For decades, moons of the outer solar system have proven fascinating subjects for scientists interested in the search for life. Forty years ago this year, NASA’s Voyager 1 spacecraft flew by Saturn’s moon Titan and took the first close images, revealing a thick orange-colored atmosphere that is the most Earth-like in the solar system. The Cassini probe then dropped off a lander on Titan called Huygens in 2004, and studied the moon in detail during its 13 years at Saturn. Now, NASA is preparing to launch the rotorcraft mission Dragonfly to Titan in the 2020s. But Titan is just one interesting moon. Ganymede may harbor an ocean under its icy crust, as does Europa, another moon of Jupiter. The European Space Agency’s upcoming JUpiter ICy moons Explorer (JUICE) mission will study Ganymede, Europa, and another moon of Jupiter called Callisto. Meanwhile, NASA’s Europa Clipper mission will provide complementary observations of Europa. A great era of exploration of the icy moons is about to begin. Athena Coustenis of the Paris Observatory talks about missions to the icy moons of the outer solar system and international collaborations with NASA and ESA. She also reveals that she holds degrees in literature and astronomy – find out why in this episode.

Jim Green:Looking for life in the outer planets, where will we go? Is it in the big gas giants or their moons?

Jim Green:Hi, I’m Jim Green, Chief Scientist at NASA. And this is Gravity Assist. On this season of Gravity Assist we’re looking for life beyond Earth.

Jim Green:I’m here with Dr. Athena Coustenis and she is the director of research with the French National Center for Scientific Research at the Paris Observatory in Muedan, France. She is involved in several space missions for the European Space Agency and for NASA. Her focus is on the gas giant planets, Saturn, Jupiter, and all their moons. And she’s considered one of the foremost experts on Saturn’s moon Titan. So today we’re going to talk about the possibility of life beyond Earth in the realm of the giant planets.

Jim Green:Welcome Athena Coustenis to Gravity Assist.

Athena Coustenis: Hi, Jim, happy to be here in chat with you today.

Jim Green:Well, you’ve gotten very involved in what we call astrobiology that topic of searching for life. How did you get so interested in it?

Athena Coustenis:Jim, it’s really easy to get interested in astrobiology, or exobiology as we used to call it, so as you say, this is a study of the, the origin, the evolution, and life in the universe in general. And we consider a question in astrobiology of whether extraterrestrial life exists, and if it does, how humans can detect it. And this is where an astrophysicist like me can play a role. We try to identify places favorable for the emergence and sustainability of life, which we call habitable worlds. So, early in my astrophysics studies at Paris Observatory and the university, I opted for planetology. Okay, so, so planetology obviously, we’re studying the planets in our solar system. And the reason I went that direction is probably because I just wanted to be able to go places, far places and check out my models, you know, preferably with a space mission. I’m hooked on space missions, you know, Like, I love imagining, building, flying and exploiting a space mission.

Jim Green:So the Voyagers 1 and 2, which were launched in the 70s, were designed to go and visit those outer planets, those gas giants, and they made fantastic gravity assists to go from one to another. How did you get involved in those?

Athena Coustenis:Jim, those missions were amazing, when you think [about] when they were launched and what technology they’re based on. I got involved because I did my PhD thesis on the Voyager infrared data from an instrument called IRIS. And [it] was the instrument that told us everything about the temperature composition of Titan’s atmosphere. And can you imagine I did that 10 years after Voyager had encountered Saturn in 1980. And it was one flyby

So anyway, Titan proved to be addictive. I was just so attracted by this world. It is, you know, this. This is a very big satellite, you know, second only to Ganymede in the solar system. And we found it had an atmosphere resembling the Earth, and it kept the secrets of its surface.

Jim Green:Well, the Voyagers were so successful, and really showed us some fabulous things about Saturn in the moons and Titan in particular, we just had to go back. And that’s where the idea of Cassini came about. And so, NASA and the European Space Agency got together and they each decided to create a role. What were those roles on Cassini for NASA and ESA?

Athena Coustenis:Yes, it was, it was amazing. Cassini is still what I believe to be the most tremendous, tremendously successful actually, international collaboration for a mission because NASA and ESA came together with shared roles, you know, NASA was going to build the orbiter spacecraft, the Cassini spacecraft, and it carried the Huygens probe which was developed by the European Space Agency. But these two worked together in every scientific aspect that we learned finally for Titan.

