Ice crystals coming off of Comet 103P/Hartley

The idea that the atoms in our body were cooked in the cores of massive stars that ultimately blew their starry guts out has already fascinated me. Now, astronomers have added a related celestial twist: the water making up earth’s ancient oceans, some of which resides inside our present day bodies, probably came from a special group of comets which normally wander lazily around the edge of the solar system:

Earth-like water has been discovered in the small oddball comet Hartley 2, which the Deep Impact/EPOXI spacecraft flew by in November 2010. This comet originated in the disk-shaped Kuiper belt, a region of the solar system beyond the orbit of Neptune, suggesting this is ultimately where much of Earth’s water came from.

What does earth-like water mean?

A depiction of earth's first steamy oceans and nearby moon circa 4 billions years ago

Water is made of hydrogen and oxygen, any high school science student can recite the formula H2O. But hydrogen comes in several basic versions called isotopes. All hydrogen atoms have one proton, that’s what makes it hydrogen. But the one proton comes with different numbers of neutrons. Earth-water has a distinctive ratio of regular light hydrogen to a heavier isotope called deuterium. This same D/H fingerprint was detected in comet 103P/Hartley, the ratio is too exact to be coincidental.

The inference? Much of earth’s water probably arrived later, after the planet had formed, delivered by comets. That in itself suggests the early solar system was a more dynamic and incredibly violent place than the relatively serene we know and love. Most comets in the Kuiper Belt are thought to have formed closer to the proto sun before being ejected by Jupiter (Or possibly its long-lost companions now residing in the sun, or those objects now known as Neptune and Uranus), but the gas giant would have also sent thousands careening into the inner solar system where a fraction would have ended up hitting earth.

We are a mixture stardust and comet water, and that’s pretty cool.