact with the ocean water. Across this temperature gradient live countless life-forms that have never seen the Sun and couldn't care less if it were there. These hardy bugs live on geothermal energy, which is a combination of the leftover heat from Earth's formation and heat continuously leaching into Earth's crust from the radioactive decay of naturally occurring yet unstable isotopes of familiar chemical elements such as Aluminum-26, which lasts millions of years, and Pota.s.sium-40, which lasts billions.

At the ocean floor we have what may be the most stable ecosystem on Earth. What if a jumbo asteroid slammed into Earth and rendered all surface life extinct? The oceanic thermophiles would surely continue undaunted in their happy ways. They might even evolve to repopulate Earth's surface after each extinction episode. And what if the Sun were mysteriously plucked from the center of the solar system and Earth spun out of orbit, adrift in s.p.a.ce? This event would surely not merit attention in the thermophile press. But in 5 billion years, the Sun will become a red giant as it expands to fill the inner solar system. Meanwhile, Earth's oceans will boil away and Earth, itself, will vaporize. Now that would be news.

If thermophiles are ubiquitous on Earth, we are led to a profound question: Could there be life deep within all those rogue planets that were ejected from the solar system during its formation? These "geo" thermal reservoirs can last billions of years. How about the countless planets that were forcibly ejected by every other solar system that ever formed? Could interstellar s.p.a.ce be teeming with life formed and evolved deep within these homeless planets? Far from being a tidy region around a star, receiving just the right amount of sunlight, the habitable zone is indeed everywhere. So the Three Bears's cottage was, perhaps, not a special place among fairy tales. Anybody's residence, even that of the Three Little Pigs, might contain a sitting bowl of food at a temperature that is just right. We have learned that the corresponding fraction in the Drake equation, the one that accounts for the existence of a planet within a habitable zone, may be as large as 100 percent.

What a hopeful fairy tale this is. Life, far from being rare and precious, may be as common as planets themselves.

And the thermophilic bacteria lived happily ever after-about 5 billion years.

TWENTY-FOUR.

WATER, WATER.

From the looks of some dry and unfriendly looking places in our solar system, you might think that water, while plentiful on Earth, is a rare commodity elsewhere in the galaxy. But of all molecules with three atoms, water is by far the most abundant. And in a ranking of the cosmic abundance of elements, water's const.i.tuents of hydrogen and oxygen are one and three in the list. So rather than ask why some places have water, we may learn more by asking why all places don't.

Starting in the solar system, if you seek a waterless, airless place to visit then you needn't look farther than Earth's Moon. Water swiftly evaporates in the Moon's near-zero atmospheric pressure and its two-week-long, 200-degree Fahrenheit days. During the two-week night, the temperature can drop to 250 degrees below zero, a condition that would freeze practically anything.

The Apollo astronauts brought with them, to and from the Moon, all the air and water (and air-conditioning) they needed for their round-trip journey. But missions in the distant future may not need to bring water or a.s.sorted products derived from it. Evidence from the Clementine Clementine lunar orbiter strongly supports a long-held contention that there may be frozen lakes lurking at the bottom of deep craters near the Moon's north and south poles. a.s.suming the Moon suffers an average number of impacts per year from interplanetary flotsam, then the mixture of impactors should include sizable water-rich comets. How big? The solar system contains plenty of comets that, when melted, could make a puddle the size of lake Erie. lunar orbiter strongly supports a long-held contention that there may be frozen lakes lurking at the bottom of deep craters near the Moon's north and south poles. a.s.suming the Moon suffers an average number of impacts per year from interplanetary flotsam, then the mixture of impactors should include sizable water-rich comets. How big? The solar system contains plenty of comets that, when melted, could make a puddle the size of lake Erie.

While one wouldn't expect a freshly laid lake to survive many sun-baked lunar days at 200 degrees, any comet that happened to crash and vaporize will cast some of its water molecules in the bottom of deep craters near the poles. These molecules will sink into the lunar soils where they will remain forever because such places are the only places on the Moon where the "Sun don't s.h.i.+ne." (If you otherwise thought the Moon had a perpetual dark side then you have been badly misled by many sources, no doubt including Pink Floyd's 1973 best-selling rock alb.u.m Dark Side of the Moon Dark Side of the Moon.) As light-starved Arctic and Antarctic dwellers know, the Sun never gets very high in the sky at any time of day or year. Now imagine living in the bottom of a crater whose rim was higher than the highest level the Sun ever reached. In such a crater on the Moon, where there is no air to scatter sunlight into shadows, you would live in eternal darkness.

