Sniffing Saturn's atmosphere

The best way to analyse a gas of course is to sniff it! Which is what, among other things, NASA’s Cassini spacecraft has been doing around Saturn. It was launched from Earth in 1997; it arrived in 2004 and since then it’s been exploring the planet Saturn, as well as it its rings and its moons, sending back breath-taking images from more than a billion kilometres across space. The mission is almost at an end and, on 15th September, Cassini will plunge headlong into Saturn and break up. With us now to discuss some of what’s been discovered during the mission is mission scientist and atmospheric specialist Ingo Mueller-Wodarg.

Ingo - The probe is about the size of a minibus and it simply is a box with large antenna on the front, a big booster rocket on the back and lots of instruments hanging out of it. Initially, it also had a little dish on the side, which was the Huygens probe, which was released into Titan in 2005.

Chris - Titan, of course, is Saturn’s largest moon, isn’t it. Why were you particularly interested in Titan?

Ingo - Titan is actually quite boring if you look at it through the telescope, it’s just a yellow ball. But the first Voyager flybys in the early 80s showed us that Titan has an atmosphere composed of nitrogen - a thick atmosphere. Nitrogen, of course, is also the main gas in the Earth’s atmosphere and, therefore, people were interested to find out more about this planet and its atmosphere.

Chris - So you send this probe onto the surface of Titan - what did it tell you?

Ingo - Huygens landed in 2005. The furthest we’ve ever landed an object from Earth away. It basically entered through the atmosphere, sniffed the atmosphere as it descended and then, most breathtakingly, it gave us the first glimpse through the global layer of haze that we normally just see through the naked eye, this yellow ball I was talking about initially. We managed, for the first time, to look underneath at the cameras on board revealed a breathtaking landscape with mountains and rivers, a smooth surface, and even lakes, but not the liquids we know from Earth. On Earth it’s water, on Titan it’s liquid methane, so it’s not something you would want to swim in.

Chris - Still, not a very nice place to go and hang out though. What about the other moons of Saturn, because Saturn has quite a few and Cassini has had the privilege of being in that system for more than a decade and taking multiple passes among them? What has it seen?

Ingo - The other most exciting of the 50 moons that Saturn has is Enceladus, a small icy body that nobody really expected to see anything interesting in. But it turned out that Enceladus had geysers of water coming out of the southern pole and these were discovered by Cassini for the very first time. We have looked at countless other moons, and the rings of Saturn, and Saturn itself.

Chris - Just talking for a minute about Enceladus because that was really a staggering finding, wasn’t it, those jets of water that spray into space; they go enormous distances? Don’t they even, in fact, contribute to one of Saturn’s rings, the material that they’re ejecting?

Ingo - Indeed, they feed one of the rings with material. In fact, this water that is emitted by Enceladus ultimately enters into the atmosphere again so it’s quite an interesting cycle of water transport over huge distances. But, looking at Enceladus as an isolated system, these geysers they come out of the cracks in the southern pole. We managed to fly through these water jets and we found that they contained molecular hydrogen, carbon dioxide, and methane amongst others. The interesting bit is that these are not chemically in equilibrium the way you would expect them to be. There’s basically too much molecular hydrogen and this could be caused by processes we also see on Earth. Basically, it’s hot water in the ocean flowing through cracks on the seafloor and then reacting with iron rich rocks to form molecular hydrogen.

Chris - Could it also be, one of the things people were very excited about, were saying there’s warm water there which is encouraging because we think water is an essential ingredient for life. Could also some of this equilibrium be because there are maybe biological processes which are exploiting that very strange environment that Enceladus is providing?

Ingo - These imbalances are not caused by microbial life, but they could indeed host microbial life because we see exactly the same happening on Earth. Whenever there’s a surplus on the ocean bed of molecular hydrogen, this can be used by microbial life as an energy source. Whether or not this life exists on Enceladus we don’t know, but we can say that the conditions for microbial life to exist are there.

Chris - Cassini has, undoubtedly, been a staggeringly successful mission, hasn’t it? It’s gone on for all these years and returned this huge amount of information and data, and these surprised like what emerged from Enceladus. Why has the decision been made to bring the mission to a close and crash land Cassin - why are you doing that?

Ingo - You’re absolutely right. It’s been a wonderful mission and all of us would love for it to continue another decade, but it’s running out of fuel. This means that there is enough fuel to power the systems, there is enough electricity on board, but there isn’t enough fuel to control the spacecraft anymore because you have to steer it on it’s orbit. We don’t want the spacecraft to get out of control and then we would risk the spacecraft landing on one of these delicate moons, for example, like Enceladus. We don’t want that to happen so that’s why we decided, while there is fuel, to bring it to a controlled end. We call it it the grande finale, because this is a whole mission again on it’s own with exciting new science to be done very close to Saturn, something which has never been done before.

Chris - So it will continue to send back data to you right up until it finally gives up the ghost as it plunges into the atmosphere of Saturn?

Ingo - Yes, indeed. Normally the data is saved onboard and then transmitted later once the high gain antenna points towards Earth but the exception will now be that it sends the data live while it plunges into the atmosphere. Because once it’s in there, it reaches a point where the spacecraft disintigrates and then we would have lost the data if it hadn’t been sent directly. It will be the first ever in-situ exploration of the atmosphere of Saturn. In-situ means that you sniff the atmosphere on location and this has never been done before on Saturn.

Chris - It seems sad to be looking forward to the demise of something but, at the same time, it will be a spectacular demise we hope, won’t it?