A Brief History of Earth: Four Billion Years in Eight Chapters

by Andrew H. Knoll

Comets, sadly, fail this test; their distinct hydrogen chemistry suggests that they can account for no more than about 10 percent of Earth’s water. The rest of our water, as well as the gases in our atmosphere and the carbon in our bodies, arrived in some of the meteorites that built the planet as a whole, especially certain types of chondritic meteorites thought to have arrived during late stages of Earth’s growth. One group of chondrites, termed carbonaceous chondrites, deserves particular attention, as they contain 3–11 percent water by mass, mostly bound chemically into clays and other minerals, as well as about 2 percent organic matter (molecules in which carbon and hydrogen are bound together), including amino acids like those found in proteins. Chondritic meteorites, then, provide a source of water and carbon, and unlike comets, they pass the hydrogen isotope test. Thus it looks like chondritic meteorites of different kinds provided most of the rock, water, and air that we call home. On the early Earth, heat would have driven water vapor, nitrogen gas, and carbon dioxide from the interior to form a hot, dense atmosphere, perhaps a hundred times more dense than the one we experience today.

On the young Earth, impacts capable of vaporizing early oceans continued for some time. Evidence for this comes not from our own planet, but from our planetary neighbor, Mars, where an ancient, cratered surface is still preserved in the southern highlands. There are some giants among these craters; a remarkable impact structure called Hellas Planitia is about 2,300 kilometers (1,400 miles) across—roughly the distance from Boston to New Orleans.

A PRIZED POSSESSION in my office desk is an old flip book assembled in 1979 by Chris Scotese (then a graduate student, now a world authority on Earth’s ever-changing geography). Each page shows the positions of continents at a given time, and when you flip through the book rapidly, landmasses appear to move, as in an early stop-action movie. Every few seconds, words like “crash,” “crunch,” and “rrri-ppp” flash by, highlighting continental collisions and breakup.

Plate tectonics is not an inevitable consequence of planet formation. Mars, for example, shows no evidence of ancient or modern plate movements, and neither does Venus. On Earth, however, plate tectonics was established early, setting in place the physical processes that sculpt Earth’s surface and, as we’ll see, maintain its environment. In consequence, Earth became more than a planet with oceans and atmosphere, mountains, and volcanoes. It became a planet capable of sustaining life.