Moisture is a nice feature, but hardly exclusive it seems (Image: Stocktrek/Getty)

Who would have guessed that Earth’s oceans are older than the sun? Much of the water on our planet and around the solar system started out as tiny grains of ice floating in interstellar space. The discovery provides important clues about not only the make-up of our solar system, but also what planets around other stars might be like.

“Water is an essential ingredient that pretty much all known forms of life on Earth need to flourish,” says Ilsedore Cleeves at the University of Michigan in Ann Arbor. “To understand where water came from tells us a little bit about how common life is in the universe.”

All water in the solar system – whether on planets, in comets or meteorites, or icy moons like Europa – contains a certain amount of deuterium – an isotope of hydrogen that has an extra neutron attached.


Ripped apart

Ice found between stars is even more deuterium-rich, so it was long suspected that it was the source of our solar system’s heavy water. But the young solar system was so violent and full of radiation that any incoming ice should have been ripped apart, recombining later into water – according to this picture.

But when Cleeves and her collaborators built a model of the early sun, they found that this explanation didn’t stand up. After their model scrapped all of the interstellar ice, they found that oxygen was being locked up in frozen carbon monoxide. And there wasn’t enough ionised, deuterium-rich hydrogen being produced either. In short, the nascent solar system wouldn’t have had the ingredients for water with the high levels of deuterium we see.

Instead, interstellar ice must have made its way into planets, moons and comets intact. Cleeves and her colleagues calculate that as much as half the water in Earth’s oceans and possibly all of the water found in comets came from this ancient source.

Ice isn’t the only thing out there in interstellar space – there’s also organic material, says Fred Ciesla, a planetary scientist at the University of Chicago. If this interstellar stuff went into the formation of our solar system’s planets, then it probably forms part of planets around other stars too – boosting the chances that many planetary systems formed with the raw ingredients for life.

“It provides the opportunity for organic materials, and the things that are important to the formation of life, to at least be accessible to all planets out there,” says Ciesla. “Whether or not it forms into aliens and little green men and women is a whole other story.”

Journal reference: Science, DOI: 10.1126/science.1258055