ALTHOUGH ICE IN the cold and dark of your freezer evaporates over time (just look at cubes in your freezer's ice tray after you've come back from a long vacation), the bottoms of these craters are so cold that evaporation has effectively stopped for all needs of this discussion. No doubt about it, if we were ever to establish an outpost on the Moon it would benefit greatly from being located near such craters. Apart from the obvious advantages of having ice to melt, filter, then drink, you can also break apart the water's hydrogen from its oxygen. Use the hydrogen and some of the oxygen as active ingredients in rocket fuel and keep the rest of the oxygen for breathing. And in your spare time between s.p.a.ce missions, you can always go ice skating on the frozen lake created with the extracted water. the cold and dark of your freezer evaporates over time (just look at cubes in your freezer's ice tray after you've come back from a long vacation), the bottoms of these craters are so cold that evaporation has effectively stopped for all needs of this discussion. No doubt about it, if we were ever to establish an outpost on the Moon it would benefit greatly from being located near such craters. Apart from the obvious advantages of having ice to melt, filter, then drink, you can also break apart the water's hydrogen from its oxygen. Use the hydrogen and some of the oxygen as active ingredients in rocket fuel and keep the rest of the oxygen for breathing. And in your spare time between s.p.a.ce missions, you can always go ice skating on the frozen lake created with the extracted water.

Knowing that the Moon has been hit by impactors, as its pristine record of craters tells us, then one might expect Earth to have been hit too. Given Earth's larger size and stronger gravity, one might even expect us to have been hit many more times. It has been-from birth all the way to present day. In the beginning, Earth didn't just hatch from an interstellar void as a preformed spherical blob. It grew from the condensing protosolar gas cloud from which the other planets and the Sun were formed. Earth continued to grow by accreting small solid particles and eventually through incessant impacts with mineral-rich asteroids and water-rich comets. How incessant? The early impact rate of comets is suspected of being high enough to have delivered Earth's entire oceanic supply of water. But uncertainties (and controversies) remain. When compared with the water in Earth's oceans, the water in comets observed today is anomalously high in deuterium, a form of hydrogen that packs one extra neutron in its nucleus. If the oceans were delivered by comets, then the comets available to hit Earth during the early solar system must have had a somewhat different chemical profile.

And just when you thought it was safe to go outside, a recent study on the water level in Earth's upper atmosphere suggests that Earth regularly gets slammed by house-sized chunks of ice. These interplanetary s...o...b..a.l.l.s swiftly vaporize on impact with the air, but they too contribute to Earth's water budget. If the observed rate has been constant over the 4.6 billion-year history of Earth, then these s...o...b..a.l.l.s may also account for the world's oceans. When added to the water vapor that we know is out-ga.s.sed from volcanic eruptions, we have no shortage of ways that Earth could have acquired its supply of surface water.

Our mighty oceans now comprise over two-thirds of Earth's surface area, but only about one five-thousandth of Earth's total ma.s.s. While a small fraction of the total, the oceans weigh in at a hefty 1.5 quintillion tons, 2 percent of which is frozen at any given time. If Earth ever suffers a runaway greenhouse effect (like what has happened on Venus), then our atmosphere would trap excess amounts of solar energy, the air temperature would rise, and the oceans would swiftly evaporate into the atmosphere as they sustained a rolling boil. This would be bad. Apart from the obvious ways that Earth's flora and fauna will die, an especially pressing cause of death would result from Earth's atmosphere becoming three hundred times more ma.s.sive as it thickens with water vapor. We would all be crushed.

Many features distinguish Venus from the other planets in the solar system, including its thick, dense, heavy atmosphere of carbon dioxide that imparts one hundred times the pressure of Earth's atmosphere. We would all get crushed there too. But my vote for Venus's most peculiar feature is the presence of craters that are all relatively young and uniformly distributed over its surface. This innocuous-sounding feature implicates a single planetwide catastrophe that reset the cratering clock by wiping out all evidence of previous impacts. A major erosive weather phenomenon such as a planetwide flood could do it. But so could widespread geologic (Venusiologic?) activity, such as lava flows, turning Venus's entire surface into the American automotive dream-a totally paved planet. Whatever reset the clock, it must have ceased abruptly. But questions remain. If indeed there was a planetwide flood on Venus, where is all the water now? Did it sink below the surface? Did it evaporate into the atmosphere? Or was the flood composed of a common substance other than water?

OUR PLANETARY FASCINATION (and ignorance) is not limited to Venus. With meandering riverbeds, floodplains, river deltas, networks of tributaries, and river-eroded canyons, Mars was once a watering hole. The evidence is strong enough to declare that if anyplace in the solar system other than Earth ever boasted a flouris.h.i.+ng water supply, it was Mars. For reasons unknown, Mars's surface is today bone dry. Whenever I look at both Venus and Mars, our sister and brother planets, I look at Earth anew and wonder how fragile our surface supply of liquid water just might be. (and ignorance) is not limited to Venus. With meandering riverbeds, floodplains, river deltas, networks of tributaries, and river-eroded canyons, Mars was once a watering hole. The evidence is strong enough to declare that if anyplace in the solar system other than Earth ever boasted a flouris.h.i.+ng water supply, it was Mars. For reasons unknown, Mars's surface is today bone dry. Whenever I look at both Venus and Mars, our sister and brother planets, I look at Earth anew and wonder how fragile our surface supply of liquid water just might be.

