Death by Black Hole: And Other Cosmic Quandaries (20 page)

What we have now is a solar system in the making, soon to comprise molecule-rich planets and molecule-rich comets. Once there’s some solid material, the sky’s the limit. Molecules can get as fat as they like. Set carbon loose under those conditions, and you might even get the most complex chemistry we know. How complex? It goes by another name: biology.

O
nce upon a time, some four billion years ago, the formation of the solar system was nearly complete. Venus had formed close enough to the Sun for the intense solar energy to vaporize what might have been its water supply. Mars formed far enough away for its water supply to be forever frozen. And there was only one planet, Earth, whose distance was “just right” for water to remain a liquid and whose surface would become a haven for life. This region around the Sun came to be known as the habitable zone.

Goldilocks (of fairy-tale fame) liked things “just right,” too. One of the bowls of porridge in the Three Bears’ cottage was too hot. Another was too cold. The third was just right, so she ate it. Also in the Three Bears’s cottage, one bed was too hard. Another was too soft. The third was just right, so Goldilocks slept in it. When the Three Bears came home, they discovered not only missing porridge but also Goldilocks fast asleep in a bed. (I forget how the story ends, but if I were the Three Bears—omnivorous and at the top of the food chain—I would have eaten Goldilocks.)

The relative habitability of Venus, Earth, and Mars would intrigue Goldilocks, but the actual story of these planets is somewhat more complicated than three bowls of porridge. Four billion years ago leftover water-rich comets and mineral-rich asteroids were still pelting the planetary surfaces, although at a much slower rate than before. During this game of cosmic billiards, some planets had migrated inward from where they had formed while others were kicked up to larger orbits. And among the dozens of planets that had formed, some were on unstable orbits and crashed into the Sun or Jupiter. Others were ejected from the solar system altogether. In the end, the few that remained had orbits that were “just right” to survive billions of years.

Earth settled into an orbit with an average distance of 93 million miles from the Sun. At this distance, Earth intersects a measly one two-billionth of the total energy radiated by the Sun. If you assume that Earth absorbs all incident energy from the Sun, then our home planet’s average is about 280 degrees Kelvin (50 degrees F), which falls midway between winter and summer temperatures. At normal atmospheric pressures, water freezes at 273 degrees and boils at 373 degrees Kelvin, so we are well-positioned for nearly all of Earth’s water to remain in a happy liquid state.

Not so fast. Sometimes in science you can get the right answer for the wrong reasons. Earth actually absorbs only two-thirds of the energy that reaches it from the Sun. The rest is reflected back into space by Earth’s surface (especially the oceans) and by the clouds. If reflectivity is factored into the equations, then the average temperature for Earth drops to about 255 degrees Kelvin, which is well below the freezing point of water. Something must be operating in modern times to raise our average temperature back to something a little more comfortable.

But wait once more. All theories of stellar evolution tell us that 4 billion years ago, when life was forming out of Earth’s proverbial primordial soup, the Sun was a third less luminous than it is today, which would have placed Earth’s average temperature even further below freezing.

Perhaps Earth in the distant past was simply closer to the Sun. But after the early period of heavy bombardment, no known mechanisms could have shifted stable orbits back and forth within the solar system. Perhaps the greenhouse effect was stronger in the past. We don’t know for sure. What we do know is that habitable zones, as originally conceived, have only peripheral relevance to whether there may be life on a planet within them.

The famous Drake equation, invoked in the search for extraterrestrial intelligence, provides a simple estimate for the number of civilizations one might expect to find in the Milky Way galaxy. When the equation was conceived in the 1960s by the American astronomer Frank Drake, the concept of a habitable zone did not extend beyond the idea that there would be some planets at the “just right” distance from their host stars. A version of the Drake equation reads: Start with the number of stars in the galaxy (hundreds of billions). Multiply this large number by the fraction of stars with planets. Multiply what remains by the fraction of planets in the habitable zone. Multiply what remains by the fraction of those planets that evolved life. Multiply what remains by the fraction that have evolved intelligent life. Multiply what remains by the fraction that might have developed a technology with which to communicate across interstellar space. Finally, when you introduce a star formation rate and the expected lifetime of a technologically viable civilization you get the number of advanced civilizations that are out there now, possibly waiting for our phone call.

