One of the mysteries of the formation of the Earth is where it got its oceans from. One popular theory is that, in the early years after the Solar System‘s formation when the inner planets were heavily bombarded with leftover material (we can still see the results in the scarred, cratered face of the Moon), Earth would have been pelted with comets. According to the theory, water ice from these comets would have vapourised and condensed into the water we have today. Astronomers using data from the Herschel infrared space observatory have finally found evidence to support this theory.
Previous attempts to test the idea involved looking at comets through a spectrograph and analysing their water content. Water is a compound of Hydrogen and Oxygen, but these elements come in different isotopes (meaning that the individual atoms can have different weights even though they remain chemically identical). Most water on Earth is made with regular hydrogen, but about 150 molecules per million are made from a heavier Hydrogen called Deuterium. This ratio of water to heavy water makes for a handy signature to identify water which comes from the same source. Unfortunately, previous looks at comets found a much higher ratio that what is found on Earth, discounting them as a source for Earth’s water.
But the Herschell data is different because it looked at comet Hartley 2, a short period comet from the Kuiper Belt. The earlier attempts had studied longer period comets which all came from the Oort cloud, out beyond Neptune. The Kuiper belt lies in a much closer region of the Solar System, and the signature of the water from comet Hartley 2 is an almost exact match to that of Earth.
Of course, in science, no questions are ever truly settled, so it remains to be seen whether other Kuiper Belt comets yield the same result. But for now, it’s great news to have an old theory revitalised, because it offers hope to finally put away an old, unanswered question.