The water that makes up most of our planet’s surface formed in the cold darkness of insterstellar space long before our Sun flared to life, according to a recent study.
Tracking the ratios of heavy water to regular water in the ice of a distant newborn star system sheds light on how water forms — and how it ends up in the oceans of a planet like Earth. And it turns out that star systems inherit their water from interstellar dust clouds, as part of a starting package of chemicals,which means watery planets like ours should be pretty common in the universe. National Radio Astronomy Observatory astronomer John Tobin and his colleagues published their work in the journal Nature.
Water, Water Everywhere? Kind Of
Around 1,300 light years away, the star V883 Ori looks a lot like our Sun would have looked in its wild younger days, back when it was footloose and fancy-free and not playing host to eight planets and a swarm of smaller objects. V883 Ori is a few billion years younger than our Sun, and it still hangs out in the center of a lively whirl of gas, dust, and ice, which is only beginning to coalesce into the clumps of material that will eventually be planets. If you want to understand how our Solar System formed, and how it grew up, V883 Ori is a good place to look. And Tobin and his colleagues, in particular, wanted to know where the water that eventually filled Earth’s oceans actually came from.
Scientists generally agree that water arrived on our little ball of rock aboard comets and asteroids that collided with the young planet early in our Solar System’s history, when our cosmic neighborhood was a lot more eventful. They don’t yet agree on whether most of that water arrived via asteroids or via comets. But Tobin and his colleagues wanted to know where the water came from even before that. Where did the water molecules that now make up about 60 percent of our bodies actually form?
According to Tobin and his colleagues’ recent study, the water in our oceans, our glasses, and our bodies first formed as an icy rime on grains of dust in chilly interstellar space, long before our Sun flared to life.
Not All Water is Created Equal
Tobin and his colleagues pointed the radio dishes of the Atacama Large Millimeter/submillimeter Array at V883 Ori to measure the radio waves coming from its accretion disk: all the gas, dust, and ice circling the young star and slowly coalescing into planets. Every atom or molecule radiates, reflects, and absorbs specific wavelengths of radiation, like a chemical fingerprint; by breaking up the light from an object into its individual wavelengths, astronomers can see exactly what sorts of molecules it contains.
In particular, they were interested in water, which is normally hard to see when it’s frozen. But V883 Ori is apparently a rather temperamental young star, and it’s in the middle of an outburst, flaring brighter and hotter than usual — so water in its accretion disk is heating up and sublimating straight from ice to water vapor, which ALMA can see pretty easily.
“Astronomers still debate whether all of them go through this, but the prevailing theory is that most stars go through some enormous outbursts like this during the course of their formation,” Tobin tells Inverse. “We don't know how often they happen; we're still studying that. But it's possible that a star like our Sun went through something like this in its formation.”
ALMA’s instruments measured how much of the water in V883 Ori’s disk was a weird type of water, called “semiheavy water.” A normal water molecule is two hydrogen atoms stuck to one oxygen atom — one of the few chemical formulae most people recognize on sight. Semiheavy water is water, but one of the hydrogen atoms is a specific type of hydrogen called deuterium, which has a neutron along with the typical proton and electron that make up a normal hydrogen atom Thanks to the neutron, a deuterium atom is a tiny bit heavier. Full-fledged “heavy water” has two deuterium atoms and an oxygen atom.
About 1 in every 6,420 hydrogen atoms in the universe ends up being deuterium, according to the International Atomic Energy Agency, so you’ve probably breathed and drunk and otherwise consumed your share of it. It’s rare but not dangerous, and it turns out to be a handy marker for astronomers to trace the origins of water.
“Water is so fundamental to everything on earth. And I think it's sort of amazing that we're able to trace the origins of water back to before our Solar System even formed,” says Tobin. “If you have a single glass of water, only a tiny fraction of it has deuterium in it, but that seemingly small amount really enables us to read the story of water and where it came from, tracing it back from here on Earth to comets, disks, protostars, and the interstellar medium.”
Some Cool Details (And Some Warm Ones)
According to physicists who use computer simulations to study how chemical reactions happen out in the darkness of interstellar space, water probably forms thanks to chemical reactions on the surface of dust grains in what’s called the interstellar medium: the sparse gas and dust that drift through the space between stars, sometimes clumping together into denser clouds. Through those chemical reactions, the dust grains end up with a thin coating of ice: some water, some organic molecules like methanol, and some other things like carbon dioxide — and even building blocks for life, like ethanolamine.
“So things like methanol, and things that form like methanol does on the surfaces of dust grains as ices [...] might be inherited relatively unchanged from the interstellar medium to the disks,” says Tobin.
And deuterium forms most easily in cold places.
“You can’t form water with the amount of deuterium that we see just from normal chemistry; it has to happen when it’s cold, and it has to happen on the surfaces of dust grains,” says Tobin. “And that's why we think that this deuterium-to-hydrogen ratio is a tool that we can use to trace back the origins of water. Because there's really only one easy way to form water with this much deuterium.”
The ratio of semiheavy water to regular water in V883 Ori’s accretion disk matches the ratio that other astronomers have measured in the clouds of material around protostars — even younger objects that haven’t quite finished becoming stars yet. And the same ratio of semi-heavy water to regular water shows up in comets from the outer reaches of our own Solar System.
In other words, the water that’s floating around in the cloud of material that collapses to form a star is the same water that’s swirling around in a young star’s accretion disk and later flying around on comets in a mature solar system. Planets seem to inherit their water from the clouds of interstellar material that first formed their stars.
There’s less deuterium here on Earth, and in some comets that have passed through the inner Solar System; the ratio of semiheavy water to regular water is much lower than it is around V88 Ori, in the Oort Cloud, or in the clouds of material around protostars. Why? Heat speeds up the chemical reactions that can strip deuterium of its neutron, turning it into plain, boring hydrogen. So we’d expect to see less deuterium in water that’s spent a lot of time basking in the heat of the Sun — even if it started its journey in the frigid void of interstellar space.
