Where did Earth's water come from?
More than two-thirds of the planet's surface is covered in it, but the origin of the most precious resource we have has long been a mystery - until perhaps now.
The discovery of comets with water similar to that found here on Earth has given life to a theory once thought dead - that water came to Earth aboard comets.
"Although comets with their icy nuclei seem like ideal candidates, analyses have so far shown that their water differs from that in our oceans," researchers at the French National Center for Scientific Research (CNRS) said in a statement.
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Comets analysed in the past appeared to contain water with a very high ratio of deuterium to hydrogen - two to three times higher than water found on Earth - so it was assumed they couldn't have been the source.
Astronomers are able to figure out the contents of objects far away in space by analysing the light they reflect that reaches Earth.
But a comet that approached Earth in December was different. Comet 46P/Wirtanen - dubbed the 'Christmas comet' - had water just like Earth's.
46P belongs to a group of comets called hyperactive, which release more water as they approach the sun than other comets of the same size. 46P - and two other hyperactive comets analysed previously - had deuterium to hydrogen ratios just like water on Earth.
"Hyperactive comets, whose water vapour is partially derived from icy grains expelled into their atmosphere, have a deuterium to hydrogen similar to that of terrestrial water, unlike comets whose gas halo is produced only by surface ice," CNRS said.
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They're now proposing that atmosphere readings of past comets may not accurately measure the deuterium to hydrogen ratio.
"If this hypothesis is correct, the water in all cometary nuclei may in fact be very similar to terrestrial water, reopening the debate on the origin of Earth's oceans."
The research was published in journal Astronomy & Astrophysics.