Where did Earth’s water come from?

It is commonly accepted that soon after formation, the Earth was a dry planet.  The water that now makes up about 70% of its surface came later via collisions with water-bearing space objects.  Those objects were thought to be have almost exclusively been asteroids, but new research suggests that some of them may have been comets.

It’s no surprise that comets contain vast amounts of water.  They are, after all, mainly dirty snowballs of ice, gases and organic compounds.  However, upon closer examination, the amount of heavy hydrogen (deuterium) present on some comets has not matched the ratio found in our oceans, indicating that those comets could not have made significant contributions to our oceans.

More recently, an international team of astronomers examined the comet Hartley 2 and found that its deuterium/hydrogen ratio does match that of Earth’s oceans.  Hartley 2 may have little in common with the previously studied comets, having most likely come from the Kuiper belt and not the Oort cloud like the rest of the comets. 

While there’s no doubt that much of our water was delivered via asteroid, this data suggests some of it may have come from comets as well.

Image of Hartley 2 comet.