As we already know, imaginative observations of the planet by Percival Lowell led him to suppose that colonies of resourceful Martians had built an elaborate network of ca.n.a.ls to redistribute water from Mars's polar ice caps to the more populated middle lat.i.tudes. To explain what he thought he saw, Lowell imagined a dying civilization that was somehow running out of water. In his thorough, yet curiously misguided treatise Mars as the Abode of Life, Mars as the Abode of Life, published in 1909, Lowell laments the imminent end of the Martian civilization he imagined he saw: published in 1909, Lowell laments the imminent end of the Martian civilization he imagined he saw: The drying up of the planet is certain to proceed until its surface can support no life at all. Slowly but surely time will snuff it out. When the last ember is thus extinguished, the planet will roll a dead world through s.p.a.ce, its evolutionary career forever ended. (p. 216) (p. 216) Lowell happened to get one thing right. If there were ever a civilization (or any kind of life at all) that required water on the Martian surface, then at some unknown time in Martian history, and for some unknown reason, all the surface water did did dry up, leading to the exact fate for life that Lowell describes. Mars's missing water may be underground, trapped in the planet's permafrost. The evidence? Large craters on the Martian surface are more likely than small craters to exhibit dried mud-spills over their rims. a.s.suming the permafrost to be quite deep, reaching it would require a large collision. The deposit of energy from such an impact would melt this subsurface ice on contact, enabling it to splash upward. Craters with this signature are more common in the cold, polar lat.i.tudes-just where one might expect the permafrost layer to be closer to the Martian surface. By some estimates, if all the water suspected of hiding in the Martian permafrost and known to be locked in the polar ice caps were melted and spread evenly over its surface, Mars would don a planetwide ocean tens of meters deep. A thorough search for contemporary (or fossil) life on Mars must include a plan to look many places, especially below the Martian surface. dry up, leading to the exact fate for life that Lowell describes. Mars's missing water may be underground, trapped in the planet's permafrost. The evidence? Large craters on the Martian surface are more likely than small craters to exhibit dried mud-spills over their rims. a.s.suming the permafrost to be quite deep, reaching it would require a large collision. The deposit of energy from such an impact would melt this subsurface ice on contact, enabling it to splash upward. Craters with this signature are more common in the cold, polar lat.i.tudes-just where one might expect the permafrost layer to be closer to the Martian surface. By some estimates, if all the water suspected of hiding in the Martian permafrost and known to be locked in the polar ice caps were melted and spread evenly over its surface, Mars would don a planetwide ocean tens of meters deep. A thorough search for contemporary (or fossil) life on Mars must include a plan to look many places, especially below the Martian surface.

When thinking about where liquid water might be found (and by a.s.sociation, life), astrophysicists were originally inclined to consider planets that orbited the right distance from their host star to keep water in liquid form-not too close and not too far. This Goldilocks-inspired habitable zone, as it came to be known, was a good start. But it neglected the possibility of life in places where other sources of energy may be responsible for keeping water as a liquid when it might have otherwise turned to ice. A mild greenhouse effect would do it. So would an internal source of energy such as leftover heat from the formation of the planet or the radioactive decay of unstable heavy elements, each of which contributes to Earth's residual heat and consequent geologic activity.

Another source of energy are planetary tides, a more general concept than simply the dance between a moon and a slos.h.i.+ng ocean. As we have seen, Jupiter's moon Io gets continually stressed by changing tides as it ambles slightly closer and then slightly farther from Jupiter during its near-circular orbit. With a distance from the Sun that would otherwise guarantee a forever-frozen world, Io's stress level earns it the t.i.tle of the most geologically active place in the entire solar system-complete with belching volcanoes, surface fissures, and plate tectonics. Some have a.n.a.logized modern-day Io to the early Earth, when our planet was still piping hot from its episode of formation.

An equally intriguing moon of Jupiter is Europa, which also happens to be tidally heated. As had been suspected for some time, Europa was recently confirmed (from images taken by the Galileo Galileo planetary probe) to be a world covered with thick, migrating ice sheets, afloat on a subsurface ocean of slush or liquid water. An ocean of water! Imagine going ice fis.h.i.+ng there. Indeed, engineers and scientists at the Jet Propulsion Laboratory are beginning to think about a mission where a s.p.a.ce probe lands, finds (or cuts or melts) a hole in the ice, and extends a submersible camera to have a peek. Since oceans were the likely place of origin for life on Earth, the existence of life in Europa's oceans becomes a plausible fantasy. planetary probe) to be a world covered with thick, migrating ice sheets, afloat on a subsurface ocean of slush or liquid water. An ocean of water! Imagine going ice fis.h.i.+ng there. Indeed, engineers and scientists at the Jet Propulsion Laboratory are beginning to think about a mission where a s.p.a.ce probe lands, finds (or cuts or melts) a hole in the ice, and extends a submersible camera to have a peek. Since oceans were the likely place of origin for life on Earth, the existence of life in Europa's oceans becomes a plausible fantasy.

In my opinion, the most remarkable feature of water is not the well-earned badge of "universal solvent" that we all learned in chemistry cla.s.s; nor is it the unusually wide temperature range over which it remains liquid. As we have already seen, water's most remarkable feature is that, while most things-water included-shrink and become denser as they cool, water expands when it cools below 4 degrees Celsius, becoming less and less dense. When water freezes at zero degrees, it becomes even less dense than at any temperature when it was liquid, which is bad news for drainage pipes, but very good news for fish. In the winter, as the outside air drops below freezing, 4-degree water sinks to the bottom and stays there while a floating layer of ice builds extremely slowly on the surface, insulating the warmer water below.

