The question of where all of Earth’s water came from has been a scientific puzzle. It was long presumed that the planet wasn’t born with it, but that it appeared some 700 million years later—likely riding piggyback on comets.
Comets originate from two places in the solar system: the Kuiper belt, a ring of icy debris just beyond the orbit of Neptune, and the Oort cloud, a spherical shell of frozen detritus much farther out. Asteroids are rocky bodies in the inner solar system, mostly between the orbits of Mars and Jupiter.
But data sent back by Rosetta from comet 67/P refutes that theory.
The water on it—which exists in the form of ice and vapor—is different from the water found on Earth: it has thrice the concentration of “heavy water”—a form of water in which its hydrogen is replaced by deuterium (a heavier version of hydrogen)—than terrestrial water.