Rosetta latest: water brought to Earth by asteroids, not comets

The water on planet Earth probably didn’t originate on a comet, but was delivered here by asteroids instead, Europe’s
comet-chasing Rosetta space probe has just revealed.

Last month Rosetta scientists pulled off the amazing feat of landing a probe on the surface of comet 67-P, some 300 million kilometres from Earth.

One of  Rosetta’s key objectives was to get to the bottom of a long-running space conundrum: how did Earth become the blue planet whose oceans of water allowed the origin of complex life?

The most recent observations of a comet’s water ice had led scientists to conclude that it closely resembled the water on Earth. The ratio of hydrogen atoms and a heavy isotope of hydrogen called deuterium in the water of comets appeared to match that of Earth.

This suggested the water on Earth was carried to our barren rocky planet by comets about 800 million years after it formed during a period called the “late heavy bombardment”.

But data just analysed from Rosetta suggests that the deuterium-to-hydrogen ratio in 67-P’s water is about twice that of Earth’s and matches earlier measurements of other comets like Halley.

“We have to conclude terrestrial water was probably brought by asteroids rather than comets,” said Prof Kathrin Altwegg from the University of Bern who analysed the data.

The results from Rosetta also reveal there is much more chemical diversity on comets from the same part of the Solar System.

This might make it a more confusing place to understand, but its the sort of thing that makes scientists happy because there are many more mysteries to solve.

A good thing, given Rosetta and its lander Philae have only just started analysing comet 67-P.

“It is a real treasure chest to explore how our solar system looked like 4.6 billion years ago,” said Altwegg.

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