Origins: Fourteen Billion Years of Cosmic Evolution (23 page)

It would be odd, however, if Earth had acquired a great deal of water, while the nearby Moon got almost none. One possibility, certainly true at least in part, is that water evaporated from the Moon’s surface much more readily than from Earth’s because of the Moon’s lesser gravity. Another possibility suggests that lunar missions may eventually not need to import water or the assortment of products derived from it. Observations by the
Clementine
lunar orbiter, which carried an instrument to detect the neutrons produced when fast-moving interstellar particles collide with hydrogen atoms, support a long-held contention that deep-frozen ice deposits may lurk beneath craters near the Moon’s north and south poles. If the Moon receives an average number of impacts per year from interplanetary flotsam, then the mixture of these impactors should, from time to time, include sizable water-rich comets, like those that strike Earth. How big could these comets be? The solar system contains plenty of comets that could melt into a puddle the size of Lake Erie.

While we can’t expect a freshly laid lake to survive many sun-baked lunar days at temperatures of 200 degrees, any comet that happened to crash in the bottom of a deep crater near one of the Moon’s poles (or happened to make a deep polar crater itself) would remain shrouded in darkness, because deep craters near its poles are the only places on the Moon where the “Sun don’t shine.” (If you thought that the Moon has a perpetual dark side, you have been badly misled by many sources, probably including Pink Floyd’s 1973 album
Dark Side of the Moon
.) As light-starved Arctic and Antarctic dwellers know, the Sun in those regions never rises high in the sky at any time of day or any season of the year. Now imagine living at the bottom of a crater whose rim rises higher than the highest altitude that the Sun ever reaches. With no air to scatter sunlight into the shadows, you would live in eternal darkness.

But even in cold darkness, ice slowly evaporates. Just look at the cubes in your freezer’s ice tray upon your return from a long vacation: their sizes will be distinctly smaller than when you departed. However, if ice has been well mixed with solid particles (as occurs in a comet), it can survive for thousands and millions of years at the bottom of the Moon’s deep polar craters. Any outpost that we might establish on the Moon would benefit greatly from being located near this lake. Apart from the obvious advantages of having ice to melt, to filter, and then to drink, we could also profit by dissociating the water’s hydrogen from its oxygen atoms. We could use the hydrogen, plus some of the oxygen, as active ingredients for rocket fuel, while keeping the rest of the oxygen for breathing. And in our spare time between space missions, we might choose to go skating.

Although Venus has
nearly the same size and mass as Earth, several attributes distinguish our sister planet from all the other planets in the solar system, notably including its highly reflective, thick, dense, carbon dioxide atmosphere, which exerts a hundred times the surface pressure of Earth’s atmosphere. Except for bottom-dwelling marine creatures that live at similar pressures, all forms of Earthlife would be crushed to death on Venus. But Venus’ most peculiar feature resides in the relatively young craters uniformly scattered over its surface. This innocuous-sounding description implies that a recent planetwide catastrophe reset the cratering clock—and thus our ability to date a planet’s surface by its buildup of craters—by wiping out the evidence of all previous impacts. A major erosive weather phenomenon such as a planetwide flood might also have done this. But so could planetwide geologic (should we say Venusologic?) activity, such as lava flows, which could have turned Venus’ entire surface into the American automotive dream—a totally paved planet. Whatever events reset the cratering clock must have ceased abruptly. But important questions remain, in particular about Venus’ water. If a planetwide flood did occur on Venus, where has all the water gone? Did it sink below the surface? Did it evaporate into the atmosphere? Or did the flood consist of a common substance other than water? Even if no flood occurred, Venus presumably acquired about as much water as its sister planet Earth. What has happened to it?

