Death by Black Hole: And Other Cosmic Quandaries by Tyson Neil deGrasse

Author:Tyson, Neil deGrasse [Tyson, Neil deGrasse]
Language: eng
Format: azw3
Publisher: W. W. Norton & Company
Published: 2007-11-16T16:00:00+00:00

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

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