Rosetta Comet Mission Reveals Clues About the Origin of Earth’s Water

Rosetta took this picture of comet 67P/Churyumov-Gerasimenko from a little more than 12 miles away. Philae’s original landing site is in the upper right corner. ESA/Rosetta/NAVCAM

One of the theories for how Earth became a watery world suitable for life now faces doubt, according to the latest results from the Rosetta spacecraft that’s now orbiting a comet 326 million miles away.
Countless comet impacts were thought to have delivered water to Earth not long after the planet formed 4.6 billion years ago. But new measurements from Rosetta, which is studying comet 67P/Churyumov-Gerasimenko, reveal that the chemical signature of water in the comet is nothing like what’s found in Earth’s oceans.

Read More:

Incredible New Photos Taken From the Surface of a Comet

This Is What It Sounded Like When We Landed on a Comet

What’s Next for the Rosetta Mission and Comet Exploration

The discrepancy suggests that comets did not bring water to Earth and that the more likely water source was asteroids, says planetary scientist Kathrin Altwegg of the University of Bern in Switzerland.


The results, which are among the first from the European Space Agency’s Rosetta mission, were published today in the journal Science.
The young Earth was a hot place—so hot that most of its surface water evaporated. At the time, about four billion years ago, the solar system was swarming with asteroids and comets. They pelted Earth’s surface, prompting scientists to hypothesize that maybe it was these objects that helped supply Earth with its oceans. Because comets are known to contain water, they seemed a likely source.
If this were the case, then the chemical signature of Earth’s water would match what is found on comets. Water, of course, is H₂O: two parts hydrogen and one part oxygen. It can be made from regular hydrogen, which consists of a proton and an electron, or from a type of hydrogen called deuterium, which has an added neutron. If Earth’s water came from comets, then the ratios of deuterium to hydrogen would be the same in both Earth and comets.
But over the past couple decades, telescope observations of several comets—and a spacecraft flyby of another, comet Halley—turned up deuterium numbers much higher than those found on Earth, causing scientists like Altwegg to doubt the cometary origin of water. These comets all came from the Oort cloud, a collection of icy bodies at the outer fringes of the solar system.
Then came a surprise in 2011. The Herschel space telescope pointed at a different comet, named Hartley 2, and discovered that the deuterium-to-hydrogen ratio is the same as Earth’s. Another recent analysis of comet 45P/Honda-Mrkos-Pajdusakova also revealed a similar signature. Unlike the previous comets, these two belong to a group called the Jupiter family, named so because their orbits are influenced by Jupiter. These comets come from the Kuiper belt, a group of objects just beyond the orbit of Neptune but closer than the Oort cloud.
Now it seemed possible that water could have come from comets after all—at least, comets from the Kuiper belt. The comet Rosetta is studying, comet 67P is also a Jupiter-family comet from the Kuiper belt, and one of the mission’s goals is to help determine whether comets like 67P could have produced Earth’s oceans. The answer, however, appears to be no. Rosetta’s measurements revealed a deuterium-to-hydrogen ratio about three times higher than that found on Earth, even higher than in the Oort cloud comets.
The assumption is that comet 67P represents at least a fraction of Kuiper-belt comets, but even a small fraction of comets with such a high deuterium-to-hydrogen ratio is enough to cast doubt on the cometary origin of water, Altwegg says. Instead, water seems more likely to have come from asteroids.
Although asteroids don’t have much water today, having been baked dry by the sun over the last four billion years or so, they likely had more in the past, she says. Previous analysis of the hydrogen in meteorites that were once part of asteroids has revealed that on average, the deuterium-to-hydrogen ratios are similar to Earth’s.
What would be better, Altwegg says, is to make direct measurements of water in an asteroid. Although most asteroids are rather inert, over the last decade astronomers have discovered more than a dozen active asteroids that spew gas and dust, providing an opportunity to measure the hydrogen in asteroids.
The Rosetta spacecraft is just beginning its primary science mission, after having sent its Philae spacecraft to make an unprecedented landing on comet 67P last month. Over the next several months, as Rosetta escorts the comet toward the sun and the comet begins to eject more gas, dust, and water, scientists will be able to make more detailed measurements. For example, they will learn if the deuterium-to-hydrogen ratio changes in different parts of the comet.
WIRED.com