Water, water: did cometary impacts deliver Earth's entire oceanic supply?

From the looks of the many dry and unfriendly-seeming spots in our solar system, you might think that water, while plentiful on Earth, is a rare commodity 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 on 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 dry, 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 during its two-week-long, 200 [degrees] F day. But during the two-week night, the temperature can drop to 250 [degrees] below zero.

The Apollo astronauts carried with them all the water and air (and air conditioning) that they needed for their journey to and from the Moon. But missions in the distant future may not need to bring water or assorted products-derived from it. Evidence from the Clementine lunar orbiter in the 1980s and more recent evidence from the Lunar Prospector strongly support a long-held contention that there are frozen lakes hiding at the bottom of deep craters near the Moon's north and south poles. Assuming the Moon suffers a certain number of impacts per year from interplanetary flotsam, then the mix of impactors should include some 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, any comet that happened to crash into a deep crater near the poles (or happened to create its own crater there) would remain in darkness, because deep polar craters are the only places on the Moon "where the Sun don't shine." (If you 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.) Light-starved Arctic and Antarctic dwellers know that 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 so high you could never see the Sun on the horizon. In such a crater on the Moon, where there is no air to scatter sunlight into shadows, you would live in eternal darkness.

Ice can evaporate even in cold darkness. Just look at cubes in your freezer's ice tray after you've come back from a long vacation: they'll be distinctly smaller than when you last looked. But if the ice were well mixed with solid particles (as it is in a comet), it could survive for thousands and millions of years in the extreme cold at the bottom of the Moon's deep polar craters. No doubt about it; if we were ever to establish an outpost on the Moon, we would benefit greatly from locating it near a frozen lake, An obvious advantage would be having ice to melt, filter, and drink. You could also dissociate the water's hydrogen and its oxygen and use them as active ingredients in rocket fuel, remembering to keep the rest of the oxygen for breathing. And in your spare time, between space missions, you could always go ice skating.

The Moon has been pummeled by solar-system debris, as its well-preserved record of craters tells us, so one might expect Earth to have been hit, too. Given Earth's larger size and stronger gravity, you would expect it to have been hit even more often. Indeed, it has been. Earth didn't just hatch from an interstellar void as a preformed spherical blob. It grew out of the condensing protosolar cloud from which the Sun and all its asteroids, comets, and planets were formed. During its first half-billion years, Earth continued to grow by accreting small solid particles and then (through incessant impacts) asteroids and water-rich comets. The impact rate of comets at the time is suspected of having been high enough to account for 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 is anomalously high in deuterium, a form of hydrogen that packs one extra neutron in its nucleus. If all the water of Earth's oceans were delivered by comets, then the comets that hit Earth during the early history of the solar system must have had a somewhat different chemical profile than the ones we observe today.

And just when you thought it was safe to go outside, a recent study of the moisture level in Earth's upper atmosphere suggests that Earth regularly gets slammed by house-sized chunks of ice. These interplanetary icebergs swiftly vaporize on impact with the air, but they nonetheless contribute to Earth's water budget. If the impact rate of the ice has been constant over the 4.6-billion-year history of Earth, then this source of water alone could account for the world's oceans. Water vapor is also out-gassed from volcanic eruptions, so Earth has had no shortage of ways of getting its supply of surface water.