Some of the most fascinating targets in the search for life in our solar system are moons of giant planets. Did you know If you had wings, you could fly on Titan, a moon of Saturn? Did you know that Europa, a moon of Jupiter, is thought to have more water than Earth under its icy shell? NASA is planning to send spacecraft to both of these places in the coming years to look for signs and ingredients of life. Another intriguing moon of Saturn is Enceladus, which is spouting a wall of water nearly 100 miles high. Morgan Cable, an astrobiologist at NASA’s Jet Propulsion Laboratory, discusses these wondrous worlds, the exotic locations where she has done fieldwork, and the research she has done on the chemistry of life that could thrive on Titan.
Jim Green: Is the Earth the only ocean world in our solar system? No, it’s not. There are moons around giant planets with enormous amounts of water under their ice shells.
Morgan Cable: If we were to land in the right spot with the right instrument, we could find life even if there were only trace amounts of it there.Jim Green:Hi, I’m Jim Green, chief scientist at NASA, and this “Gravity Assist.” On this season of “Gravity Assist” we’re looking for life beyond Earth.
Jim Green: I’m here with Dr. Morgan Cable and she is a research scientist and group supervisor in the astrobiology and ocean worlds group at the Jet Propulsion Laboratory. Welcome, Morgan.
Morgan Cable: Hello! Thank you so much, Jim. I’m so happy to be here.
Jim Green: You know, when we look in the outer solar system at Jupiter and Saturn, we see these spectacular ocean moons. How come our Moon isn’t an ocean moon?
Morgan Cable: I know. Why doesn’t Earth have one, huh? Well, it’s actually because of science and because of physics. And it has to do with the fact that some things boil off more quickly and turn into what we call the gas phase, vapor, than others. And so as our solar system was forming, things that melted or boiled off more easily tended to move further out from our Sun. And so that means that things like metal and rock tend to be concentrated more closer to our sun like Mercury, Venus, Earth, and Mars.
Morgan Cable: And then as you step further and further out in our solar system, now you get to these gas giants that have a lot of these volatiles. So Jupiter, Saturn, those systems have a lot of things like methane, water, ammonia, and those are what coalesced and congealed and formed these moons. And so it turns out there’s a lot of water that is liquid further out than we ever thought when we first started exploring the solar system.
Jim Green: You’ve been analyzing data from Cassini, looking for chemical signatures of life. Now Cassini orbited Saturn for about 13 years and observed some really exciting moons. Tell us about your work on the moon Enceladus.
Morgan Cable: Oh so Jim, Enceladus, as you know, is one of my favorite places in the solar system. It is one of the smaller moons around Saturn and in our solar system in general, but small can be mighty and Enceladus has absolutely proven that to be true. I was very fortunate to get involved in the Cassini mission right near the end, around the last two years of Cassini’s travels in the Saturn system and over Cassini’s mission, we made some very surprising discoveries at Enceladus. We found that it has this giant plume of material, mostly water, spewing out of its south pole. This comes out of these four giant cracks that we call the tiger stripes. They’re massive. They’re about, let’s see, 120, 130 kilometers long. What is that? Like three marathons, I think. And they’re about a marathon spaced apart, about 42 kilometers. Jim, you know 42 is the answer to everything.
Morgan Cable: Yeah, out of these cracks is coming liquid water but also some really tantalizing evidence of the ocean below. We find salts, some salts that are similar to Earth’s oceans and we’ve also found organic molecules, not just small tiny ones. We found ones as large as the instruments aboard Cassini could detect. Now when we built Cassini, it was meant to study the Saturn system, but at that time we didn’t know there was liquid water out that far. We had no idea there was complex organic chemistry going on and so its instruments weren’t designed to look for life. But we’re hoping that a future mission might be able to answer that question in the Saturn system one day.
Jim Green: Well those tiger stripes, as you say, are really exciting. They only exist in the Southern hemisphere and we believe they’re open all the time and so water is constantly pouring out of them. Now think about that. As you said, they go up 100 or 150 kilometers. That’s almost like a wall of water going from the surface of the Earth up to space station. Well, we don’t see that here on Earth. So that moon is doing an incredible job of showing us that there’s huge amounts of water in these ocean worlds
Jim Green:Well, there’s another moon of Saturn that may also hold great promise to where life might exist and that’s Titan. So what aspects of Titan really excites you and other astrobiologists to be looking for life there?
Morgan Cable: Oh goodness. There are so many fascinating things about Titan. Did you know it’s bigger than Mercury?
Jim Green: I did.
