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What are comets made of? (Image: CESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)

COMET might be the proper term, but “dirty snowball” would be more descriptive. Travelling in large, elliptical orbits that periodically bring them from the solar system’s outer reaches to its inner regions, these mountain-sized lumps of ice and rock boil off when bathed in the sun’s rays, giving rise to their trademark tails.

The wonder – and fear – these cosmic wanderers have inspired over centuries is reason enough to seek a closer encounter. But comets are also windows on the solar system’s ancient past, being bits left over from the formation of the planets billions of years ago. So planetary scientists were expecting a treasure trove of information when, late last year, the mission beamed back its first data from 510 million kilometres away, as it arrived at the comet 67P Churyumov-Gerasimenko.

They haven’t been disappointed. The mission has already revealed that the comet, unexpectedly, has almost no magnetic field, while some of its surface features seem to have been formed by wind. But most intriguing is what has emerged about the water 67P contains – and what that might mean for how Earth itself became wet.

Comets have long been crucial for explaining Earth’s water. In our planet’s early days, water would just have boiled away from its hot surface. To get around this problem, a well-established theory is that our planet received an express delivery of life’s vital solvent later on, when it was bombarded by icy bodies like comets. Although atmospheric weathering and tectonic activity have erased obvious evidence from Earth, the moon’s cratered face tells us that such bombardments did happen during the solar system’s convulsive early aeons. This cometary ice also contained carbon-rich molecules, supplying Earth with a starter pack for life, freeze-dried for freshness.

But that’s not the story Rosetta tells. “Even before we did any analysis we could see that the cometary water was completely different from the Earth water,” says Kathrin Altwegg of the University of Bern in Switzerland. She is in charge of Rosetta’s ROSINA instrument, which scrutinised the water it encountered when passing through 67P’s tail.

Ordinary water is made of two hydrogen atoms bound to an oxygen atom. But just occasionally, one or both of those hydrogens is replaced by deuterium, a stable isotope of hydrogen with an extra neutron in its nucleus. On Earth, there are roughly 160 molecules of this “heavy water” for every million ordinary molecules, a number known as the D/H ratio.

The D/H ratio of 67P was triple that of Earth – confirming a discrepancy that has been in the air a while. In 1986, Altwegg was a young researcher on the European Space Agency’s Giotto mission when it rendezvoused with and photographed Halley’s comet. Indirect spectroscopic measurements suggested Halley’s D/H ratio was twice Earth’s. Similar studies of around a dozen comets since have indicated something similar: only one, named Hartley 2, contains water with a similar D/H ratio to Earth’s oceans.

That would seem to indicate comets weren’t the source of Earth’s water. So what was?

“It seems comets can’t have been the source of Earth’s water. So what was?”

Asteroids are one alternative – one that had long been ruled out. The closest, brightest of these bodies, which orbit in a belt between Mars and Jupiter, are bone dry. And while some meteorites found on Earth, which are thought to have been flung from more distant members of the asteroid belt, do contain water with the right D/H ratio, there isn’t nearly enough to supply Earth’s oceans.

At least that was the view when Laurence O’Rourke, Rosetta’s science operations coordinator, started using another of ESA’s missions, the infrared space observatory Herschel, to look for water on asteroids. Specifically, he was targeting the largest of the asteroids, Ceres, some 1000 kilometres in diameter and now classed as a dwarf planet.

Ceres orbits in the middle of the asteroid belt, following an elliptical path that varies in distance from the sun from about 2.6 to 3 astronomical units (1 AU is the distance of Earth from the sun). This crucial region straddles the solar system’s snow line, the boundary beyond which the sun’s rays are so weak that any water condenses to ice crystals.

“Ceres is in this really special place, where ice was starting to become a major planet-building material. As you went further from the sun, you got more and more ice crystallising,” says , a planetary scientist at the Georgia Institute of Technology in Atlanta. With a density around 2000 kilograms per cubic metre that is neatly in between that of ice and rock, Ceres, and its more distant asteroid siblings generally, could plausibly be half rock and half ice. Such asteroids could have supplied significant amounts of water to the early Earth.

Sadly, O’Rourke’s first measurements in November 2011 revealed a disappointingly desiccated body. It was a different story when he looked again in October 2012. “We got a very strong detection of water,” he says – the first confirmed detection of water in the asteroid belt.

What had changed was Ceres’s position. In 11 months, its orbit had taken it from 3 AU to 2.7 AU from the sun, enough to come within the snowline and turn some of its ice to vapour. About 6 kilograms of water was being released every second, surrounding the asteroid in a tenuous atmosphere rather like a comet’s. In fact if comets are dirty snowballs, asteroids such as Ceres look increasingly like they might be snowy dirt balls.

NASA’s Dawn mission, which was captured by Ceres’s gravity on 6 March to begin a 16-month study of the dwarf planet, is one probe that could provide more clues to its composition (see “Asteroid ahoy!“). But regardless of what Dawn finds, it’s unlikely to be as simple a picture as one in which bombardment by material from asteroids, rather than comets, brings water and organic molecules to Earth. “That can’t be the whole story,” says of the Open University in the UK.

Four in a row: Rosetta has delivered the most detailed picture of a cometary surface yet (Image: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)

The reason is written once again in the moon’s cratered face. Most models of the solar system’s evolution involve the giant planets – Jupiter, Saturn, Uranus and Neptune – migrating closer or farther away from the sun before settling into their final orbits. These movements would have upset comets’ orbits, throwing some into the inner solar system.

So there must be at least some cometary water with its heavy make-up mixed into Earth’s oceans. “It’s like paint, you mix red and yellow and you get orange. So, if you have a component that’s three times ‘heavier’ than Earth water and a component that’s three times ‘lighter’ and you mixed them 50:50, you’d get Earth water,” says Wright.

The answer to where that lighter component might come from might lie in our still-emerging understanding of the solar system’s “water cycle”: a sequence of events that began when the sun and the planets were nothing more than a frigid mixture of gases drifting through space. In these interstellar clouds, atoms occasionally stick together and form molecules. By a quirk of chemistry, the isotopic composition of any water formed depends on temperature: the colder it is, the more deuterium it ends up containing. Interstellar clouds are typically at a frigid -220° C to -260° C, which will yield D/H ratios at least three times those on Earth.

But under the warming influence of the nascent sun, some water molecules would have been broken up and reformed, lowering the D/H ratio as they did so. So in all probability bodies formed across the solar system with a huge range of D/H ratios, as indeed the observations of comets so far suggest – and Earth got bombarded with a selection of them all. That, combined with some moisture baked out of our planet’s interior by a process known as outgassing, might have supplied the right sort of water. The fit is still not perfect: bodies with a Ceres-like distance are likely to have lower D/H ratios, but models show that making asteroids with exactly the right low D/H ratio takes too long.

Whatever the full story, the tale of the wrong water does shed new light on comet 67P itself. Its extraordinarily high D/H ratio suggests we have, more by luck than judgement, stumbled on a body that has more claim than most to represent the pristine material that made the solar system – its water is, in Altwegg’s words, “more or less purely interstellar”.

More detail might have to wait for the hoped-for reawakening of Rosetta’s lander, currently languishing in a crevasse (see “Come in, comet“). But it’s all the more reason to regard these bodies with awe and wonder. “The beauty about studying a comet is that the stuff that has been there for 4.5 billion years,” says Wright. “You warm it up and you can basically say that’s the stuff that was on the surface of the Earth. I think that is fantastic.”

Read more: “Rosetta’s real revolution is right here on Earth“

This article appeared in print under the headline “What Rosetta did: The wrong water”