Jim Green:Well, how long was Cassini in orbit around Saturn before we decided to drop off the Huygens probe into the Titan atmosphere?

Athena Coustenis:So, Jim, the Cassini mission, the whole spacecraft arrived in around Saturn and went into orbit in July 2004. And it started immediately making observations. Christmas time 25th of December 2004. It launched the Huygens probe towards Titan. And the probe went down, made a descent, a beautiful descent through the atmosphere of Titan on the 15th of January 2005, landing on the surface and sending back all the beautiful images and data we got during the descent and after we had landed, which was not exactly expected at the time, that we would land and survive, and all of this data was relayed by the Cassini orbiter.

Jim Green:Yeah, that was a fantastic landing. You only needed the parachute because the density of the atmosphere is larger than here on Earth, even though it’s dominated by nitrogen just like the Earth’s atmosphere. That must have been an exciting time. Tell us about that.

Athena Coustenis:So the first image I saw was the one after we had landed where we sell those pebbles. You know, sprawled around the surface on something that looked very orange and dark. And I looked up, I looked at the image and I said, “Who put that Mars image on the screen? you know, move it away we’re waiting for Titan. And then my colleagues turned around said it is Titan. And I said, “Oh my god, oh my god.” I think I couldn’t breathe. It was amazing. It was it was enormous that we could see this surface that we had speculated on for so much time. And during the descent, we saw the channels, we saw the channels, we saw the hills on the side of what ended up being the landing site, which was a dry lake on Titan, recognized immediately you know, by Martin Tomasko, the PI of DISR, who knows about dry lakes in Arizona.

Jim Green:So those channels were rivers that were feeding into that landing spot, that dried lake, is that right?

Athena Coustenis:absolutely, we could even see the base, we could see shores. Um, all of this, you know has been put together in, in, in videos and films the team put together. But for us at that precise moment it was, it was like incredible. We could also identify, at the time we didn’t know it, but we could identify the dunes you know, a little further up the beyond the hills. And it was so amazing to find all these features so similar to what we have on Earth, in a faraway object that sits 10 times further from the sun than our own planet.

Jim Green:Well, you know, we now know there’s liquid on the surface of Titan, but that’s not liquid water. It’s methane. So, were those rivers of methane, do you think?

Athena Coustenis:Absolutely. Methane plays the role of water on Titan. If you think water on the Earth, you have the hydrological cycle you have the water evaporating from the oceans going into the atmosphere, condensing, producing rain, producing haze and condensates and clouds and falling on the surface in the form of rain. We have exactly the same thing on Titan. It’s amazing, but with methane.

Jim Green:Well, do you think would find life on Titan? And if so, how would it be so different than here on Earth?

Athena Coustenis:Well, I don’t know if we could find life on Titan. But there are several criteria that are satisfied. And make us think that Titan is probably the most habitable environment that we have in the solar system, because it has a stable substrate, you know, there’s their surface where an organism can live. It has available energy sources. That’s another criteria we need. That’s from solar radiation. Although it’s a hundredth of what we get on our own planet, but it has some solar radiation. It has solid body tides caused by Saturn and even perhaps radiogenic energy production. It has organic chemistry, this fabulous organic chemistry producing prebiotic molecules in the atmosphere, like hydrogen cyanide, which is a key molecule for prebiotic chemistry and a precursor molecule actually for amino acids. And it has two kinds of solvents. At present, inside Titan, we have the water ocean, the subsurface liquid water ocean with perhaps a fraction of ammonia to keep it like that.

Athena Coustenis: But I think if we find life, you know, um, and in spite of all the harsh temperature conditions, minus 180 degrees on the surface, and light conditions, like I said, there isn’t very much light. I think it would be different from what we know on our own planet. But then it gets so interesting. Amazing.

Jim Green:Well, you know, NASA eventually decided that we needed to eliminate Cassini from orbiting Saturn completely, so that it wouldn’t crash on Titan. And I had a little something to do with that. So, we decided that we needed to ditch Cassini and Saturn in in 2017. So where were you on that day? And how did you feel about that whole idea?