Morgan Cable: Yeah. And it’s actually, if you count its atmosphere, it is the largest moon in our solar system. Its atmosphere extends over 1,000 kilometers. It’s huge.
Jim Green: That’s cute. Yeah. I hadn’t thought of that, but indeed that would make it bigger than Ganymede, which is our largest moon.
Morgan Cable: You’re right. But we’ll let Ganymede have a win because Titan is so fascinating. It doesn’t need to take that title of largest moon.
Jim Green: No it doesn’t.
Morgan Cable: The pressure at standing at the surface of Titan, if you were there standing at the surface of Titan, it will be about if you took a breath and went down to the deep end of your swimming pool. That would be about the pressure that you would feel and because that atmosphere is thicker and the gravity is less, it’s about one-sixth, one-seventh that of Earth. If you were standing on the surface of Titan and you had wings and you flapped them, you could fly, which I just think is very cool.
Morgan Cable: But in terms of astrobiology, there are so many fascinating things about Titan. Titan’s atmosphere is made of nitrogen with a little bit of methane and because of solar radiation and a bunch of charged particles from Saturn’s magnetosphere that whack into these molecules, it splits them up and they recombine into pretty much any combination of carbon, hydrogen and nitrogen you can think of.
Morgan Cable: So if you have your chemistry set at home, like I do, any listeners, if you have that, try to make sort of any combo. Small tiny molecules to really massive ones, the size of proteins. We have found all of those, at least some evidence of them in Titan’s atmosphere and we think they’re raining depositing down on the surface too. And so Titan seems to have all of this organic goop. We call it a massive chemical inventory. And this is really exciting for us because a lot of these molecules, if you make them in the lab in a way similar to how they’re made in Titan’s atmosphere, if you make those, we call them tholin. Actually, Carl Sagan was one of the first scientists to make these. So he coined the term. It’s from the Greek tholos, which means muddy or not clear, which I love because they’re muddy color, the reddish brown.
Morgan Cable: But it’s also not clear exactly what’s in them because it’s so complicated. But if you take that tholin and you dissolve it in liquid water, you make amino acids like that. You make a bunch of other prebiotic molecules, things that life as we know it uses. And so since Titan also, most people don’t realize this, Titan has a liquid water ocean underneath all the cool stuff on its surface. So if any of these organic molecules are getting sucked or pulled down into that ocean, you could have some really fascinating chemistry that could lead to life as we know it.
Jim Green: Well, there’s an upcoming mission that you’re also involved in. It’s called Dragonfly and that is a quadcopter.
Morgan Cable: Oh, I am so excited to be a part of this mission. Yeah, so Dragonfly, actually Jim, it technically it’s an octacopter because it has four sets of two counter-rotating blades.
Jim Green: Oh that’s right. Okay.
Morgan Cable: Picture the size of one of our big Mars rovers, like Curiosity or the Perseverance, the Mars 2020 rover. It’s that size, but take off the wheels and put on two skis like you’re going skiing instead. And then it’s got these, like I said, these four sets of two helicopter blades that are big. They’re about the size, one of them, of my arm. I’m not a super large human, I’m like five-foot-two but you can picture that. These are big. That’s just one blade of this giant system. And because of Titan’s atmosphere being so thick, it’s actually more efficient to do these flying hops that Dragonfly is planning across the surface than it would be to drive like our traditional rovers. This allows us to sample a lot of different places of Titan’s really diverse terrain that it has, features across the surface.
Jim Green: So how many hops will it do and how far will it go?
Morgan Cable: We don’t have an exact number of hops explicitly stated. I think we’re going to start by collecting samples first when we land and do as much as we can and then we’ll see how far we can get during the nominal mission lifetime. Dragonfly is going to be powered and planned, for at least three years, hopefully longer than that, and we are hoping to do a lot of really cool science along the way.
Jim Green: That sounds great. I’m really excited about that mission. Now that will launch in this late decade or early next and then land on Titan and then radio its data directly back to Earth. Now I know you’ve helped discover a variety of minerals, but one in particular, a hydrated mineral that might actually exist on Titan. What is it and how did you do that?
Morgan Cable: Oh, this is so cool. Okay, so I’m a chemist and I work in a lab and a lot of the laboratory work that we’re doing can help inform or be informed by a lot of the space missions that we have going out to visit these worlds.
Morgan Cable: And what we’ve done is we’ve started taking a lot of these simple molecules that we know are abundant in Titan’s atmosphere and we think are solids on the surface. Now, these are things that you have to wrap your brain around what it’s like on Titan to understand because most of these are either liquids or gases at Earth conditions, things like acetylene or butane or ethane.