Without this density inversion below 4 degrees, whenever the outside air temperature fell below freezing, the upper surface of a bed of water would cool and sink to the bottom as warmer water rose from below. This forced convection would rapidly drop the water's temperature to zero degrees as the surface begins to freeze. The denser, solid ice would sink to the bottom and force the entire bed of water to freeze solid from the bottom up. In such a world, there would be no ice fis.h.i.+ng because all the fish would be dead-fresh frozen. And ice anglers would find themselves sitting on a layer of ice that either was submerged below all remaining liquid water or was atop a completely frozen body of water. No longer would you need icebreakers to traverse the frozen Arctic-either the entire Arctic ocean would be frozen solid or the frozen parts would all have sunk to the bottom and you could just sail your s.h.i.+p without incident. You could walk around, fearless of falling through. In this altered world, ice cubes and icebergs would sink, and in 1912, the t.i.tanic t.i.tanic would have steamed safely into its port of call in New York City. would have steamed safely into its port of call in New York City.

The existence of water in the galaxy is not limited to planets and their moons. Water molecules, along with several other household chemicals such as ammonia and methane and ethyl alcohol, are found routinely in cool interstellar gas clouds. Under special conditions of low temperature and high density, an ensemble of water molecules can be induced to transform and funnel energy from a nearby star into an amplified, high-intensity beam of microwaves. The atomic physics of this phenomenon greatly resembles what goes on with visible light inside a laser. But in this case, the relevant acronym is M-A-S-E-R, for microwave amplification by the stimulated emission of radiation. Not only is water practically everywhere in the galaxy, it occasionally beams at you, too.

While we know water to be essential for life on Earth, we can only presume it to be a prerequisite for life elsewhere in the galaxy. Among the chemically illiterate, however, water is a deadly substance to be avoided. A now-famous science fair experiment that tested ant.i.technology sentiments and a.s.sociated chemical-phobia was conducted in 1997 by Nathan Zohner, a 14-year-old student at Eagle Rock Junior High School in Idaho. He invited people to sign a pet.i.tion that demanded either strict control of, or a total ban on, dihydrogen monoxide. He listed some of the odious properties of this colorless and odorless substance: - It is a major component in acid rain - It eventually dissolves almost anything it comes in contact with - It can kill if accidentally inhaled - It can cause severe burns in its gaseous state - It has been found in tumors of terminal cancer patients Forty-three out of 50 people approached by Zohner signed the pet.i.tion, six were undecided, and one was a great supporter of dihydrogen monoxide and refused to sign. Yes, 86 percent of the pa.s.sersby voted to ban water (H2O) from the environment.

Maybe that's what really happened to all the water on Mars.

TWENTY-FIVE.

LIVING s.p.a.cE.

If you ask people where they're from, they will typically say the name of the city where they were born, or perhaps the place on Earth's surface where they spent their formative years. Nothing wrong with that. But an astrochemically richer answer might be, "I hail from the explosive jetsam of a mult.i.tude of high-ma.s.s stars that died more than 5 billion years ago."

Outer s.p.a.ce is the ultimate chemical factory. The big bang started it all, endowing the universe with hydrogen, helium, and a smattering of lithium: the three lightest elements. Stars forged all the rest of the ninety-two naturally occurring elements, including every bit of carbon, calcium, and phosphorus in every living thing on Earth, human or otherwise. How useless this rich a.s.sortment of raw materials would be had it stayed locked up in the stars. But when stars die, they return much of their ma.s.s to the cosmos, sprinkling nearby gas clouds with a portfolio of atoms that enrich the next generation of stars.

Under the right conditions of temperature and pressure, many of the atoms join to form simple molecules. Then, through routes both intricate and inventive, many molecules grow larger and more complex. Eventually, in what must surely be countless billions of places in the universe, complex molecules a.s.semble themselves into some kind of life. In at least one cosmic corner, the molecules have become so complex that they have achieved consciousness and attained the ability to formulate and communicate the ideas conveyed by the marks on this page.

Yes, not only humans but also every other organism in the cosmos, as well as the planets or moons on which they thrive, would not exist but for the wreckage of spent stars. So you're made of detritus. Get over it. Or better yet, celebrate it. After all, what n.o.bler thought can one cherish than that the universe lives within us all?

TO COOK UP some life, you don't need rare ingredients. Consider the top five const.i.tuents of the cosmos, in order of their abundance: hydrogen, helium, oxygen, carbon, and nitrogen. Take away chemically inert helium-which is not fond of making molecules with anybody-and you've got the top four const.i.tuents of life on Earth. Awaiting their cue within the ma.s.sive clouds that lurk among a galaxy's stars, these elements begin making molecules as soon as the temperature drops below a couple thousand degrees Kelvin. some life, you don't need rare ingredients. Consider the top five const.i.tuents of the cosmos, in order of their abundance: hydrogen, helium, oxygen, carbon, and nitrogen. Take away chemically inert helium-which is not fond of making molecules with anybody-and you've got the top four const.i.tuents of life on Earth. Awaiting their cue within the ma.s.sive clouds that lurk among a galaxy's stars, these elements begin making molecules as soon as the temperature drops below a couple thousand degrees Kelvin.