The answer seems to be that Venus lost its water by growing too hot, a result attributable to Venus’ atmosphere. Although carbon dioxide molecules let visible light pass by, they trap infrared radiation with great efficiency. Sunlight can therefore penetrate Venus’ atmosphere, even though atmospheric reflection reduces the amount of sunlight that reaches the surface. This sunlight heats the planet’s surface, which radiates infrared, and which cannot escape. Instead, the carbon dioxide molecules trap it, as the infrared radiation heats the lower atmosphere and the surface below. Scientists call this trapping of infrared radiation the “greenhouse effect” by loose analogy to their glass windows, which admit visible light but block some of the infrared. Like Venus and its atmosphere, Earth produces a greenhouse effect, essential for many forms of life, that raises our planet’s temperature by about 25 degrees Fahrenheit over what we would find in the absence of an atmosphere. Most of our greenhouse effect arises from the combined effects of water and carbon dioxide molecules. Since Earth’s atmosphere has only one ten-thousandth as many carbon dioxide molecules as the atmosphere of Venus does, our greenhouse effect pales in comparison. Nevertheless, we continue to add more carbon dioxide by burning fossil fuels, so we steadily increase the greenhouse effect, performing an unintended global experiment to see just what deleterious effects arise from the additional trapping of heat. On Venus, the atmospheric greenhouse effect, produced entirely by carbon dioxide molecules, raises the temperature by hundreds of degrees, giving Venus’ surface furnacelike temperatures close to 500
o
Celsius (900
o
Fahrenheit)—the hottest in the solar system.

How did Venus reach this sorry state? Scientists apply the apt term “runaway greenhouse effect” to describe what happened as the infrared radiation trapped by Venus’ atmosphere raised the temperatures and encouraged liquid water to evaporate. The additional water in the atmosphere trapped infrared even more effectively, increasing the greenhouse effect; this in turn caused even more water to enter the atmosphere, ratcheting up the greenhouse effect still farther. Near the top of Venus’ atmosphere, solar UV radiation would break the water molecules apart into hydrogen and oxygen atoms. Because of the high temperatures, the hydrogen atoms would escape, while the heavier oxygen combined with other atoms, never to form water again. With the passage of time, all the water that Venus once had on or near its surface has been essentially baked out of the atmosphere and lost to the planet forever.

Similar processes occur on Earth, but at a much lower rate because we have much lower atmospheric temperatures. Our mighty oceans now comprise most of Earth’s surface area, though their modest depth gives them only about one five-thousandth of Earth’s total mass. Even this small fraction of the total allows the oceans to weigh in at a hefty 1.5 quintillion tons, 2 percent of which is frozen at any given time. If Earth should ever undergo a runaway greenhouse effect like the one that has occurred on Venus, our atmosphere would trap larger amounts of solar energy, raising the air temperature and making the oceans evaporate swiftly into the atmosphere as they sustained a rolling boil. This would be bad news. Apart from the obvious ways that Earth’s flora and fauna would die, an especially pressing cause of death would result from Earth’s atmosphere growing three hundred times more massive as it thickens with water vapor. We would be crushed and baked by the air we breathe.

Our planetary fascination
(and ignorance) are hardly limited to Venus. With its long dry, still preserved meandering riverbeds, floodplains, river deltas, networks of tributaries, and river-eroded canyons, Mars must once have been a primeval Eden of water in motion. If any place in the solar system other than Earth ever boasted a flourishing water supply, it was Mars. For reasons unknown, however, today Mars has a bone-dry surface. Close examination of Venus and Mars, our sister and brother planets, forces us to look at Earth anew and to wonder how fragile our surface supply of liquid water may turn out to be.

Early in the twentieth century, imaginative observations of Mars by the noted American astronomer Percival Lowell led him to suppose that colonies of resourceful Martians had built an elaborate network of canals in order to redistribute water from Mars’ polar ice caps to the more populated middle latitudes. To explain what he thought he saw, Lowell imagined a dying civilization that was exhausting its supply of water, like Phoenix discovering that the Colorado River has its limits. In his thorough yet curiously misguided treatise entitled
Mars as the Abode of Life,
published in 1909, Lowell lamented the imminent end of the Martian civilization that he imagined he saw.

