Earth's Water Didn't Come from Comets Like Rosetta's

Europe’s Rosetta spacecraft has found that water shedding from its host comet has a different chemical fingerprint than water on Earth, raising serious doubts that Kuiper Belt cousin comets were the source of baby Earth’s oceans.

Scientists have long wondered how Earth got its water. Any indigenous water molecules likely were lost to space during the planet’s hot and violent formation.

The leading theory is that the water came later, during a period roughly 800 million years after the solar system’s formation when the inner planets were being blitzed by asteroids, comets and other small bodies. It was during this time, known as the “late heavy bombardment” that a Mars-sized object is believed to have crashed into Earth, sending clouds of debris into space that later consolidated into the moon.

With their icy bodies, comets were an early first choice to be Earth’s water-bearers, an idea buttressed by studies three years ago when Europe’s Herschel spacecraft made chemical measurements of water in another Kuiper Belt transplant, Comet Hartley 2. Its water matched Earth’s perfectly.

Comet 67P/Churyumov-Gerasimenko, the subject of the ongoing Rosetta studies, tells a different story. Its water has three times more of the hydrogen isotope deuterium than Earth’s water, one of the highest concentrations ever measured in a solar system body, said University of Bern’s Kathrin Altwegg, lead researcher for one of Rosetta’s science instruments.

“That now probably rules out Kuiper Belt comets as bringing the water on the Earth,” Altwegg said.

Even a mixture of water from Hartley 2-type comets and those chemically similar to 67P would have too much deuterium to match what exists on Earth today.

The ratio of deuterium to regular hydrogen in 67P is so high “you need only a small fraction of these comets to (spoil) the Hartley 2 results if you mix them up to make terrestrial water,” Altwegg told Discovery News.

Additional measurements from other comets may shed more light on the mystery. The new findings, the first reported since Rosetta arrived at Comet 67P in August, increase the odds that asteroids were the source of Earth’s water.

“Today asteroids have very limited water, that’s clear. But that was probably not always the case. The late heavy bombardment was 3.8 billion years ago and at that time asteroids could well have had much more water than they have today. They have just lived in the vicinity of the sun for 4.6 billion years” and lost water due to heating, Altwegg said.

Related studies show the line between asteroids and comets is blurring. Astronomers have discovered comet-like jetting asteroids, tail-less comets that look like asteroids and asteroids with comet-like tails.

“Comets also are amazingly asteroid-like … in their non-ice components,” added astronomer Donald Brownlee, with the University of Washington and lead researcher for NASA’s Stardust mission, which in 2006 returned samples from a comet.

The Rosetta results also show that comets hailing from the Kuiper Belt, a region of icy bodies beyond Neptune’s orbit, formed at different distances from the sun and under different conditions.

That idea is consistent with results from the Stardust mission, which found large-scale mixing of materials formed in the inner and outer solar system.

Rosetta scientists and engineers meanwhile continue the hunt for the Philae lander, which last month touched down on the comet’s surface, bounced twice and finally came to rest at a still-to-be-determined location, most likely in a crater.

“One side of the lander appears to be in a hole. We see the legs sticking up in the air. We can see a bit of the sky. We also see an overhanging cliff-like structure, so it gives us an idea of what the local terrain is like,” said Rosetta project scientist Matt Taylor with the European Space Agency.

Read more at Discovery News