Astronomers appear to have found the missing link that traces the water here on Earth and throughout our solar system back billions of years to when it formed in interstellar space!
By now, we've likely all paused for a moment to contemplate how the water we were drinking, washing with, or swimming in, was likely dinosaur urine at some point. However, have you ever wondered where that water came from originally?
Before Earth's water collected on the planet's surface, filling up the oceans and allowing life to kickstart on this world, it was all probably locked up in icy comets. Scientists figured this out by looking at the exact composition of the water here and comparing that to the water we see elsewhere in the solar system.
In science class, we're taught that water's chemical formula is H2O — two hydrogen atoms and one oxygen atom bonded together. However, the complete story (that we tend to learn later) is that in addition to that 'simple' water, there is also a small amount of naturally occurring 'heavy' water that goes along with it. Heavy water's chemical formula is HDO, where one of the hydrogen atoms in the molecule is replaced by an atom of deuterium (an isotope of hydrogen with an extra neutron). And there's something particularly neat about this. Since H2O and HDO require different conditions to form naturally, if you can find the ratio of H2O to HDO molecules in a sample of water, you have a good chance of tracing that sample back to its original source.
That's how scientists figured out that Earth's water probably arrived here due to comet impacts, billions of years ago. The ratio of H2O to HDO in Earth water and comet water is very similar. Additionally, astronomers have detected water among the gas and dust between the stars (the interstellar medium) and in the clouds of this stuff that eventually collapse to form stars, and also found that same or similar ratio of H2O to HDO.
This artist's impression illustrates how a cloud of interstellar gas and dust collapses to form a protostar with a disc around it, which eventually develops into a planetary system. Credit: ESO/L. Calçada
So, that's two pieces of the puzzle we have right there. However, where did the water in the comets come from? Is it the same water from interstellar space? The answer wasn't definitive. A lot goes on during solar system formation. It's possible that when a protostar ignites into a true star, the energy produced may destroy all the water that collapsed along with the gas cloud, reducing it to its component atoms. Only after the ignition would the water naturally reform from the hydrogen and oxygen over time. Still, it wouldn't be the same water that formed in interstellar space.
To find out for sure, we'd have to look back at our solar system in its infancy, when it was still a volatile protostar surrounded by a disk of planet-forming debris. We don't need a time machine to do this, though. Many baby star systems out in space are going through the very processes that our solar system would have been going through at that time.
One such protostar, V883 Orionis, located around 1,300 light years away in the constellation Orion, has now given astronomers the missing link to complete the puzzle and trace our water back to its origins.
This simulation of the night sky shows the location of protostar V883 Orionis (V883 Ori) in the constellation Orion. Credit: Stellarium
"It is known that the bulk of the water in the interstellar medium forms as ice on the surfaces of tiny dust grains in the clouds," Margot Leemker, a Leiden University astronomer who co-authored a new study on V883 Orionis, explained in an NRAO press release. "When these clouds collapse under their own gravity and form young stars, the water ends up in the disks around them. Eventually, the disks evolve and the icy dust grains coagulate to form a new solar system with planets and comets. We have shown that water that is produced in the clouds follows this trail virtually unchanged. So, by looking at the water in the V883 Ori disk, we essentially look back in time and see how our own Solar System looked when it was much younger."
As it turns out, V883 Orionis presented astronomers with a rare lucky break for solving this mystery. According to the researchers, getting a strong enough signal to read the ratio of H2O to HDO in the water surrounding a protostar over a thousand light years away is particularly difficult.
While there is certainly water in protoplanetary disks, much of it is locked up in comets and ice. However, there must be enough water in vapour form, so when the protostar's light shines through the gas, it picks up the telltale signs of the water's presence that can be read with telescopes here on Earth. However, in a typical protoplanetary disk, the only gaseous water tends to be located closer to the protostar, inside a boundary known as the 'snow line'. There, it is masked by the bulk of the disk and thus is next to impossible to read.
This artist's depiction of V883 Ori includes an inset showing the presence of both H2O and HDO molecules in the debris disk. Credit: ESO/L. Calçada/S. Sutherland
V883 Orionis is different, though. The research team said this protostar recently emitted a burst of energy that pumped extra heat into its planet-forming disk. This vapourized a good portion of the ice, pushing the snow line much farther out and presenting astronomers with a stronger signal to work with.
Taking advantage of this good fortune, the authors of this study used the Atacama Large Millimeter/submillimeter Array (ALMA), a powerful radio telescope array located at the European Southern Observatory in the mountains of northern Chile, to detect the water vapour around V883 Ori.
Computing the ratio of H2O to HDO in the protoplanetary disk's water, they found that it matched the ratio seen in solar system comets and in the water here on Earth.
"V883 Orionis is the missing link in this case," John Tobin, the National Radio Astronomy Observatory astronomer who led the research, said in an ESO press release.
"The composition of the water in the disk is very similar to that of comets in our own Solar System. This is confirmation of the idea that the water in planetary systems formed billions of years ago, before the Sun, in interstellar space, and has been inherited by both comets and Earth, relatively unchanged."
So, next time you have a glass of water or wash your hands, pause to think about that. That water may have been dino pee at some point, but it likely formed in the dark spaces between the stars well over 4.6 billion years ago, before the Earth or the Sun even existed!