Morgan Cable: These are actually flammable or sometimes hazardous gases. But around Titan, there’s not a lot of oxygen. It’s all trapped as water, so it’s frozen. We just basically dissolve things together or mix them together at these cold temperatures to see what happens. And about half the time, we get surprised.
Morgan Cable: Now one of these minerals you talked about, this is, we call it a hydrated mineral because that’s the role it’s taking on Titan. Here hydrated minerals means they have water that are liquid trapped in their crystal structure. But on Titan, water is frozen solid. It’s not a liquid.
Morgan Cable: Instead, you have methane and ethane that form the liquid phase and just like water does on Earth, they form clouds. They rain or even snow. They carve gullies and features on Titan’s surface and they pool in lakes at the poles. Now what we found is one of those molecules, ethane, liquid ethane, if you mix it together with a rather hazardous compound, benzene, here on Earth, we do this very carefully in the lab.
Jim Green: You better.
Morgan Cable: Yeah. When you mix these two together, it actually makes a new structure where the benzene molecules rearrange to let ethane inside of its crystal structure. And so it’s just like a hydrated mineral on Earth but made of different stuff.
Jim Green: Wow, that sounds fascinating. So in reality, looking for life on Titan is not going to be life like us because we use water as a liquid because on Titan, as you say, water is in solid form. But methane as a liquid, that’s a completely new dimension.
Morgan Cable: Well, so there are two different ecosystems on Titan. Right? That’s a really fascinating thing. There’s this liquid methane and ethane on the surface, and so that would be life as we don’t know it, but don’t forget Titan still has that liquid water ocean deep down inside. And so if we were able to, say, find a place where some of that liquid water ocean has squirted up and frozen on the surface, we might be able to search for life as we know it too.
Jim Green: Wow.
Morgan Cable: That’s one of the reasons I’m so fascinated by Titan because we could find potential for two completely different genesis. Genesi? What’s a plural of genesis? Of life. It would be really cool and so hopefully Dragonfly will give us some hints that may be able to tell us whether or not that’s happening.
Jim Green: Well, one of the moons of Jupiter I’ve always been fascinated with is Europa. And that of course is an ocean world. We know that it has a large amount of water, maybe twice the amount of water Earth has below its icy crust. So how are Europa and Enceladus so much alike?
Morgan Cable: So yeah, Europa is another really fascinating place that I’m really passionate about exploring. Europa and Enceladus share a lot of similarities. They both seem to have these global liquid subsurface water oceans, liquid water oceans underneath an icy crust. They also seem to have, we know for Enceladus for sure has hydrothermal activity. So that means that down at the sea floor, there seem to be places where liquid water is interacting with hot rock, temperatures of boiling point of water, 100 degrees C or above. And that’s really exciting for potential for life because here on Earth at our sea floor, we find these rich communities of life that exist, not just bacteria, but crabs, tube worms, multicellular life, all existing off of geothermal energy. And we think that the same thing may be happening on Europa as well. And so these moons seem to share these characteristics of having liquid water, chemistry and energy, the three ingredients for life as we know it.
Jim Green: Well, how is Europa and Enceladus so different?
Morgan Cable: They do share some differences which means that we need to explore both of them in my opinion with dedicated missions at some point. So Europa is much larger than Enceladus and so we can explore differences in how energy is distributed in these oceans over time. Enceladus’ crust is a little bit thinner and we know that there are active plumes on Enceladus. We have tantalizing evidence for maybe some plume activity on Europa, too. And boy, wouldn’t it be nice if a mission was going to go and look for that?
Jim Green: And that mission’s coming up.
Morgan Cable: Yes! It is!
Jim Green: Yes! What’s the name of that mission?
Morgan Cable: That mission is Europa Clipper. We’re so excited that we’ll be sending this amazing payload of instruments to go and explore all aspects of Europa doing flybys. So we’ll be orbiting around Jupiter and we’ll map the entire surface. We have instruments that can tell us what that surface is made of, whether it’s organics or salts or ice or a combination of them. We have some instruments that will essentially be able to stick out their tongue and taste some of the particles that are being sputtered off the surface. So we’ll be able to get composition without having to land. It’s going to be fascinating. We’ll be exploring how habitable the Europan environment could be and hopefully get a lot of hints about more information of what that ocean is like.
Morgan Cable: There are some theories that since there seems to be an abundance of electrons, we think down, at the ocean core interface, so where the ocean meets the rock and there seems to be a lack of electrons, we think at the ice ocean interface. That may mean that all of Europa, its whole ocean could act like a giant battery. And this is exciting for life because a lot of organisms can basically just take those electrons and move them from one place to the other and put a little energy tax on there so they can survive. But that can be a great way for an ecosystem to live. And so we’re hoping to explore some of those possibilities with the Europa Clipper mission.
