There are skeptics. People were saying that, yes, we found all of these ingredients necessary for life, as we know it, but it’s too much food out there, so maybe nothing consumes it…

Yes, one of the points that was made as soon as our discovery was announced, and in full disclosure I’m a co-author on that hydrogen paper, was, well, gee, you got so much hydrogen that nothing is there eating it up. That’s possible. It’s possible the environment is habitable, but not inhabited. But another possibility is that so much hydrogen is being produced that the organisms that are there are not numerous enough to consume it. And in fact there are places in the Earth’s oceans, on the seafloor of the Earth, where there are hydrothermal systems where so much hydrogen is coming out that the organisms present simply can’t consume it all. And so, it’s too much of a feast and a lot of food gets wasted and, in the case of Enceladus, blasted into space. But it simply may be that the amount of life is limited enough that it can’t consume all that hydrogen. We can speculate on these things, but the only way to know is to go back and look for the molecular signs of life in the plume.

We’re pretty sure we’re gonna go back, it’s too much of an attractive target. With the exception of a catastrophe on Earth, I’ll bet the international agencies, NASA, the Europeans, Japanese, Chinese or Russians, will send, together or alone, a mission there, to be the first to find aliens, be they microbes.

And you did all these discoveries with ‘old’ technology, without being disrespectful to those great engineers. But, naturally, we have much better tech now. What would you put on a future orbiter to find further signs of life?

I’ve thought quite a lot about the question of the next step for Enceladus, so much so that I have a proposal into NASA for a concept called Enceladus Life Finder, or ELF. Well, the most robust way to look for life and to avoid at the same time contaminating that ocean is to do the measurements in the plume of Enceladus, to fly through the plume and not try to land and not try to go into the ocean or even perch on the edge of one of the fractures, because that always leaves open the possibility of contamination by terrestrial organisms. So in flying through the plume we have to work with very little material, tens of nano-grams, and the best technique for dealing with that kind of tenuous sample is mass spectrometry, which is just what Cassini used. But the mass spectrometers on Cassini are a quarter century old, when they were designed, and mass spectrometers that can be flown in space today are much more powerful, they have much better sensitivity and much greater mass range and they can look at the masses to a much, much finer degree, looking at fractional masses associated with binding energies and the nucleus, which allow you to distinguish molecules that might otherwise be regarded as having the same mass. So with mass spectrometry — a tried and true technique in solar system exploration — we can look for signs of life and we can also determine to a much better extent exactly how habitable and for what kind of life is the Enceladus ocean inhabitable.

The plumes coming out from the ice fractures at the south pole of Enceladus, on Nov. 30, 2010. Credit: NASA/JPL-Caltech/Space Science Institute

And what kind of life could be there? People in the recent NASA conference joked about ‘Enceladus shrimp’. And it didn’t seem that strange, because when we hear of hydrothermal vents on the bottom of the ocean on Enceladus, we all imagine the lush environments around our terrestrial hydrothermal vents. But are we jumping the gun here, dreaming of such alien crustaceans? As a serious astrobiologist, how optimistic are you about this sea food?

An artist’s rendering showing a cutaway view into the interior of Saturn’s moon Enceladus. Credit: NASA/JPL-Caltech

Well, we’re probably jumping the gun as far as shrimp go. But if you want to imagine hydrothermal vents at the bottom of Enceladus’ ocean, where microbes, single celled organisms, are living and extracting energy from the hydrogen and other food stuffs, I think that’s entirely reasonable. We need to know how much phosphorus and sulphur are present, elements that Cassini couldn’t measure, but which could be measured by ELF. That’s very important to know, how much life could be present there, how many microorganisms could be there.

But certainly, it’s not at all far fetched to imagine microbial communities sitting at the base of this ocean, enjoying the chemical reactions going on between the rock and the water.

Tiny Enceladus was a wonderful surprise for everybody. The initial target for investigations, among the Saturnian moons, was Titan, and we’re very proud, as Europeans, of the Huygens probe. That proved to be an incredible planet. As a Titan expert, surely you wanna go back.