Indeed, Mars seems certain to dry up to the point that its surface can support no life at all. Slowly but surely, time will snuff life out, if it has not done so already. When the last living ember dies away, the planet will roll on through space as a dead world, its evolutionary career forever ended.

Lowell happened to get one thing right. If Mars ever had a civilization (or any kind of life at all) that required water on the surface, it must have faced catastrophe, because 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—though in the past, not the present—that Lowell described. What happened to the water that flowed abundantly over Mars’ surface billions of years ago remains an outstanding mystery among planetary geologists. Mars does have some water ice in its polar caps, which consist mainly of frozen carbon dioxide (“dry ice”), and a tiny amount of water vapor in its atmosphere. Although the polar caps contain the only significant amounts of water now known to exist on Mars, their total content of ice falls far below the amount needed to explain the ancient records of flowing water on Mars’ surface.

If most of Mars’ ancient water did not evaporate into space, its most likely hiding place lies underground, with the water trapped in the planet’s subsurface permafrost. The evidence? Large craters on the Martian surface are more likely than small craters to exhibit dried mud spills over their rims. If the permafrost lies deep underground, to reach it would require a large collision. The deposit of energy from such an impact would melt this subsurface ice upon contact, causing it to splash upward. Craters with this mud-spill signature are more common in the cold, polar latitudes—just where we might expect the permafrost layer to be closer to the Martian surface. According to optimistic estimates of the Martian permafrost’s ice content, the melting of Mars’ subsurface layers would release enough water to give Mars a planetwide ocean tens of meters deep. A thorough search for contemporary (or fossil) life on Mars must include a plan to search in many locations, especially below the Martian surface. So far as the chance of finding life on Mars is concerned, the great question to be resolved asks, Does liquid water now exist anywhere on Mars?

Part of the answer leaps from our knowledge of physics. No liquid water can exist on the Martian surface, because the atmospheric pressure there, less than 1 percent of the value on the surface of Earth, does not allow it. As enthusiastic mountaineers know, water vaporizes at progressively lower temperatures as the atmospheric pressure decreases. At the summit of Mount Whitney, where the air pressure falls to half of its sea-level value, water boils not at 100 but at 75 degrees Celsius. On top of Mount Everest, with air pressure only a quarter of its sea-level value, boiling occurs at about 50 degrees. Twenty miles high, where the atmospheric pressure equals only 1 percent of what you feel on the sidewalks of New York, water boils at about 5 degrees Celsius. Rise a few miles higher, and liquid water will “boil” at 0 degrees—that is, it will vaporize as soon as you expose it to the air. Scientists use the word “sublimation” to describe the passage of a substance from solid to gas without any intervening liquid stage. We all know sublimation from our youth, when the ice cream man opened his magic door to reveal not only the delicacies inside but also the chunks of “dry” ice that kept them cold. Dry ice offers the ice cream man a great advantage over familiar water ice: It sublimates from solid to gas, leaving no messy liquid to clean up. An old detective story conundrum describes the man who hanged himself by standing on a cake of dry ice until it sublimated, leaving him suspended by a noose, and the detectives without a clue (unless they carefully analyzed the atmosphere in the room) as to how he did it.

What happens to carbon dioxide on Earth’s surface happens to water on the surface of Mars. No chance for liquid exists there, even though the temperature on a warm day of the Martian summer rises well above 0 degrees Celsius. This seems to draw a sad veil over the prospects for life—until we realize that liquid water could exist beneath the surface. Future missions to Mars, intimately bound up with the possibility of finding ancient or even modern life on the red planet, will direct themselves toward regions where they can drill into the Martian surface in a search for the flowing elixir of life.

Elixir though it may appear, water represents a deadly substance among the chemically illiterate, to be avoided sedulously. In 1997, Nathan Zohner, a fourteen-year-old student at Eagle Rock Junior High School in Idaho, conducted a now famous (among science popularizers) science fair experiment to test antitechnology sentiments and associated chemical phobia. Zohner invited people to sign a petition that demanded either strict control or a total ban of 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