Data from Europe's comet-chasing Rosetta probe has cast further doubt on the theory Earth's oceans were seeded by icy space rocks pummeling our planet.
In its ten-year game of catchup with Comet 67/P, the craft has been using its Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) to sample stuff flying off the speeding cosmic boulder.
And it's found the water on the comet is subtly different from that on Earth.
"We knew that Rosetta's in situ analysis of this comet was always going to throw up surprises for the bigger picture of Solar System science, and this outstanding observation certainly adds fuel to the debate about the origin of Earth's water," beamed Matt Taylor, ESA's Rosetta project scientist.
The key differentiator is the amount of deuterium (a hydrogen isotope with an extra neutron in its nucleus) in the comet's water. Here on Earth a tiny percentage of water is "heavy" with deuterium, but the findings from Comet 67/P show the water has a different deuterium-hydrogen mix.
It's theorized that when the Earth formed about 4.5 billion years ago, it was too hot for water to exist in a liquid state. Many scientists believe the water that is now here came from moist comets crashing through the atmosphere and disposing their watery cargo on land. Arthur C Clarke proposed pushing the cosmic rocks onto a collision course with Venus to cool the planet for eventual colonization.
In order for this theory to work, however, nearby comets, which usually form in the Kuiper Belt beyond Neptune that encircles our solar system, should have the same water as we find here. But Comet 67/P, a child of Kuiper, contains five times as much heavy water, according to the results published in the journal Science.
So far scientists have sampled the water on 11 comets and only one of them, Comet 103P/Hartley 2, showed a similar deuterium-hydrogen ratio to our precious bodily fluids.
That comet, 103P, didn't form in the Kuiper Belt, but in the Oort cloud outside our solar system – giving hope to boffins who reckon Earth's water came from outer space.
The Rosetta scientists haven't ruled out the bombardment theory, either. And if comets weren't responsible for your bottle of Evian, there are plenty of asteroids whizzing around with smaller amounts of water on board that have similar deuterium-hydrogen ratios to Earth's water.
"This surprising finding could indicate a diverse origin for the Jupiter-family comets – perhaps they formed over a wider range of distances in the young Solar System than we previously thought," says Kathrin Altwegg, principal investigator for ROSINA and lead author of the paper.
"Our finding also rules out the idea that Jupiter-family comets contain solely Earth ocean-like water, and adds weight to models that place more emphasis on asteroids as the main delivery mechanism for Earth's oceans." ®
Don't get too excited, Tumblr: there is no suggestion snappy dresser Dr Taylor was wearing a controversial shirt this time.