Shores of Cosmic Oceans

Our home planet is exceptional for being a warm rocky planet with plenty of water on its surface. There have been several proposed origins for Earth’s water, such as that it originated within the primordial materials of our planet, or that it was brought by cometary or meteor impacts. We know from measuring isotopic ratios in Earth’s water that much of it was formed before even our solar system, and that it seems to match water found in certain asteroids. So the most popular model for Earth’s water is that it was brought to our planet by asteroid-like meteors. But did asteroids in our young planetary system really have enough water to produce our oceans?

It would seem the answer is yes, and new research tends to support that conclusion.¹ In this work, the authors looked at the spectra of a white dwarf and found evidence of hydrogen at oxygen in its atmosphere. A white dwarf is a sun-like star that has reached the end of its life. After is has consumed much of its hydrogen, fusing it to heavier elements like helium and carbon) it collapses under its own weight until it is roughly the size of Earth (but still the mass of the Sun). Because a star will enter a red giant stage before collapsing to a white dwarf, most of the lighter elements like hydrogen would be cast off. Likewise, heavier elements like oxygen would tend to settle into the core of the star. So one would think we wouldn’t see much of either hydrogen or oxygen in the spectra of a white dwarf.

It turns out we do tend to see hydrogen, which could be an indication that some of that light element didn’t get thrown off during the red giant stage. But the presence of oxygen would seem to indicate this material was accreted by the star relatively recently on a cosmic scale. The authors found evidence of other elements such as silicon and iron, which are common in asteroids. One explanation for this is that the white dwarf accreted an asteroid, and its remnants are seen in the star’s spectra.

So the authors calculated the size of such an asteroid from the spectra, and found that it would have been about the size of Ceres. If that’s the case, then the strength of the oxygen and hydrogen spectra would imply that the asteroid was about 38% water when it was accreted. That’s a sizable amount. We know that asteroids in our solar system such as Ceres and Vesta contain water, but the amount is still under investigation. If this roughly 1/3 fraction is common, then there was plenty of water for a young Earth to gain by meteor impacts. The fact that this was seen around a white dwarf is also an indication that water-bearing asteroids may be common in planetary systems.

This isn’t conclusive proof that Earth’s water did in fact come from meteor impacts, but it adds to a growing pool of evidence supporting that model.