Extraterrestrial Oceans

One of the amazing things scientists can do at Diamond is to recreate conditions of other parts of the Universe. Recently they used this remarkable ability to peer into the salty waters hidden underneath kilometres of ice on Enceladus, one of Saturn’s moons.

Credit: LPG-CNRS-U. Nantes/Charles U., Prague.

In September, NASA ended the Cassini mission in spectacular fashion, crashing the spacecraft into Saturn. For twenty years, Cassini brought us closer to our gas giant neighbour and its moons. The probe made astonishing discoveries about one of them: Enceladus. This small moon has plumes of gas erupting from its surface, it has a rocky core covered in a thick layer of ice, and in between lies a deep, salty ocean. It is one of the most promising places to look for extraterrestrial life. Enceladus is one of the few places in the Solar System where liquid water is known to exist.

Spacecraft aren’t our only way of exploring the solar system, and Stephen leads a team of experimental astrophysicists based at Diamond and Keele University (UK), who have been recreating the conditions in Enceladus’s salty ocean right here in Harwell. They have been using Diamond’s astoundingly bright light to investigate one of the more mysterious properties of water – its ability to form clathrates when water is cooled under pressure. Clathrates are ice-like structures that behave like tiny cages, and can trap molecules such as carbon dioxide and methane.

The conditions on Enceladus may be just right for the formation of clathrates, and understanding more about how they form could provide clues about what is happening in Enceladus’s ocean. In order to predict where the clathrates may be on Enceladus, experiments on Earth have to mirror real conditions as closely as possible. Thanks to the Cassini probe we know that Enceladus’ ocean is full of magnesium sulfate (salt). For the experiments at Diamond, scientists filled tiny tubes with water, and different amounts of magnesium sulfate. The tubes were cooled down, and then carbon dioxide (gas) was fed into the frozen water, where it became trapped in the clathrates that formed in the tubes.

Enceladus, one of Saturn’s moons is surrounded by an icy crust. Beneath the surface lies a hidden ocean containing water, carbon dioxide and various salts. This mixture holds out the tantalising prospect that life might have evolved in the oceans, and alien organisms might be living underneath the ice. But finding them might be another matter. Imagine a life-hunting space probe landing in the middle of Antarctica. It would be easy to conclude that Earth is lifeless as that region is frigid and unfertile. Yet we know our planet is teeming with life. Any future visitor to Enceladus needs to know where to look.

Credit: NASA/JPL/Space Science Institute

Shining Diamond’s high energy X-rays into the tubes allows the scientists to examine what was happening, using a technique called X-ray Power Diffraction. The X-rays are deflected by the contents of the sample tube and in doing so tell the researchers a lot about how the molecules of water, gas and salt are interacting. Previous experiments with clathrates have used pure water, and the scientists thought that the presence of the magnesium sulphate would change the way that clathrates form. This is indeed happening. The salt interferes with the clathrate formation in much the same way that putting table salt on a slippery path in winter melts the ice.

The researchers also found that the salt causes subtle changes that make clathrates more likely to sink in Enceladus’s ocean. Some microbes on Earth make use of carbon dioxide, and if clathrates filled with carbon dioxide are sinking to the bottom of the ocean, that may be a good place to start looking for signs of life. Their work will guide others studying Enceladus, helping pin down what might happen and rule out what cannot. All very helpful when you are studying an ocean seven times further from the Sun than we are. The advantage of using Diamond is that lots of experiments can be done in a short time, lots of information gathered very quickly. PhD student Emmal Safi from Keele University analyses a lot of data. “I’m the first person to see the results” she says “and get to see something that we didn’t expect to see.” Sarah Day, Senior Support Scientist at Diamond, is excited to be able to match results from experiments here on Earth with what we already know about Enceladus.

“Creating that link is very exciting, matching up with what might be happening on other worlds,” says Sarah.

There is also the bonus that clathrates are very important on Earth too. Millions of tonnes of methane are tied up in clathrates in the deep oceans and arctic permafrost. Understanding their behaviour could be very important as climate change warms up these previously frozen regions. Ultimately, we will need to send another spacecraft to Enceladus to find out whether these experimental results really match up with what’s happening on the moon, and whether there’s life in its salty waters. As Stephen puts it “all bets are off until we go there and look directly into the ocean”.

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