Molecules made of just two atoms form early: carbon monoxide and the hydrogen molecule (hydrogen atoms bound together in pairs). Drop the temperature some more, and you get stable three-or four-atom molecules such as water (H2O), carbon dioxide (CO2), and ammonia (NH3)-simple but top-shelf ingredients in the kitchen of life. Drop the temperature even more, and hordes of five-and six-atom molecules form. And because carbon is both abundant and chemically enterprising, most of the molecules include it; indeed, three-quarters of all molecular "species" sighted in interstellar s.p.a.ce have at least one carbon atom.

Sounds promising. But s.p.a.ce can be a dangerous place for molecules. If the energy from stellar explosions doesn't destroy them, ultraviolet light from nearby ultraluminous stars will. The bigger the molecule, the less stable it is against a.s.sault. Molecules lucky enough to inhabit uneventful or s.h.i.+elded neighborhoods may endure long enough to be incorporated into grains of cosmic dust, and ultimately into asteroids, comets, planets, and people. Yet even if none of the original molecules survives the stellar violence, plenty of atoms and time remain available to make complex molecules, not only during the formation of a particular planet but also on and within the planet's nubile surface. Notables on the short list of complex molecules include adenine (one of the nucleotides, or "bases," that make up DNA), glycine (a protein precursor), and glycoaldehyde (a carbohydrate). Such ingredients, and others of their caliber, are essential for life as we know it and are decidedly not unique to Earth.

BUT ORGIES OF organic molecules are not life, just as flour, water, yeast, and salt are not bread. Although the leap from raw ingredients to living individual remains mysterious, several prerequisites are clear. The environment must encourage molecules to experiment with one another and must shelter them from excessive harm as they do so. Liquids offer a particularly attractive environment, because they enable both close contact and great mobility. The more chemical opportunities an environment affords, the more imaginative its resident experiments can be. Another essential factor, brought to you by the laws of physics, is a generous supply of energy to drive chemical reactions. organic molecules are not life, just as flour, water, yeast, and salt are not bread. Although the leap from raw ingredients to living individual remains mysterious, several prerequisites are clear. The environment must encourage molecules to experiment with one another and must shelter them from excessive harm as they do so. Liquids offer a particularly attractive environment, because they enable both close contact and great mobility. The more chemical opportunities an environment affords, the more imaginative its resident experiments can be. Another essential factor, brought to you by the laws of physics, is a generous supply of energy to drive chemical reactions.

Given the wide range of temperatures, pressures, acidity, and radiation flux at which life thrives on Earth, and knowing that one microbe's cozy nook can be another's house of torture, scientists cannot at present stipulate additional requirements for life elsewhere. As a demonstration of the limits of this exercise, we find the charming little book Cosmotheoros Cosmotheoros, by the seventeenth-century Dutch astronomer Christiaan Huygens, wherein the author speculates that life-forms on other planets must grow hemp, for how else would they weave ropes to steer their s.h.i.+ps and sail the open seas?

Three centuries later, we're content with just a pile of molecules. Shake 'em and bake 'em, and within a few hundred million years you might have thriving colonies of organisms.

LIFE ON EARTH is astonis.h.i.+ngly fertile, that's for sure. But what about the rest of the universe? If somewhere there's another celestial body that bears any resemblance to our own planet, it may have run similar experiments with its similar chemical ingredients, and those experiments would have been ch.o.r.eographed by the physical laws that hold sway throughout the universe. is astonis.h.i.+ngly fertile, that's for sure. But what about the rest of the universe? If somewhere there's another celestial body that bears any resemblance to our own planet, it may have run similar experiments with its similar chemical ingredients, and those experiments would have been ch.o.r.eographed by the physical laws that hold sway throughout the universe.

Consider carbon. Its capacity to bind in multiple ways, both to itself and to other elements, gives it a chemical exuberance unequalled in the periodic table. Carbon makes more kinds of molecules (how does 10 million grab you?) than all other elements combined. A common way for atoms to make molecules is to share one or more of their outermost electrons, creating a mutual grip a.n.a.logous to the fist-shaped coupler between freight cars. Each carbon atom can bind with one, two, three, or four other atoms in this way, whereas a hydrogen atom binds with only one, oxygen with one or two, and nitrogen with three.

By binding to itself, carbon can generate myriad combinations of long-chain, highly branched, or closed-ring molecules. Such complex organic molecules are ripe for doing things that small molecules can only dream about. They can, for example, perform one kind of task at one end and another kind at the other; they can coil and curl and intertwine with other molecules, creating no end of features and properties. Perhaps the ultimate carbon-based molecule is DNA: a double-stranded chain that encodes the ident.i.ty of all life as we know it.

What about water? When it comes to fostering life, water has the highly useful property of staying liquid across what most biologists regard as a fairly wide range of temperatures. Trouble is, most biologists look to Earth, where water stays liquid across 100 degrees of the Celsius scale. But on some parts of Mars, atmospheric pressure is so low that water is never liquid: a freshly poured cup of H2O boils and freezes at the same time! Yet in spite of Mars's current sorry state, its atmosphere once supported liquid water in abundance. If ever the Red Planet harbored life on its surface, it would have been then.

