Just how old is our water?

A new study out of the University of Michigan and University of Exeter reveals that the water in our solar system - on the planets, moons, and comets - predates the formation of the sun.

Earlier research had investigated the introduction of water to our own planet, a contentious topic among astronomers. While many scientists agree that water was actually delivered to Earth after its formation, competing theories exist as to how the water got here from elsewhere in the solar system. Some argue that water-rich comets bombarded our atmosphere during Earth's early years. Others say that oxidation of a hydrogen-rich atmosphere produced the planet's water.

Ilsedore Cleeves, astronomy PhD student at the University of Michigan, chose to go even farther back in time. Instead of asking how water came to our planet, Ms. Cleeves and her colleagues looked at how water came to our solar system.

"We were trying to understand where the water in comets and the water in meteorites came from prior to that delivery stage," Cleeves told the Monitor.

The research group established two possible scenarios. One is that some ice survived the violent formation of the solar system, which was then incorporated into planetesimal bodies. The other is that the solar nebula, the swirling "dust disk" that formed the sun, was capable of producing the water levels we have today without any inheritance of water ice from an earlier period.

Cleeves's team couldn't go back in time to observe our solar system's dust disk, so instead they observed stars that are forming today and modeled the conditions of their disks to calculate which scenario was right.

Using samples of "heavy water" ices, which contain high levels of a heavy isotope of hydrogen called deuterium, Cleeves and her colleagues worked backward to see if conditions in the solar nebula could have yielded the current measurements of heavy water in the solar system.

It turned out that the first scenario, where the solar nebula produced our water's current levels of deuterium on its own, had to be crossed off the list.

"The punch line was that heavy water was just not produced abundantly," says Cleeves. "It didn't even sort of tip the scale at all."

Cleeves says that no matter which samples of deuterium you look at, all of the values are higher than what you would expect if they had been created following the formation of the sun.

From this discovery, the research team could determine by process of elimination that a large fraction of our water - up to half of the water in our oceans - predates the solar system by about a million years. And if water ices survived the turbulent formation of our sun and planets, these ices should be present in other planetary systems.

"By identifying the ancient heritage of Earth's water, we can see that the way in which our solar system was formed will not be unique, and that exoplanets will form in environments with abundant water," Professor Tim Harries, a member of the research team, said in a release. "Consequently, it raises the possibility that some exoplanets could house the right conditions, and water resources, for life to evolve."

Many scientists regard water, with its abilities to hold heat, dissolve almost anything, remain liquid over a wide range of temperatures, and, when liquid, to easily transport nutrients, as an essential ingredient of life.

"Certainly there might be other possible recipes for life in other systems," says Cleeves. "But as far as we know, every single form of life we have required water."

Cleeves and her team are now looking to study water ices in planetary disks, another critical piece of the puzzle for understanding water's journey across space and time. Cleeves says while they have direct measurements of water in older interstellar environments as well as in already-formed solar systems (such as our own), scientists are still missing measurements from the in-between period, when a disk has not yet formed its planets.

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"That would be pretty amazing if it's actually found," says Cleeves "Because it would be the first heavy water detection in the materials before they go into planets."

The results of the study appear in Thursday's issue of Science.