Athena Coustenis:First, thank you Jim. I think it’s a wonderful idea to preserve Titan and preserve the environment of Titan.

Jim Green:Right.

Athena Coustenis:It was such a fitting end for such a wonderful mission. It was a great idea this Grand Finale, you know, brave plunge into Saturn. But not before you know, it had accomplished another 22 orbits between the planet and the rings, sending us information up until the end when it burned into Saturn’s atmosphere. I was at JPL. I was there with colleagues and friends and watching actually, on the screen yourself and other people describing and talking about the mission. And also looking at the signal that Cassini was sending back a little bit that like what you have in a hospital, with a patient and seeing this signal, disappearing little by little until, the mission was declared dead. And in the French delegation how we had brought a bottle of champagne and some glasses. And we drank to Cassini’s success, you know, and to more such missions in the future.

Jim Green:Well, you know, Cassini and the Huygens probe in particular, did such a fantastic job looking at Titan, we just absolutely have to go back.

Athena Coustenis:Oh, of course.

Jim Green:Yeah, now we’re moving towards that. That mission is called Dragonfly. So are you going to be involved in Dragonfly?

Athena Coustenis:Jim, I would be involved in any mission that would return to Titan. I would even fly there if I could. Um, we have so much more to learn. You know, about 10 years ago I proposed a mission to return to Titan with an orbiter, a lander and a balloon. It was called TANDEM and became the Titan-Saturn system mission in collaboration between ESA and NASA. And many colleagues joined that effort at the time. Dragonfly is a new generation. It’s a great mission. My God, so modern, so fashionable, you know, a drone that goes there.

Athena Coustenis:What can be more fashionable than a drone that goes there and explores the surface of Titan, hopping from one place to the other, you know, to get to get different sites of interest. And it’s a rotorcraft lander mission to sample materials and determine the surface composition in different geologic settings because with Huygens we only went to one place and it will also characterize the habitability of Titan’s environment, you know, to investigate how far this prebiotic chemistry we’re talking about has progressed and to search for chemical signatures that could be indicative of water-based or hydrocarbon-based life for organisms but at least study the conditions. I love that concept.

Jim Green:Mm-hmm. I do too. I can’t wait for Dragonfly. But [the] European Space Agency’s moving forward with a spectacular mission they call JUICE and so what is JUICE all about? And where is it going?

Athena Coustenis:So JUICE is the first large class mission in ESA’s Cosmic Vision, it’s called Cosmic Vision 2015-2025 Program and it’s planned for launch in 2022 arrival in Jupiter around 2029, 2030, and it will spend more than three years making detailed observations of Jupiter, and three of its largest moons with a focus on Ganymede, but also Callisto and Europa.

Athena Coustenis: And it’s to characterize the conditions that may have led to the emergence of habitable environments among Jovian icy satellites with again a special emphasis on Ganymede, because Ganymede provides us with such a natural laboratory for the analysis of, of the nature, evolution, and potential habitability of icy worlds in general. But also, it is a class of objects in the universe, in our galaxy that we call the ocean worlds that have these liquid water oceans underneath the surfaces. It’s amazing. I was involved from the beginning in the definition of the development of the mission, as a European colleague scientist and I’m sitting on the edge of my seat to see the launch in time.

Jim Green:Yeah 2022 is coming up pretty fast.

Jim Green:Well JUICE is going to end up orbiting Ganymede, and Ganymede turns out to be one of my favorite moons. It’s the largest moon in the solar system. What else about Ganymede is so exciting?

Athena Coustenis:Absolutely. It’s one of my favorite moons also. Jim, it’s amazing. Ganymede is one of the objects also where we find indication on the surface of resurfacing from something that was previously in liquid form, probably under the surface. Where we have indication by the fact that this is a satellite that has an induced magnetic field, which actually interacts with the magnetosphere of Jupiter and so on. But it has an induced magnetic field that indicates the presence of a liquid water ocean underneath its surface. It’s the best indication we have today of that. It also has auroras. it’s amazing that you find that around such a distant object. So we do need to go back and look closely at Ganymede, not only because, you know, it’s our biggest satellite the solar system, because we want to disentangle all of these interactions of the magnetic field in the magnetosphere of Jupiter and try to discover exactly what it’s telling us on its ocean properties.