Earth, of course, happens to have a goodly-and occasionally deadly-amount of water on its surface. Where did it come from? As we saw earlier, comets are a logical source: they're chock full of (frozen) water, the solar system holds countless billions of them, some are quite large, and they would regularly have been slamming into the early Earth back when the solar system was forming. Another source of water could have been volcanic outga.s.sing, a frequent phenomenon on the young Earth. Volcanoes erupt not simply because magma is hot, but because hot, rising magma turns underground water to steam, which then expands explosively. The steam no longer fits in its subterranean chamber, and so the volcano blows its lid, bringing H2O to Earth's surface from below. All things considered, then, the presence of water on our planet's surface is hardly surprising.

ALTHOUGH EARTH-LIFE takes multifarious forms, all of it shares common stretches of DNA. The biologist who has Earth-on-the-brain may revel in life's diversity, but the astrobiologist dreams of diversity on a grander scale: life based on alien DNA, or on something else entirely. Sadly, our planet is a singular biological sample. Nevertheless, the astrobiologist may glean insights about life-forms that dwell elsewhere in the cosmos by studying organisms that thrive in extreme environments here on Earth. takes multifarious forms, all of it shares common stretches of DNA. The biologist who has Earth-on-the-brain may revel in life's diversity, but the astrobiologist dreams of diversity on a grander scale: life based on alien DNA, or on something else entirely. Sadly, our planet is a singular biological sample. Nevertheless, the astrobiologist may glean insights about life-forms that dwell elsewhere in the cosmos by studying organisms that thrive in extreme environments here on Earth.

Once you look for them, you find these extremophiles practically everywhere: nuclear dump sites, acid-laden geysers, iron-saturated acidic rivers, chemical-belching vents on the ocean floor, submarine volcanoes, permafrost, slag heaps, commercial salt-evaporation ponds, and a host of other places you would not elect to spend your honeymoon but that may be more typical of the rest of the planets and moons out there. Biologists once presumed that life began in "some warm little pond," to quote Darwin (1959, p. 202); in recent years, though, the weight of evidence has tilted in favor of the view that extremophiles were the earliest earthly life-forms.

As we will see in the next section, for its first half-billion years, the inner solar system resembled a shooting gallery. Earth's surface was continually pulverized by crater-forming boulders large and small. Any attempt to jump-start life would have been swiftly aborted. By about 4 billion years ago, though, the impact rate slowed and Earth's surface temperature began to drop, permitting experiments in complex chemistry to survive and thrive. Older textbooks start their clocks at the birth of the solar system and typically declare that life on Earth needed 700 million or 800 million years to form. But that's not fair: the planet's chem-lab experiments couldn't even have begun until the aerial bombardment lightened up. Subtract 600 million years' worth of impacts right off the top, and you've got single-celled organisms emerging from the primordial ooze within a mere 200 million years. Even though scientists continue to be stumped about how life began, nature clearly had no trouble creating the stuff.

IN JUST A FEW dozen years, astrochemists have gone from knowing nothing of molecules in s.p.a.ce to finding a plethora of them practically everywhere. Moreover, in the past decade astrophysicists have confirmed that planets...o...b..t other stars and that every exosolar star system is laden with the same top four ingredients of life as our own cosmic home is. Although no one expects to find life on a star, even a thousand-degree "cool" one, Earth has plenty of life in places that register several hundred degrees. Taken together, these discoveries suggest it's reasonable to think of the universe as fundamentally familiar rather than as utterly alien. dozen years, astrochemists have gone from knowing nothing of molecules in s.p.a.ce to finding a plethora of them practically everywhere. Moreover, in the past decade astrophysicists have confirmed that planets...o...b..t other stars and that every exosolar star system is laden with the same top four ingredients of life as our own cosmic home is. Although no one expects to find life on a star, even a thousand-degree "cool" one, Earth has plenty of life in places that register several hundred degrees. Taken together, these discoveries suggest it's reasonable to think of the universe as fundamentally familiar rather than as utterly alien.

But how familiar? Are all life-forms likely to be like Earth's-carbon-based and committed to water as their favorite fluid?

Take silicon, one of the top ten elements in the universe. In the periodic table, silicon sits directly below carbon, indicating that they have an identical configuration of electrons in their outer sh.e.l.ls. Like carbon, silicon can bind with one, two, three, or four other atoms. Under the right conditions, it can also make long-chain molecules. Since silicon offers chemical opportunities similar to those of carbon, why couldn't life be based on silicon?

One problem with silicon-apart from its being a tenth as abundant as carbon-is the strong bonds it creates. When you link silicon and oxygen, for instance, you don't get the seeds of organic chemistry; you get rocks. On Earth, that's chemistry with a long shelf life. For chemistry that's friendly to organisms, you need bonds that are strong enough to survive mild a.s.saults on the local environment but not so strong that they don't allow further experiments to take place.

