A recently completed analysis of comet 67P/C-G by the Rosetta Space Craft has effectively ruled out comets as the primary source of Earth’s water. The probe analyzed the ratio of deuterium to regular hydrogen in the comet’s water and found that it was not similar to the deuterium/hydrogen (D/H) ratio in Earth’s water. Earth has more water than any known body in the universe. So where did it come from?
The search starts back in the very beginning, when the Earth itself was formed. In a 2006 article in Philosophical Transactions: Biological Sciences, Jonathan Lunine explains about the harsh, dry conditions present on early Earth, a sharp contrast to our present blue home. Lunine explains how Earth was formed by violent collisions of ever-larger objects, starting with nebular dust up to moon-sized planetoids that collided, bounced, and eventually fused together. The gravitational influence of our mighty planetary neighbor, Jupiter, condensed this process into a reasonable time frame (only a few tens of millions of years, give or take, as opposed to billions).The violence of these collisions vaporized all liquid, so Earth simply could not have had any water at all until things settled down and literally cooled off.
Therefore, water had to be brought to Earth later in its formation, and from elsewhere in the solar system. Comets were viewed as a reasonable possibility simply because they contain a relatively high amount of water. The problem, according to Lunine, is that the physics don’t add up. Comets have a more distant orbit than many solar objects, and Jupiter knocked them even farther out toward the edges of the solar system. There was no force able to draw enough comets into the inner solar system to hydrate the Earth.
If comets didn’t bring water and it wasn’t here to begin with, that leaves asteroids. Certain asteroids do contain trace amounts of water in a compatible range of D/H ratios, and many of these objects were attracted towards the inner solar system. The problem with the asteroid theory is that not enough of Earth is of meteoric origin to account for all of the water, so there is a possibility that comets did add to the total. In some scenarios, Jupiter might actually have knocked icy bodies inwards, helping to resolve one of the dilemmas noted by Lunine. Asteroids are dry now after billions of years of orbiting the sun, but in the past might might have been wetter, requiring fewer meteors to deliver enough water. For now, the Rosetta project highlights that in scientific inquiry, it’s often process of elimination at work rather than active discovery. It’s amazing what you can learn from not learning.