Jim Green:In addition, with ESA’s JUICE, NASA is launching the Clipper mission and Clipper is going to Jupiter. But it’s going to study Europa. So when you think about JUICE at Ganymede and, and, and the NASA Clipper mission at Europa, the synergy is going to be fantastic. We’re going to learn all kinds of things and I can’t wait for that to happen.

Athena Coustenis:Amazing. Can you imagine two missions, two missions in the Jupiter system, it deserves it. And I think more than different they’re very complimentary. You know, Europa and Ganymede are the divas, you know, in the Jupiter system. If you’re looking for habitable worlds. And while the instruments, some of the instruments are similar, there’s overlapping observations that we’re going to do, there’s a lot of complementarity.

Jim Green:So, you know, all these outer planet moons have energy from tidal forces, and they have fabulous environments. Could they actually have habitable environments?

Athena Coustenis:Absolutely. I mean, I think these icy moons are so far from the, the notions we had that they were dead bodies. You know, a few decades back we thought that these moons are just dead bodies, they have no activity and so on. We know today that they’re very much alive. They have processes in there like tidal forces that create cryovolcanismm for instance. This is this is a phenomenon we only find out there, where you have volcanoes that actually do not eject lava, they eject ice. Cryovolcanism is a source of energy very important for these satellites. They have organic chemistry that we find every time we look at their atmospheres, or exospheres. They have liquid water oceans underneath their surfaces. All of these elements put together make those satellites very, very habitable environments that we really need to go back and investigate. These are not things we can simulate in a laboratory on Earth.

Athena Coustenis:So we need to go back. But which one is the best? I think every scientist has their own favorite object. And of course, you know mine. I think Titan is the most fantastic candidate for a habitable environment. But then Europa and Enceladus and Ganymede are all in my favorite places list.

Jim Green: Yeah, I agree. Titan is so exciting because it’s so diverse and if it was going to have life, it’s going to be life completely different than what we have here on Earth.

Jim Green:Well, you know, I heard that you’ve gotten degrees, not only in astrophysics, but in literature. I mean, I had a tough enough time just getting a physics degree. How did this happen?

Athena Coustenis:So, not without a challenge. Well, I was very much interested in English literature at school at the same time as in physics and astronomy. So when I got my baccalaureate, I came to France and enrolling in two, Paris universities: Sorbonne on one side and Pierre and Marie Curie, you know, for sciences on the other side, because my family wasn’t at all convinced that there was a chance for me to get a job in astronomy. So they say, well, do English literature, you can always find a job with that. And so I started my English literature PhD at the same time as the astrophysics one, but haven’t finished it yet, Jim. I have to admit, I hope to do that sometime. Maybe when I retire and I’m sitting in front of the sea on some Greek island.

Jim Green: That sounds great. Well, you know, I always ask my guests to tell me, what was that event or person, place or thing that got them so excited about being the scientist they are today? I call that a gravity assist. So, Athena Coustenis, what was your gravity assist?

Athena Coustenis:Jim, I decided to become an astronomer when I was 15, and never wavered or changed my mind. So, but I’ve got a huge boost in my life from a person and a place. The place is Greece, a home of Icarus, you know, who flew through space and went all the way to the sun and, and it’s a land of dreams, coming true also, so because the people there know how to survive in the face of difficult conditions. But I would strongly encouraged by my family and friends but in particular by my father, and my father, Panagiotis, was a pilot in the Greek Air Force, who trained in the States, in Dayton, Ohio, and later became Major General and he had a passion for flying. And he shared that with me and I think somehow he managed to instill this in my mind. My brother is also a pilot. So you see, we come from a family looking at the skies, but I decided to follow and even fly higher and further, but I really got gravity assists from them.

Jim Green:Wow, that’s great. Well, Athena, thanks so much for joining me in discussing this really fascinating object, Titan.

Athena Coustenis:Thank you, Jim. It was wonderful talking to you today.Jim Green:Well, join me next time as we continue our journey to look for life beyond Earth. I’m Jim Green, and this is your gravity assist.