After we discovered the geysers on Enceladus, why did it take all of this time to find their source?
Other than to look for plumes coming off Enceladus, which we first saw in February 2005 in a set of images designed to look for [these ejections], we had no other plans in the nominal four-year mission for specific observations of geysering activity. How could we? We weren’t sure [the ocean] existed. So once we found them, we altered the nominal mission as much as we could to get a closer look and, of course, we designed many observations in the extended mission to develop a much more complete picture about the phenomenon. This included a set of flybys to measure the mass distribution within the outer layers of the moon in the south polar region, as well as very high-resolution images of the geyser field and Enceladus’ surface.
It wasn’t until 2013, at which point the gravity results were adequately analysed, that we had confirmation of a liquid layer about ten kilometres (6.21 miles) thick under the south polar terrain. We realised that it was at least as wide as the south pole region, but a global ocean couldn’t be ruled out. Under certain reasonable assumptions, some analyses suggested it should be global. But that conclusion didn’t carry a high degree of confidence. As we saw, it wasn’t until last month that my team members finally finished their work utilising our high-resolution images of the surface. We managed to tease out a small wobble in the rotation of Enceladus that is best explained by a global ocean.
These analyses are all, at this stage in the mission, very sophisticated, requiring more than just a look/ see engagement with an image, but a lot of work. And good, careful work takes time.
Could you tell us a bit more about the techniques used to find the ocean?
The first confirmation that there was a liquid layer about 35 kilometres (21.75 miles) below the south polar terrain came from the gravity signature. The distribution of mass under the south polar terrain was detectable by carefully tracking the motion of the Cassini spacecraft as it flew closely over Enceladus’ south pole. Doing three such flybys allowed the gravity team to figure out that the water layer had to be about 35 kilometres (21.75 miles) beneath the surface and about ten kilometres (6.21 miles) thick.
The confirmation that the ocean is global came from noticing how the rotation of Enceladus was not uniform but sometimes slightly faster and sometimes slightly slower. To do this, my team members were using features on the surface as benchmarks – kind of like painting a white stripe on your car tyre – and comparing the positions of the features at various times to see how they compared to where they would have been if there were no wobble.
And the magnitude of the wobble indicates something about the interior – in particular, whether the ice shell moves independently of the core or is attached to it. In this particular case, they found that the ice shell has to be moving independently and that can only be the case if it is floating on a liquid layer.
What do you think the ocean is likely to be made up of?
We know from those instruments on Cassini that have sampled the Enceladus plume that the source of the ocean is mostly water, with trace amounts of salt and organic compounds.
Does the confirmation of an ocean make Enceladus an additional target for life?
Once we knew Enceladus had any liquid at all, but especially that it’s liquid is in contact with a core, we knew it was a target for searching for extraterrestrial biology. So we’ve known that for quite a while and for about ten years. This new observation, however, makes the story richer, and may tell us that there is more energy being injected into Enceladus over the course of time than we had previously thought. So if it does anything, it may mean that any living organisms there may have had a longer time to evolve than we originally suspected. Maybe when we get back there to look closely, instead of finding micro-organisms, we’ll find lobster and sushi!
If you could hazard a guess, what do you think life on Enceladus would be like?
It’s fair to say we all have micro-organisms on the brain. I’d fall off my chair, assuming I’m still able to sit on chairs when the time comes, if there were anything as complex as shrimp or sea horses, wow, that would be a stunner.
Similarly, we’re interested in Jupiter’s moon Europa for its oceans. Is there now going to be difficulty in deciding which moon we should send a dedicated-mission to first?
In my mind, Enceladus was the clear winner if the goal was to sample an extraterrestrial habitable zone – a candidate environment for life – as soon and as easily as possible. Why? The operative word with Enceladus is accessibility. You don’t have to land, scratch, drill or bunker your spacecraft in lead [to protect it from radiation] in order to search for evidence of life. However, other factors like political and technical momentum won and a mission to Europa was chosen and is now under way. So I say: great! Now that we have that mission out the door, the next one in line needs to go to Enceladus. A lot of curious minds want to know if there are microbes snowing at the south pole on that lovely little ice ball.
Would a mission to Enceladus be similar to the Europa Clipper?
The next mission, the Europa Clipper, will essentially bring our knowledge of Europa up to the same level as that of Enceladus, which we have been studying closely now for 11 years. We need to complete the mapping of [Europa’s] surface in visible and near- infrared wavelengths to see if there is any thermal emission coming from the surface among other things – all of the things we’ve already done at Enceladus with Cassini.
If Europa has plumes, the Clipper will carry instrumentation to sample them, though at the moment it seems there are no plumes so it’s now not clear how effective that will be.
A group of us have planned a small Discovery-class mission to return to Enceladus. We aim to do one thing and one thing only: fly through the plume repeatedly with only a few instruments to collect samples and look for chemical evidence of life. If that mission is chosen, that’s what the next mission to Enceladus will look like. Assuming we haven’t found the place to be as dead as a door, the only thing that makes sense would be a more sophisticated mission to do both in-situ sampling and to return a sample to Earth. I hope I live to see the day!