Where did Earth's water come from? Comets? Asteroids?

New data from the Rosetta spacecraft exploring Comet 67P/Churyumov-Gerasimenko show that comets once thought responsible for seeding Earth with water might not have delivered most of the planet's water after all. The new finding is giving scientists a more nuanced view of the solar system and its plethora of cosmic bodies.

An instrument called ROSINA on the European Space Agency's Rosetta has found that the molecular makeup of the water on Comet 67P/C-G is very different from the water found in Earth's oceans. This deuterium-to-hydrogen ratio throws a hitch into the theory that comets from Comet 67P/C-G's region of space brought water to the Earth not long after the solar system formed, Kathrin Altwegg, principal investigator for ROSINA, said. [See comet images from Rosetta]

If even a small number of comets like 67P/C-G impacted Earth in the early days of the solar system, it still would have greatly changed the molecular composition of the planet's water today, according to Altwegg. Therefore, it seems unlikely that these kinds of comets brought water to Earth. Altwegg thinks it's more probable that asteroids brought water to 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 Earths water," Matt Taylor, ESAs Rosetta project scientist, said in a statement.

While asteroids are dry, rocky bodies now, it's possible that these space rocks were water-rich during the early days of the solar system. Altwegg thinks that asteroids may have bombarded the Earth about 800 million years after the formation of the solar system, bringing water to the early planet once it cooled after formation.

Scientists have measured the D to H ratio in meteorites from asteroids and found that the water content in these tiny samples is comparable to Earth's water composition, Altwegg said.

Thanks to Rosetta, scientists now think that Kuiper Belt comets found orbiting the sun beyond Neptune are much more diverse than expected. In other words, not all comets are the same.

Although Comet 67P/C-G has a 6.5-year orbit that brings it near Jupiter, researchers still think that it originated in the Kuiper Belt.

Scientists measured the comet Hartley 2's D to H ratio in 2011 and found that it was very close to that of Earth's, leading scientists to conclude that comets like Hartley 2 (a Kuiper Belt comet) may have delivered water to the early planet. But because the ratio for Comet 67P/C-G is so off, it doesn't seem like the comets from the Kuiper Belt could have seeded the planet with water.

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Rosetta Spacecraft's Comet Water Discovery: What It Means for Earth