And how important is liquid water? Is it the only medium suitable for chemistry experiments-the only medium that can shuttle nutrients from one part of an organism to another? Maybe life just needs a liquid. Ammonia is common. So is ethanol. Both are drawn from the most abundant ingredients in the universe. Ammonia mixed with water has a vastly lower freezing point (around100 degrees Fahrenheit) than does water by itself (32 degrees), broadening the conditions under which you might find liquid-loving life. Or here's another possibility: on a world that lacks an internal heat source, orbits far from its host star, and is altogether bone-cold, normally gaseous methane might become the liquid of choice.

IN 2005, the European s.p.a.ce Agency's 2005, the European s.p.a.ce Agency's Huygens Huygens probe (named after you-know-who) landed on Saturn's largest moon, t.i.tan, which hosts lots of organic chemistry and supports an atmosphere ten times thicker than Earth's. Setting aside the planets Jupiter, Saturn, Ura.n.u.s, and Neptune, each made entirely of gas and having no rigid surface, only four objects in our solar system have an atmosphere of any significance: Venus, Earth, Mars, and t.i.tan. probe (named after you-know-who) landed on Saturn's largest moon, t.i.tan, which hosts lots of organic chemistry and supports an atmosphere ten times thicker than Earth's. Setting aside the planets Jupiter, Saturn, Ura.n.u.s, and Neptune, each made entirely of gas and having no rigid surface, only four objects in our solar system have an atmosphere of any significance: Venus, Earth, Mars, and t.i.tan.

t.i.tan was not an accidental target of exploration. Its impressive resume of molecules includes water, ammonia, methane, and ethane, as well as the multiringed compounds known as polycyclic aromatic hydrocarbons. The water ice is so cold it's as hard as concrete. But the combination of temperature and air pressure has liquefied the methane, and the first images sent back from Huygens Huygens seem to show streams, rivers, and lakes of the stuff. In some ways t.i.tan's surface chemistry resembles that of the young Earth, which accounts for why so many astrobiologists view t.i.tan as a "living" laboratory for studying Earth's distant past. Indeed, experiments conducted two decades ago show that adding water and a bit of acid to the organic ooze produced by irradiating the gases that make up t.i.tan's hazy atmosphere yields sixteen amino acids. seem to show streams, rivers, and lakes of the stuff. In some ways t.i.tan's surface chemistry resembles that of the young Earth, which accounts for why so many astrobiologists view t.i.tan as a "living" laboratory for studying Earth's distant past. Indeed, experiments conducted two decades ago show that adding water and a bit of acid to the organic ooze produced by irradiating the gases that make up t.i.tan's hazy atmosphere yields sixteen amino acids.

Recently, biologists have learned that planet Earth may harbor a greater bioma.s.s belowground than on its surface. Ongoing investigations about the hardy habits of life demonstrate time and again that it recognizes few boundaries. Once stereotyped as kooky scientists in search of little green men on nearby planets, investigators who ponder the limits of life are now sophisticated hybrids, exploiting the tools of not only astrophysics, biology, and chemistry but also geology and paleontology as they pursue life here, there, and everywhere.

TWENTY-SIX.

LIFE IN THE UNIVERSE.

The discovery of hundreds of planets around stars other than the Sun has triggered tremendous public interest. Attention was driven not so much by the discovery of exosolar planets, but by the prospect of them hosting intelligent life. In any case, the media frenzy that continues may be somewhat out of proportion with the events. Why? Because planets cannot be all that rare in the universe if the Sun, an ordinary star, has at least eight of them. Also, the newly discovered planets are all oversized gaseous giants that resemble Jupiter, which means no convenient surface exists upon which life as we know it could live. And even if they were teeming with buoyant aliens, the odds against these life-forms being intelligent may be astronomical.

Ordinarily, there is no riskier step that a scientist (or anyone) can take than to make sweeping generalizations from just one example. At the moment, life on Earth is the only known life in the universe, but compelling arguments suggest we are not alone. Indeed, most astrophysicists accept the probability of life elsewhere. The reasoning is easy: if our solar system is not unusual, then there are so many planets in the universe that, for example, they outnumber the sum of all sounds and words ever uttered by every human who has ever lived. To declare that Earth must be the only planet with life in the universe would be inexcusably bigheaded of us.

Many generations of thinkers, both religious and scientific, have been led astray by anthropocentric a.s.sumptions, while others were simply led astray by ignorance. In the absence of dogma and data, it is safer to be guided by the notion that we are not special, which is generally known as the Copernican principle, named for Nicolaus Copernicus, of course, who, in the mid-1500s, put the Sun back in the middle of our solar system where it belongs. In spite of a third-century B.C B.C. account of a Sun-centered universe, proposed by the Greek philosopher Aristarchus, the Earth-centered universe was by far the most popular view for most of the last 2,000 years. Codified by the teachings of Aristotle and Ptolemy, and later by the preachings of the Roman Catholic Church, people generally accepted Earth as the center of all motion and of the known universe. This fact was self-evident. The universe not only looked that way, but G.o.d surely made it so.

