Rosetta Data Tells Us Earth's Water Likely Didn't Come from Comets
Scientists have long thought that a bombardment of comets billions of years ago may have originally brought water to Earth. Now, data from the Rosetta spacecraft, which is currently studying comet 67P, indicates that comet water is distinctly different from Earth's water, lending credence to alternate theories.
The spacecraft has been studying the comet since August, and its research was significantly bolstered by data from the Philae lander which made history last month when it became the first spacecraft to land on a comet. However, the lander soon ran out of batteries after a troubled landing, and so the data in question was gathered by Rosetta's Rosina instrument, consisting of two mass spectrometers that analyze the gas emitted from the surface.
In order to determine whether Earth's water is comparable to comet water, Rosina analyzes the ratio between "heavy" and "light" water. "Heavy" water refers to water in which there is a larger than normal amount of a hydrogen isotope called deuterium. Heavy water has all the same physical properties as normal or "light" water, except that it is heavier in mass. Earth has a very specific signature ratio: three deuterium atoms per every 10,000 molecules of water. The Rosetta data found that comet water has a significantly higher ratio of heavy to light water.
Principal investigator for Rosina Kathrin Altwegg claimed that these results are definitive: "This ratio between heavy and light water is very characteristic. You cannot easily change it and it stays for a long time. If we compare the water in comets with the water we have on Earth, we can definitely say if the water on Earth is compatible with the water on comets."
According to the prevailing theories of Earth's beginnings, Earth contained water at the time of its formation, but that water later boiled away, which means that the water we see today came from an outside source. Altwegg and her co-authors claim in their study that, with comets eliminated as a possibility, asteroids are the most likely candidate, while there is also an outside possibility that Earth retained some of its original water in polar ice caps. Other experts are in disagreement about whether the study definitively rules out comets as Earth's water source, with some claiming that more analysis needs to be done before coming to a conclusion.