New Data from Rosetta Spacecraft Sheds Light on Origins of Earth's Oceans

ESA's Rosetta spacecraft snapped this selfie with comet 67P/C-G. (Rosetta/ESA)
ESA's Rosetta spacecraft snapped this selfie with comet 67P/C-G. (Rosetta/ESA)

After several months of analysis of Comet 67P/Churyumov-Gerasimenko, the European Space Agency's Rosetta spacecraft has yielded some intriguing, and maybe unexpected, results. This data is refueling a long-running debate in the scientific community about a matter closer to home: the origin of Earth's oceans.

It has long been debated exactly how Earth acquired its oceans. Were the waters of our oceans part of the Earth's original stock of materials, or was it added later? Was it a combination of these? It is thought that any water present in the original formation of the Earth should have boiled away due to Earth's hot, molten-rock temperatures--in which case some, if not most, of the ocean's waters must have arrived after Earth cooled.

After four and a half billion years, how could we possibly tell where the water came from?

The answer is in chemistry—particularly the chemistry of the hydrogen contained in water molecules. Hydrogen comes in different forms, or isotopes, the simplest of which contains a single proton in its nucleus. The hydrogen isotopes deuterium and tritium each contain a proton, plus one and two neutrons, respectively.

The proportion of water molecules containing deuterium atoms compared to "normal" water molecules possessing only hydrogen is a key ratio that can be used to match one sample to another—for example, matching the hydrogen-deuterium ratio in Earth's ocean water to that sampled from a particular comet, sort of like matching the DNA found at a crime scene to an individual suspect. In this case, the "crime" being investigated is the appearance of Earth's oceans—so we can probably be lenient on any suspects we match.

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The hydrogen-deuterium ratio in a sample of water is an indicator of the conditions that prevailed when the water formed, and so varies depending on where it originated.

After sampling the water chemistry of 11 different comets, including the most recent measurements by Rosetta, some unexpected results have surfaced. Of the sampled comets, only one of them matched the chemistry of Earth's ocean water: the comet 103P/Hartley 2, a Jupiter-family comet. A Jupiter-family comet is found within the orbit of Jupiter, circling the sun in less than 20 years, a class of comet once believed to have originated in the Kuiper Belt, beyond the orbit of Neptune. So, Hartley 2's contribution to the debate, on its surface, suggested that Earth's ocean water, at least in part, came from Kuiper Belt comets.

Rosetta, however, has measured the hydrogen-deuterium ratio of 67P/Churyumov-Gerasimenko, also a Jupiter-family comet, as not only three times higher than that of Hartley 2 and Earth's water, but higher also than samples obtained from comets that originated in the Oort Cloud, the vast shell of distant comets far beyond the Kuiper Belt. This suggests that Jupiter-family comets may have more diverse origins than originally thought, composed of members that came from different regions of the solar system. Yet, if the waters of Earth were delivered by a mixture of comets of different lineage, its chemistry should reflect that fact.

The new data from Rosetta has not only put into question the extent to which comet collisions may have contributed to our oceans, it has strengthened an idea that some, if not much, of Earth's ocean water came not from comets, but from a source much closer to home. Measurements of the water hydrogen-deuterium ratio in samples of meteorites that originated in the Main Asteroid Belt have also shown a positive match to Earth water chemistry, fingering asteroid impacts as a potential major culprit in the watering of our planet.

As the comet and spacecraft glide closer to the Sun in the months ahead, reaching a closest and warmest approach to the sun next August, Rosetta will continue to gather data as the comet heats up, spewing materials into space that have been frozen in it since the earliest times of our solar system's formation.