While the Copernican principle comes with no guarantees that it will forever guide us to cosmic truths, it's worked quite well so far: not only is Earth not in the center of the solar system, but the solar system is not in the center of the Milky Way galaxy, the Milky Way galaxy is not in the center of the universe, and it may come to pa.s.s that our universe is just one of many that comprise a multiverse. And in case you're one of those people who thinks that the edge may be a special place, we are not at the edge of anything either.

A WISE CONTEMPORARY posture would be to a.s.sume that life on Earth is not immune to the Copernican principle. To do so allows us to ask how the appearance or the chemistry of life on Earth can provide clues to what life might be like elsewhere in the universe. posture would be to a.s.sume that life on Earth is not immune to the Copernican principle. To do so allows us to ask how the appearance or the chemistry of life on Earth can provide clues to what life might be like elsewhere in the universe.

I do not know whether biologists walk around every day awestruck by the diversity of life. I certainly do. On this single planet called Earth, there coexist (among countless other life-forms), algae, beetles, sponges, jellyfish, snakes, condors, and giant sequoias. Imagine these seven living organisms lined up next to each other in size-place. If you didn't know better, you would be challenged to believe that they all came from the same universe, much less the same planet. Try describing a snake to somebody who has never seen one: "You gotta believe me. Earth has an animal that (1) can stalk its prey with infrared detectors, (2) swallows whole live animals up to five times bigger than its head, (3) has no arms, legs, or any other appendage, yet (4) can slide along level ground at a speed of two feet per second!"

Given this diversity of life on Earth, one might expect a diversity of life exhibited among Hollywood aliens. But I am consistently amazed by the film industry's lack of creativity. With a few notable exceptions such as the aliens of The Blob The Blob (1958), in (1958), in 2001: A s.p.a.ce Odyssey 2001: A s.p.a.ce Odyssey (1968), and in (1968), and in Contact Contact (1997), Hollywood aliens look remarkably humanoid. No matter how ugly (or cute) they are, nearly all of them have two eyes, a nose, a mouth, two ears, a head, a neck, shoulders, arms, hands, fingers, a torso, two legs, two feet-and they can walk. From an anatomical view, these creatures are practically indistinguishable from humans, yet they are supposed to have come from another planet. If anything is certain, it is that life elsewhere in the universe, intelligent or otherwise, should look at least as exotic to us as some of Earth's own life-forms. (1997), Hollywood aliens look remarkably humanoid. No matter how ugly (or cute) they are, nearly all of them have two eyes, a nose, a mouth, two ears, a head, a neck, shoulders, arms, hands, fingers, a torso, two legs, two feet-and they can walk. From an anatomical view, these creatures are practically indistinguishable from humans, yet they are supposed to have come from another planet. If anything is certain, it is that life elsewhere in the universe, intelligent or otherwise, should look at least as exotic to us as some of Earth's own life-forms.

The chemical composition of Earth-based life is primarily derived from a select few ingredients. The elements hydrogen, oxygen, and carbon account for over 95 percent of the atoms in the human body and all known life. Of the three, the chemical structure of carbon allows it to bond readily and strongly with itself and with many other elements in many different ways, which is why we are considered to be carbon-based life, and which is why the study of molecules that contain carbon is generally known as "organic" chemistry. Curiously, the study of life elsewhere in the universe is known as exobiology, which is one of the few disciplines that attempts to function with the complete absence of firsthand data.

Is life chemically special? The Copernican principle suggests that it probably isn't. Aliens need not look like us to resemble us in more fundamental ways. Consider that the four most common elements in the universe are hydrogen, helium, carbon, and oxygen. Helium is inert. So the three most abundant, chemically active ingredients in the cosmos are also the top three ingredients in life on Earth. For this reason, you can bet that if life is found on another planet, it will be made of a similar mix of elements. Conversely, if life on Earth were composed primarily of, for example, molybdenum, bis.m.u.th, and plutonium, then we would have excellent reason to suspect that we were something special in the universe.

Appealing once again to the Copernican principle, we can a.s.sume that the size of an alien organism is not likely to be ridiculously large compared with life as we know it. There are cogent structural reasons why you would not expect to find a life the size of the Empire State Building strutting around a planet. But if we ignore these engineering limitations of biological matter we approach another, more fundamental limit. If we a.s.sume that an alien has control of its own appendages or, more generally, if we a.s.sume the organism functions coherently as a system, then its size would ultimately be constrained by its ability to send signals within itself at the speed of light-the fastest allowable speed in the universe. For an admittedly extreme example, if an organism were as big as the entire solar system (about 10 light-hours across), and if it wanted to scratch its head, then this simple act would take no less than 10 hours to accomplish. Sub-slothlike behavior such as this would be evolutionarily self-limiting because the time since the beginning of the universe may be insufficient for the creature to have evolved from smaller forms of life over many generations.

HOW ABOUT INTELLIGENCE? When Hollywood aliens manage to visit Earth, one might expect them to be remarkably smart. But I know of some that should have been embarra.s.sed at their stupidity. During a four-hour car trip from Boston to New York City, while I was surfing the FM dial, I came upon a radio play in progress that, as best as I could determine, was about evil aliens who were terrorizing Earthlings. Apparently, they needed hydrogen atoms to survive so they kept swooping down to Earth to suck up its oceans and extract the hydrogen from all the H2O molecules.