


One of the arguments for water’s prevalence on Earth is that the planet was bombarded with planetesimals, asteroids, and comets very early in its history. Since we know that comets mostly consist of frozen water, it was theorized that these early objects might have played an important role in the process. The Rosetta orbiter’s analysis of the water on comet 67P suggests this isn’t true, at least not to any significant degree.

ESA infographic showing the deuterium/hydrogen ratios of various stellar bodies

ESA infographic showing the deuterium/hydrogen ratios of various stellar bodies, including comet 67P (click to zoom in)

Scientists can determine the “fingerprint” of a given water sample by measuring the amount of deuterium in a water sample. Ordinary water consists of two hydrogen atoms and a single oxygen atom, where each hydrogen atom has one proton and one electron. Deuterium, in contrast, is a stable isotope of hydrogen which contains one proton, one neutron, and one electron. On Earth, deuterium is rare — but previous measurements of past comets, including Halley, Hyakutake, and Hale-Bopp have all pointed towards substantially higher deuterium ratios. The new data from Rosetta confirms this trend. Like previous comets, 67P contains far more deuterium than is present on Earth or in the soil samples brought back from the Moon.

Most of the comets previously measured are thought to have originated in the Oort Cloud. 67P, in contrast, is from the Kuiper Belt. The Kuiper Belt is thought to extend 30-50 astronomical units (AU) from the Sun — so, pretty close-by to Earth — whereas the Oort Cloud may reach as far as 50,000 AU.

According to the researchers, the ratio on Rosetta is even higher than that on previous comets. Where previous cometary measurements had suggested a ratio of deuterium 2x higher than that of Earth, Rosetta’s ratio is more than 3x higher. Comets may still have played a part in bringing water to our planet, but the overall contribution must have been very small.

Apollo 8, Earthrise

The famous Earthrise photo, taken aboard Apollo 8. As you can see, Earth is pretty watery.

So, where did Earth’s water come from?

If the water on Earth didn’t come from cometary bombardment, where’d it come from? Analysis of the oldest rocks on Earth suggest that moisture was present virtually from the start, but there are substantial questions as to its genesis.

The fact that the Earth and Moon share the same deuterium ratio is part of why the latter is believed to have been created by some kind of catastrophic cleaving. Proposed explanations for Earth’s abundance of water range from the biological processes of primitive microorganisms, to delivery courtesy of Theia, the hypothetical protoplanet that slammed into us and created the Moon. It’s also possible that Jupiter once migrated inwards, towards the Sun, and dragged icy asteroids in its wake — thereby setting the stage for substantial water accretion from non-cometary sources.

In a case like this, knowing where water didn’t come from is as valuable as knowing which bodies within the Solar System share common traits today. The Philae lander may have only run for 60 hours, but its data continues to contribute to our understanding of the universe. This new data isn’t exactly a slam-dunk against the theory of panspermia, which generally holds that life (or molecules vital to the eventual formation thereof) were distributed to Earth from another source, but it does imply that there’s no currently known smoking gun that would tie Earth’s water supply to a vital extraterrestrial source.

Now read: The Earth and Moon, as seen from elsewhere in the universe