Comets are an obvious culprit if you want to understand how water gets from one part of the Solar System to another. They’re famously known as “dirty snowballs”, for their mixture of water ice, ices made of other molecules, and a few organic molecules thrown in to give them a dark gray color. Since very early Earth was hot enough to boil away any surface water, one popular idea is that comets brought enough water to fill the oceans during a period of time known as the Late Heavy Bombardment.
That hypothesis is nice and simple, but it’s not universally accepted among planetary scientists, because comets don’t seem to be playing nicely. For instance, the most recent measurement taken by the Rosetta probe in orbit around Comet 67P/Churyumov-Gerasimenko shows that its water contains roughly three times the abundance of deuterium — the isotope of hydrogen with one neutron attached — than we see on Earth. I covered this story for The Daily Beast:
However, it’s wrong to say the new Rosetta results rule out comets as the origin of the oceans. That’s partly because we’ve only gotten this kind of measurement for a small number of comets, and only one of those—Comet 103P/Hartley 2—had the same relative deuterium content as Earth. Both 103P and 67P are Jupiter-family comets, which are widely thought to have formed in the outer Solar System and plunged into smaller orbits thanks to Jupiter’s gravity. The difference in their chemistry makes the story even messier, so it’s likely we’ll be talking about the origin of Earth’s oceans for many years to come. [Read more…]