TIME Science space You Can Quit Thanking Comets for Your Water Comet 67P: Does this thing look like it could quench your thirst? ESA A new finding from the Rosetta spacecraft upsets a longstanding theory

There was no shortage of drama when the European Space Agencys probe Philae set down on a comet last monththe first such landing in history. First Philae bounced, then it bounced again, ending up with one of its three legs sticking up in the air, and in the shadow of a cliff that prevented its solar panels from recharging its batteries. For two days, the probe hurried to complete whatever science it could.and then everything went black.

But that hardly spelled the end of the mission. Philaes mother ship, Rosetta, has continued to orbit comet 67P/ChuryumovGerasimenko, as its been doing since August, taking measurements and images of unprecedented quality. And with nearly a year of close-up observations to go, Rosetta has already come up with one result, described in a new paper in Science, that chief scientist Matt Taylor, of the European Space Agency, labeled fantastic: Earths oceans, the scientists have concluded, were evidently not created by impacts from comets rich with water ice, despite earlier evidence to the contrary. We have to conclude instead, said lead author Kathrin Altwegg, a planetary scientist at the University of Bern, at a press conference, that the water came from asteroids.

Thats a big reversal from what scientists were thinking just a few years ago. Back in 2011, the European Herschel space telescope took a hard look at Comet Hartley 2 and determined that its own cache of water, detected as vapor boiling away as Hartley approached the Sun, had a chemical composition very similar to what we see on Earth. Its all H2O, but some of the H is a rare form of hydrogen known as deuterium, whose atoms carry not just a proton like the ordinary stuff, but a neutron as well. Water molecules made with deuterium are known as heavy water, and about three in a thousand water molecules on Earths surface are the heavy kind.

Measurements of Halleys Comet back in the mid-80s showed a deuterium-to-hydrogen ratio about twice that high, which argued against the idea that comets delivered water to a bone-dry Earth early in the Solar Systems history. But Halleys came from the Oort Cloud, a spherical swarm of proto-comets orbiting at the far edges of the Solar System. Hartley 2 came from the Kuiper Belt of comets, which lies just beyond Neptunenot exactly nearby, but a whole lot closer. Given what Herschel found at Hartley 2, it appeared that Kuiper belt comets are chemically different from those that hail from the Oort cloud. If so, our water could have cometary origins after all.

The new results from Rosetta say no: Comet 67P, which also comes from the Kuiper belt, has an even greater proportion of heavy water than Halleys and other Oort cloud objects. Even if significant numbers of comets do have Earthlike water, some clearly dontand even a relative few would have made Earths proportion of heavy water higher than it is. Its arguable that 67P is pretty much unique among its Kuiper Belt brethren in having so much deuterium. Thats not impossible, said Altwegg dubiously but.

If comets didnt bring us water, and if the Earth was too hot in its youth to hold on to what surface water it might have started out with, theres still one plausible water carrier. Today, said Taylor at the press conference, we know asteroids have very little water, but that was probably not always the case. The solar system was bombarded by asteroids early in its history, and if they were indeed wetter than they are now, that explains where the water in our oceans, in our seltzer bottles, in our bodies and everywhere else comes from.

Important as this new finding is, its likely to be only the first of many Rosetta will make as it rides along with 67P for the next year or so, watching carefully as the warming rays of the Sun bring the comet to life. Its a nice start to the science phase of the mission, Taylor said.

And if you think youve heard the last of the Philae lander, think again. Mission controllers are still trying to pinpoint Philaes precise location on 67Ps surface. That will allow scientists to do at least one more experiment: theyll send radio pings from Rosetta through body of the comet to bounce off Philae and back to Rosetta. By examining how the radio beams are altered en route, they will be able to figure out whether 67Ps insides are rock-solid or held together relatively loosely.

Locating Philae would also allow scientists to calculate whether the lander might be brought back from the dead six months from now. Its just possible, said Taylor, that a change in 67Ps orientation could bring Philae back into the sunlight, allowing its solar panels to recharge its batteries. If that happens, the prospects for extraordinary science from this already wildly successful mission will be even greater.

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You Can Quit Thanking Comets for Your Water