Small, cool, low-luminosity stars live for hundreds of billions and even possibly trillions of years, which ought to allow plenty of time for the planets around them to evolve a life-form or two, but their habitable zones fall very close to the host star. A planet that forms there will swiftly become tidally locked and always show the same face toward the star (just as the Moon always shows the same face to Earth) creating an extreme imbalance in planetary heating—all water on the planet’s “near” side would evaporate while all water on the planet’s “far” side would freeze. If Goldilocks lived there, we would find her eating oatmeal while turning in circles (like a rotisserie chicken) right on the border between eternal sunlight and eternal darkness. Another problem with the habitable zones around these long-lived stars is that they are extremely narrow; a planet in a random orbit is unlikely to find itself at a distance that is “just right.”

Conversely, large, hot, luminous stars have enormous habitable zones in which to find their planets. Unfortunately these stars are rare, and live for only a few million years before they violently explode, so their planets make poor candidates in the search for life as we know it—unless, of course, some rapid evolution occurred. But animals that can do advanced calculus were probably not the first things to slither out of the primordial slime.

We might think of the Drake equation as Goldilocks mathematics—a method for exploring the chances of getting things just right. But the Drake equation as originally conceived misses Mars, which lies well beyond the habitable zone of the Sun. Mars displays countless meandering dry riverbeds, deltas, and floodplains, which constitute in-your-face evidence for running water in the Martian past.

How about Venus, Earth’s “sister” planet? It falls smack dab within the Sun’s habitable zone. Covered completely by a thick canopy of clouds, the planet has the highest reflectivity of any planet in the solar system. There is no obvious reason why Venus could not have been a comfortable place. But it happens to suffer from a monstrous greenhouse effect. Venus’s thick atmosphere of carbon dioxide traps nearly 100 percent of the small quantities of radiation that reach its surface. At 750 degrees Kelvin (900°F) Venus is the hottest planet in the solar system, yet it orbits at nearly twice Mercury’s distance from the Sun.

If Earth has sustained the continuous evolution of life through billions of years of storm and drama, then perhaps life itself provides a feedback mechanism that maintains liquid water. This notion was advanced by the biologists James Lovelock and Lynn Margulis in the 1970s and is referred to as the Gaia hypothesis. This influential, yet controversial idea requires that the mixture of species on Earth at any moment acts as a collective organism that continuously (yet unwittingly) tunes Earth’s atmospheric composition and climate to promote the presence of life—and by implication, the presence of liquid water. I am intrigued by the idea. It has even become the darling of the New Age movement. But I’d bet there are some dead Martians and Venusians who advanced the same theory about their own planets a billion years ago.

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.

Another recent breakthrough in our concept of a habitable zone are the newly classified 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 classify 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 Potassium-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 space? 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 space 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.

F
rom 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 constituents 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 assorted products derived from it. Evidence from the
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. Assuming 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 shine.” (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 album
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 space 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 snowballs 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 snowballs may also account for the world’s oceans. When added to the water vapor that we know is out-gassed 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 mass. 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 massive 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 flourishing 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 canals to redistribute water from Mars’s polar ice caps to the more populated middle latitudes. 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,
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 space, its evolutionary career forever ended.
(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
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. Assuming 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 latitudes—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 association, 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 sloshing 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 title of the most geologically active place in the entire solar system—complete with belching volcanoes, surface fissures, and plate tectonics. Some have analogized 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
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 fishing there. Indeed, engineers and scientists at the Jet Propulsion Laboratory are beginning to think about a mission where a space 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 class; 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 fishing 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 ship without incident. You could walk around, fearless of falling through. In this altered world, ice cubes and icebergs would sink, and in 1912, the
Titanic
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.