I cut my scientific teeth on Titan, back 35 years ago. Voyager 1 flew by in 1980 and Voyager 2 in ’81 and told us that this atmosphere of this giant moon of Saturn was very dense, that it had nitrogen and methane. But didn’t tell us about the surface. And one way to understand the data was to imagine that the surface of Titan is cloaked in a giant ocean of ethane and methane. Well, Cassini popped that beautiful bubble when it got to Titan… But it did discover that there are seas and lakes in the northern hemisphere and a few lakes in the southern hemisphere of Titan. And again, just like with Enceladus, when Cassini discovered those lakes and seas it was actually able to follow up and tell us what they’re composed of and how deep they are. With the radar system, which was really never designed to do this, Cassini was able to probe the depths of the great seas of Titan, like Ligeia Mare, or Punga and Kraken Mare. And then, by measuring the weakness of the signal coming back from the depths, could tell us that indeed these seas are made of liquid methane and a little bit of ethane and nitrogen. So an enormous amount of hydrocarbon, hundreds of times more than the known oil and gas reserves on the Earth. And then Huygens, this fantastic probe built by the European Space Agency, landed on Titan in 2005 and took wonderful pictures of gullies in a nearby hillside, an icy hill carved by methane rain into these beautiful gullies. But as soon as the probe landed, methane, ethane and some other hydrocarbons came pouring out of the ground into the heated inlet of the instrument, the mass spectrometer. So Titan is full of methane and all of the discoveries that Huygens and Cassini made tell us that there’s the equivalent of an active hydrologic cycle on Titan. When we say hydrologic, we think of water, but in the case of Titan it is methane. It forms the clouds, the rain, the rivers and the seas of Titan, and so it’s a truly bizarre and exotic world, but very, very active.

On Jan. 14, 2005, ESA’s Huygens probe made its descent to the surface of Saturn’s hazy moon, Titan. Carried to Saturn by NASA’s Cassini spacecraft, Huygens made the most distant landing ever on another world, and the only landing on a body in the outer solar system. This video uses actual images taken by the probe during its two-and-a-half hour fall under its parachutes.

Can it also form life?

And the big question is this one, yes — could there be a form of biochemistry, a form of life that could develop and be sustained in liquid methane? And, of course, the answer to that is: we don’t know. But we also can’t demonstrate that is physically impossible. We don’t understand the limits beyond which chemistry might or might not be able to form life in a given kind of liquid. I believe that the way to go back to Titan is to land on one of the great seas and float around. It’s a mission concept that a few of us proposed a few years ago, called Titan Mare Explorer (TiME), which was unfortunately not selected. But the idea still stands, that you could splash down into one of these seas. In fact, Huygens could have splashed down if it would have been deployed to the high latitudes rather than the equator. And it would’ve floated in that material. So, send a capsule, splash it down, let it float around in the sea until it finally gets to a shore and measure the detailed composition, see if it’s anything going on that might be suggestive of life being formed or even life existing in that sea.

Artist’s impression of TiME lake lander

We published an interview last year with Ellen Stofan, former Chief Scientist of NASA and another supporter of Titan Mare Explorer. You’re also saying, please don’t forget about it, fund it, because we need another probe on Titan.

There are a lot of things that can be done in the Saturnian system. Of course, Enceladus Life Finder is, in my view, the most imperative thing to do. But getting back to Titan and landing on the seas also is critically important. To some extent, I would say that we have very difficult choices, because we’ve been so successful. Cassini has made so many discoveries in the Saturn system, that we really can’t follow up on them all, at least not in our lifetime. So it makes for some very, very difficult choices and interesting dilemmas to which of these fantastic discoveries do we follow up on first.

Isn’t it great that we got this far, to fight over such attractive targets! The most common discussion is that between Enceladus versus Europa. In popular imagination, Europa was the one place where we thought we ‘must’ attempt a landing on. You’re also involved in Europa Clipper mission, an over the pond sister of the European mission JUICE. We had a talk last week with some of the people from the JUICE team and they were excited about visiting not only Europa, but also Callisto and, mainly, Ganymede, other moons with possible subsurface oceans. But you’re going to focus all your efforts on Europa. Tell us about your mission and your objectives there.

The very first ocean within an icy moon that was discovered was of course that of Europa. It was discovered back in the mid to late 1990s by the Galileo orbiter, which was primarily a NASA mission, with some involvement by Germany as well, which orbited Jupiter from the mid 1990s ‘till about 2003. One of the really spectacular discoveries of Galileo was that Europa produces a distortion in Jupiter’s magnetic field, as it orbits around Jupiter, and it’s exactly the distortion you would expect if there’s a large body of salt water under the surface of Europa and not very far below the surface, in fact. Now, Europa is a much bigger moon than Enceladus, it’s hundreds of times greater in volume, so the water that we’re talking about is the equivalent of twice the volume of water in the Earth’s oceans. And we do know that it’s salty, but there’s a wide range of possible values for the saltiness. So that’s all we know. We don’t know the salt content in detail, we don’t know which salts are there, we don’t know if there are organic molecules in the ocean, carbon burning molecules. A lot of things we know about Enceladus we don’t know about Europa, because Galileo didn’t find a plume, and even it had, it didn’t have the instruments to follow up by flying through that plume. So, beginning in 1998–1999, many of us pushed for a mission to Europa. There was nearly one, but it was cancelled by NASA because they felt it had gotten too costly, and so here we are now 17 years after that cancellation and we finally have a mission that is being prepared to go.

This artist’s rendering shows NASA’s Europa mission spacecraft, which is being developed for a launch sometime in the 2020s.

