Rosetta's first results: Comets unlikely to have brought water to Earth

The European Space Agency’s Rosetta mission captivated the world by touching down a lander, Philae, on a comet for the first time on Nov. 13, after a 4 billion-mile, decade-long journey across the asteroid belt.

On Wednesday, researchers released the first results of a chemical analysis from the comet that may offer hints about one of science’s most baffling questions: How the universe gave our planet its most precious resource, water.

Since any water present on Earth in its earliest days would have evaporated due to the planet’s high temperatures, scientists have been debating for decades whether — and how — water might have been brought here from some other source, such as comets or asteroids.

“The question is, who brought this water? Was it comets, or was it something else?” said Kathrin Altwegg of the University of Bern in Switzerland. Altwegg is the principal investigator for the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), a sensor-laden instrument on Rosetta that is meant to determine the comet’s composition.

Comets — sometimes called “dirty snowballs” by scientists because of their jumble of ice, dust and gasses — remain largely the same after they’re formed. This means they act as de facto time capsules from the birth of the solar system. The goal of Rosetta’s far-flung journey is to track 67P as it moves toward the sun and examine what it is made up of.

“It’s a real treasure chest to explore how our solar system looked like when it was formed 4.6 billion years ago,” Altwegg said Tuesday in a conference call with reporters.

ROSINA’s evidence from Comet 67P/Churyumov-Gerasimenko seems to point away from this type of comet as a cosmic water-bearer, Rosetta scientists said in an article published Wednesday in the journal Science. They said the water ROSINA tested on 67P has a hydrogen-heavy composition, which is different than the makeup of Earth’s oceans.

ROSINA’s mass spectrometers measured the amounts and weights of hydrogen in the water on 67P. Scientists were then able to calculate the ratio of deuterium, a heavy form of hydrogen, on the comet. They determined that this comet’s water was far heavier than the hydrogen-light water found in the Earth’s oceans.

That means it’s unlikely water was brought to earth from a Jupiter-family comet such as 67P, which is part of the Kaipur belt of comets scattered between the orbit of Neptune and the sun.

“The terrestrial water was probably brought by asteroids more likely than comets,” Altwegg said.

Asteroids, like comets, orbit the sun and can sometimes stray close to Earth, but they’re made up of different materials. Asteroids are chunky rocks with fewer compounds — like large amounts of ice, for example — that vaporize when heated. As such, asteroids only rarely develop the glowing tails that illuminate the night sky.

Altwegg also said that scientists were shocked in 2011 when the ESA’s Herschel Space Observatory discovered that the water composition of another comet, 103P/Hartley 2, was extremely similar to that of Earth’s oceans — which brought back into popularity the idea that a comet could have brought water to Earth.

But ROSINA’s observations are in line with what the scientists originally expected, leading them back to the asteroid theory.

What they also learned about 67P is that it was probably formed at extremely low temperatures, and that it most likely “preserved original material from the very, very earliest time of the solar system,” Altwegg said.