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.