The origin of Earth’s water and, hence, how old it actually is, is one of the biggest open questions in astronomy. Water molecules originally form in cold interstellar clouds of dust and gas where stars form. But is this pristine water the same that we’re drinking today, or have these molecules been destroyed and reformed on their journey here? Researchers have now found a new piece of this puzzle.

To understand how old the water in our little rock is, we first need to start where the water journey also begins…

This diagram illustrates how a cloud of gas collapses to form a star with a disc around it, out of which a planetary system will eventually form. Water molecules can be found in all these stages, but are these the same molecules, or have they been destroyed and reformed in this process?

Credit: ESO/L. Calçada

Tiny droplets in an ocean of matter

In the cold, dense clumps of gas and dust that form molecular clouds, the first step of water creation occurs. Oxygen atoms sit on grains of dust within the cloud, and interact with free-floating hydrogen atoms to form water molecules. The water that is formed isn’t liquid, but remains frozen on the surface of the dust grains.

In the next phase gravity slowly takes over and the clump of molecular gas gets denser and denser until it collapses, forming a ‘proto-star’. A lot of heat is generated from this collapse and further transferred to the surrounding material. As the dust grains heat up in the central parts of the cloud, the frozen water that coats them sublimates, instantly turning to gas without first becoming liquid. But further out, the bulk of the water reservoir remains frozen. This water ice is harder to detect than gaseous water, but it’s also less prone to being altered by chemical processes. Water thus becomes the second most abundant molecule (after molecular hydrogen) in this warm cloud around the baby star, which contains 10 000 times more water than the oceans on Earth combined.

As the matter in the central parts of the cloud collapses, it starts to spin faster and flattens out into a disc around the proto-star. Most of the water in this ‘proto-planetary disc’ remains frozen around dust grains at this stage. Dust grains are in that sense something like the Guardians of the Water History.

Comets are the next step in the journey of water. These icy bodies form out of dust grains in the proto-planetary disc, and they can potentially deliver water to planets. But how can we be sure that water molecules in comets are chemically the same as those originally formed in the parent cloud? The way to trace the path of water is through something called heavy water.

This diagram follows the path of water from clouds where stars are born all the way down to planetary systems. Step 1: water molecules first form in giant clouds of gas and dust. Oxygen atoms, shown here as blue circles, sit on dust grains. When they interact with free-floating hydrogen atoms (yellow) they form water molecules. Some of these hydrogen atoms (gray) are actually a heavier isotope called deuterium. Step 2: as the star-forming cloud collapses, the central region heats up, sublimating some of the water ice into gas, which makes it easier to detect. But the bulk of the water remains frozen on dust grains. Step 3: due to the initial rotation of the cloud, a flat disc forms around the star. Dust grains start to coalesce and form larger solid bodies. Step 4: a planetary system is now formed, containing planets, comets and asteroids orbiting around the central star.

Credit: ESO/M. Duffek

What’s heavy water and where does it come from?

Just like normal water, heavy water comprises an oxygen atom and two hydrogen atoms. But the hydrogen in heavy water is not really hydrogen: it’s deuterium, a heavier isotope of hydrogen with one proton and one neutron instead of just one proton, hence its name.

The puzzling thing is that deuterium is only produced in the first few seconds after the Big Bang, and in very small amounts: there are about 100 000 times more normal hydrogen atoms than deuterium ones. If this fixed abundance of deuterium were to be uniformly distributed in water molecules, the expected ratio of heavy to normal water would be much lower than what we actually observe in the Solar System today.

It turns out that there are many more deuterium atoms on the surfaces of dust grains than expected from the deuterium/hydrogen ratio in the Universe. Since water molecules first form on dust grains, this high ratio of heavy to normal water acts like a chemical fingerprint that allows us to trace the subsequent journey of water. In other words: by measuring the relative amount of heavy and normal water in different environments – molecular clouds, proto-planetary discs, comets, planets – we can figure out if water molecules were altered along this trail.

Single or double?

Heavy water comes in two flavours so to speak: semi-heavy water, or HDO, where only one of the two hydrogen atoms is deuterium, and doubly-deuterated heavy water, or D₂O, which contains two deuterium atoms.

Two years ago, a group of astronomers detected HDO in the proto-planetary disc around V883 Ori, a proto-star 1300 light-years away. Using the ESO partnered Atacama Large Millimeter/submillimeter Array (ALMA), they measured an abundance of HDO very similar to that of comets in our Solar System. Assuming that this proto-planetary disc is not too different from the one that birthed our Solar System, this finding provided key evidence that the water we see today is the same pristine water formed in the Sun’s parent cloud.

But this wasn’t definitive proof, because HDO molecules can still be destroyed by the star’s radiation and then reform, thus somewhat blurring its past history. This is much less likely to happen with D₂O, which is thus a more robust tracer of water’s history.

Now, the same team of astronomers have detected D₂O in the disc around the V883 Ori star using ALMA. Their results, published in a Nature Astronomy paper led by Margot Leemker, an astronomer at the University of Milan, Italy, show a high abundance of D₂O. This abundance is similar to that found in molecular clouds and one comet [1], suggesting that the water we find in the current Solar System is the same water that existed long before the Sun had formed.

How water actually makes it to planets, including Earth, is a complex issue that needs more research, but this new finding is a key piece in the puzzle. So the next time you enjoy a refreshing glass of water, take a second and consider this: at least some of the water you are tasting not only passed through the dinosaurs but in fact predates the Earth itself and even the Sun!

Notes

[1] While the abundance of HDO has been measured in several comets, we only have D₂O measurements in one.