Earth's water piggybacked on asteroids, not comets

Whether comets or asteroids were the source of Earth's water has long been the subject of debate. Now an analysis of the composition of meteorites suggests the water did not originate in the outer solar system, a finding that favours asteroids as the vehicle for its arrival.

Both asteroids and comets are found in a region of the solar system known as the asteroid belt, which occupies a wide swathe of space between the orbits of Mars and Jupiter. However, comets with their icy tails would have been born in the chillier region of space between Saturn and Jupiter and then migrated into the asteroid belt.

To find out whether comets or asteroids were the parents of carbonaceous chondrites: rare meteorites which delivered water and volatile elements such as nitrogen, carbon and hydrogen to Earth, a team led by Conel Alexander from the Carnegie Institution of Washington in Washington DC measured the amount of deuterium &nash; a heavy isotope of hydrogen – in 86 chondrite samples found on Earth.

The further from the sun an object was formed, the more deuterium-rich it tends to be. The chondrites Alexander tested turned out to contain significantly less deuterium than comets, indicating that the chondrites most likely originated in a different place. "So, they probably formed closer in to the sun," says Alexander, most likely in the asteroid belt itself.

Despite the finding, exactly where the chondrites formed remains an open question – one that is particularly difficult to answer, says Fred Ciesla, a researcher at the University of Chicago, Illinois, who models the formation of planets. "You can't just say, you formed something at this one location and it sat there for 4.5 billion years. Things move around all the time," he says