Saturn's moon could support life

25 April 2017

Interview with

Professor David Rothery, Open University

In our own cosmic backyard is Enceladus, a small moon which, as it orbits Saturn, gets buckled by gravitational forces. This generates heat maintaining a liquid ocean inside the moon that periodically sprays jets of salty ice into space. By looking at what those plumes are made of, scientists have concluded that the inside of Enceladus could have all the right ingredients for life. Chris Smith spoke to Open University planetary geologist David Rothery about the findings.

David - Enceladus is a 502 kilometer diameter moon of Saturn. So it’s an icy body, ice covered surface, we think some rock inside, and we’re pretty convinced that there’s an internal ocean. The ice surface and the interior rock are disconnected by this zone of liquid water and there must be some tidal heating going on. There are plumes being erupted through cracks in the ice near Enceladus’ south pole, which were discovered when the Cassini spacecraft got to Saturn twelve years ago now and have been studied ever since by multiple fly throughs and so on.

The evidence has been building up, and up, and up about what’s going on in the ocean and, in particular, the interaction between the water and the warm rock at the bottom of the ocean. That’s what’s got people excited because the last bits of evidence have slotted into place that the water and the rock are reacting chemically and producing conditions down there on the ocean floor that would be habitable by the right kind of microbes.

Chris - First of all: how are these plumes being monitored?

David - We have a spacecraft orbiting Saturn called Cassini, which will crash into Saturn this September, but it’s had a dozen or so flythroughs of the plume. It wasn’t designed to fly through plumes but once they discovered the plumes they said “OK, we’ve got to fly through these and see what they are.” So the retasked the mass spectrometer to analyse what’s in the plumes and sure enough, pretty early on, they found there’s water there. Then they found ammonia and methane and carbon dioxide and, two or three years ago, they found tiny little rock particles, little silica particles about 20 nanometres across.

They said what’s happening here to get these tiny little nanoparticles of silica, there must be chemical reactions going on with the rock? If that’s happening, it’s a reaction called serpentinisation, you make a mineral serpentine. If that happens you should be liberating hydrogen gas. If there’s hydrogen gas being produced down there, then we’ve got a metabolic pathway. There are microbes on the Earth’s ocean floor called archaea that can munch on rocks provided there’s hydrogen that they can use as well, and they can make their body mass out of that and extract energy from this, but the hydrogen had never been discovered.

So on the final flythrough in October 2015, they retasked their mass spectrometer in a special mode and it took them 18 months to get the results published which have said there is hydrogen there. Everybody’s convinced that the hydrogen is real that proves the metabolic pathway is viable. You could take microbes from Earth, stuff them down at the bottom of the ocean, and they’d be quite happy there.

Chris - Is there any other rational explanation for why this hydrogen could be there?

David - You can get hydrogen by a little bit of radioactivity can break water apart, but it seems it can’t produce hydrogen at the necessary rate.

Chris - Does this mean then that given it is the end of Cassini this year, and our opportunity to study this further is now effectively at an end that we need to push up the agenda for the European Space Agency, also other space agencies around the world, new missions like Cassini and focus on Enceladus? Is that where the money should go?

David - Well, you're right. There’s now more information to come about Enceladus from Cassini. Enceladus isn’t the only place to study; there’s Europa, a big icy moon of Jupiter. Bigger than Enceladus, and plumes haven’t been seen by a spacecraft around Europa, but you can see them by the Hubble space telescope. So there are spacecraft heading to Jupiter: there’s the European Space Agency mission called Deuce, there’s the Europa Clipper funded by NASA, going to Jupiter to study Europa and the other moons.

But Enceladus should also have a mission; there’s one called the Enceladus Life Finder or ELF, which is a lovely name which was proposed in a recent round to NASA not funded. NASA’s funding two asteroid missions this round but I suspect that ELF will come back next next year and perhaps will get funded because Enceladus really is a high priority target. So Enceladus and Europa, either of those are places to go. And if we can nail down the fact that life does exist there that ought to change our outlook on life in the universe. Because if it’s in these other places it will be all over the place around other stars too.

Chris - Presumably it must focus minds in terms of being extraordinary careful about contamination?

David - Yes. We certainly don’t want to accidently seed anywhere with earth life. This why at the end of the life of Cassini it is being crashed into Saturn. It will burn up in Saturn’s atmosphere and no bugs, which have accidentally hitched a ride from Earth and have survived 20 years in space, will get down to anywhere habitable. Space agencies are careful. They have to be by international treaty not to accidently take Earth microbes anywhere where they could survive.

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