Death By Black Hole & Other Cosmic Quandaries

by Neil DeGrasse Tyson

  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 formati
on 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.

  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 antitechnology sentiments and associated 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 petition 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 petition, six were undecided, and one was a great supporter of dihydrogen monoxide and refused to sign. Yes, 86 percent of the passersby voted to ban water (H2O) from the environment.

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

  TWENTY-FIVE

  LIVING SPACE

  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 multitude of high-mass stars that died more than 5 billion years ago.”

  Outer space 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 assortment of raw materials would be had it stayed locked up in the stars. But when stars die, they return much of their mass 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 assemble 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 nobler 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 constituents 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 constituents of life on Earth. Awaiting their cue within the massive 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 space have at least one carbon atom.

  Sounds promising. B
ut space 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 assault. Molecules lucky enough to inhabit uneventful or shielded 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.