Europa Clipper is designed to be a Jupiter orbiter but it makes multiple flybys of Europa, it’s got a fantastic instrument complement including two mass spectrometers that will measure the plume composition, if there are plumes, but they can even detect material that is just being evaporated off the surface, even if there are no plumes, and so with those, plus a remote sensing instruments in the ultraviolet and near-infrared cameras and so forth, we will be able to find out if there are patches of organics on the surface that have been pushed up, welled up through the water escaping from the ocean, and there’s a radar system that will make measurements that will tell us how deep the ocean is — is it down a kilometer or is it down fifty kilometers — what’s the variation. So Europa Clipper will give us everything we need to takes the next step, should we choose to do so, in directly exploring the ocean of Europa and looking for life there. But it is the essential first step, because we do not have for Europa the kind of information we have on Enceladus, from Cassini. Yes, there’s a little bit of a ‘war of the worlds’ that goes on, some people go for Europa, others go for Enceladus. My view is that nature has gifted us with two worlds in the outer system that might have life and wouldn’t it be fantastic if we went to both and discovered that not only is life in both oceans, Europa and Enceladus, but in each case some little difference in the biochemistry that tells us that these had independent origins, and indeed had independent origins from life on Earth. That would be an incredible richness of extraterrestrial biology that we could study.

These composite images show a suspected plume of material erupting two years apart from the same location on Jupiter’s icy moon Europa. Both plumes, photographed in ultraviolet light by NASA’s Hubble’s Space Telescope Imaging Spectrograph, were seen in silhouette as the moon passed in front of Jupiter. Image credit: NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center

You joked in one of your presentations that ‘I’m a middle aged man, so please hurry with the budgets and the missions’. But you seem optimistic about it and there seem to be plans for new missions in search for life. Who knows, you might end up as an author on the paper announcing life there…

That would really be wonderful! Joking aside, I don’t really care that much if I’m the one who writes the article, but I would like to actually find out, I would like this to happen before I die. And one’s perspective is very different at 57 than at 27, I have to say. To me, the most frustrating part of this is that the trip times to the Jupiter system and the Saturn system are very long, with the rockets that NASA has available for these missions. It’s five years to Europa and it’s ten years to Enceladus. We could image using larger rockets that are under development, both the NASA Space Launch System, or rockets in private industry. But those (travel times) are daunting, particularly for Enceladus. Nonetheless, what’s ironic is that both life detection events might happen at about the same time because with Europa we have to wait somewhat for the results of Clipper, and for Enceladus it just simply takes longer to get there even though we’re ready to do a life detection mission right now. So I would love this to be happening in the 2020s, but I have a feeling that the first detection of life in the outer solar system, if it’s there, is going to be in the 2030s.

You’re also working as a co-investigator in the Juno mission science team.

Juno is in orbit around Jupiter now, it’s the first solar powered orbiter of a giant planet, part of the New Frontiers class of medium size missions, conceived of by Principal Investigator Scott Bolton, of Southwest Research, and his science team. Juno is all about Jupiter. While we think of many giant planets missions as exploring the moons or rings and so forth, Juno is really all about Jupiter. And what we are discovering as we speak - I used to talk until now in the future tense, but now we’re there so it’s the present tense — we are discovering what the structure of Jupiter is, how the circulation patterns inside Jupiter are organised - we do that through the gravity data; we’re looking ultimately to find out whether Jupiter has a solid core, that will tell us how the planet formed — was it a lengthy two step process, with forming a kind of a super-Earth, and then the gas collapsing on top; or did it form all at once, from an instability in the disk that could have happened very quickly? We also want to understand the magnetic field of Jupiter, the geometry of it, because that will tell us whether it’s generated in the metallic hydrogen region of Jupiter, or the pressures are so great that hydrogen becomes a metal and that in turn will give us more information on the structure. We’re getting beautiful images of the atmosphere, of storm systems, of the poles, both in the optical and also in the near-infrared — a wonderful instrument built and provided by the Italian Space Agency is doing these beautiful infrared observations. And then, with another instrument, we are gradually building up the data needed to measure the abundance of water, deep inside of Jupiter. The Galileo probe, which was delivered by Galileo in the mid-1990s, did not find the amount of water we expected and the thinking is that the water was actually dried out or removed because the probe fell into an unusual region, a hotspot, where there is a lot of dry, sinking air, subsiding air. So water is the primary carrier of oxygen, oxygen is the most abundant of the heavy elements, the non-hydrogen and helium, so if we want to understand the material that went into Jupiter at the beginning, the building blocks of this giant planet, we need to determine the water abundance. Juno will do that, as well as other things, pictures of auroras, measurements of magnetic field, and the key to all of this is that Juno is in the kind of orbit that Cassini has just entered in its proximal orbits - a very, very eccentric orbit, very close to the cloud tops of Jupiter at the close point, and then it goes very far away from Jupiter the most of the orbit in order to avoid getting too much radiation from the very prodigious radiation belts of Jupiter. It’s a remarkable mission, going very well, and I think the discoveries are going to be remarkable.