Jim Green: Yeah. In fact, Europa is about the size of our own moon. Now when you think about it, if a crack opens up and a wall of water comes spurting out, the gravity of Europa is so great that most of that material, maybe 99.9%, of it will fall back to the moon. So we don’t anticipate these plumes to get very high, but we do expect them to be there and we want Europa Clipper flying through them.
Jim Green: Now, you’ve been doing a lot of field work and getting out and doing a variety of chemical analysis. So can you tell us about some of your favorite moments out in the field?
Morgan Cable: Oh gosh. Okay. There are two. My all-time favorite field work experience actually happened when I was a graduate student. I was lucky enough as a grad student at Cal Tech, I had two advisors, one at Cal Tech and one at JPL. And because of that partnership, I was able to go on a field expedition to the top of Mount Kilimanjaro as a grad student. And it was just amazing. So the reason Kilimanjaro is interesting is because at the very top is a glacier, is an ice field. And this glacier has been around, we think for about 10,000 years. And over that 10,000 year cycle, it actually grows and then recedes, grows and then recedes. And so it’s melting now and it’s exposing ice that probably hasn’t been exposed for thousands of years. And so our mission was to go and hack into that ice and collect some samples and see if we could understand the kinds of life that were trapped inside. Now, Kilimanjaro is really high. It’s about 20,000 feet. It’s 58 95 meters at the top.
Jim Green: Wow.
Morgan Cable: And so some of the solar radiation that changes the ice can actually be a decent analog for some of these ice moons like Enceladus and Europa because there’s less atmosphere between that ice and space. And so that was a really fascinating trip. Oh my gosh. The views from the top were just amazing. But my second field experience that I really loved doing is a partnership with a bunch of early career scientists and engineers. This has been funded through the PSTAR program, the planetary science and technology for astrobiology research. And we of course came up with a fancy acronym called FELDSPAR, which is a type of volcanic mineral. And the reason for that is because we were studying the volcanoes of Iceland. We’d been traveling to these fresh lava fields in Iceland.
Morgan Cable: It’s one of the most volcanically active places in the entire world. And we’ve been searching for what kind of life colonizes a fresh lava field, what comes in first, what moves in next and we’ve been looking at this over time because these lava fields actually make great analogs for places like Mars to try to understand when you go and collect a sample such as Mars 2020 is going to do, the Perseverance rover is going to be caching some samples to bring back to Earth.
Jim Green: Wow, that’s super. I wish I went with you on some of these trips. Well-
Morgan Cable: Oh, we would love to have you on, Jim.
Jim Green: Might take you up on that.
Jim Green: Okay. So Morgan, do you think we’ll find life beyond Earth in your lifetime?
Morgan Cable: Well, Jim, with all of the amazing instruments that we’ve been developing just over the last few years, we’re finally at the point now where we have limits of detection such that if we were to land in the right spot with the right instrument, we could find life even if there were only trace amounts of it there. So I really do think that if we continue to support these missions to explore ocean worlds, that we’ll find life in our lifetime.
Jim Green: All right, so I’m going to answer that question—
Morgan Cable: There we go.
Jim Green: By saying yes, and leave it to you to find it.
Morgan Cable: Yes, sir, Jim. We’ll get right on that.
Jim Green:Well Morgan, I always like to ask my guests to tell me what was the event or person, place, activity or thing that got them so excited that they became the scientist they are today. And I call that event a gravity assist. So Morgan, what was your gravity assist?
Morgan Cable: Oh man, I have been lucky to have a lot of amazing mentors that have given me a boost, a gravity assist in my career. But there’s one in particular that comes to mind. So I grew up in Florida, actually in Cape Canaveral, right next to Kennedy Space Center. And so I had rockets going off in my backyard all the time. And that inspired me.
Morgan Cable:In eighth grade, I did a science project about life on Mars and it was a great way to get my feet wet learning about planetary science. And during that time, I emailed a scientist who at that time worked at Arizona State University. His name was Dr. Ken Edgett. And now people may know him as one of the main scientists that works on the Mars hand lens imager MAHLI that’s been on a bunch of our rovers, but I emailed him out of the blue as an eighth grader and was like, “Hello, I am interested in doing a science project.”
Morgan Cable: And we struck up a correspondence. He ended up shipping me this giant box of a mineral called basalt that I could use for my experiments. And then I ended up actually getting to meet him later that year because that was the launch of Pathfinder. And so those of you who are good at math can look up and figure out how old I am based on this. But it was such an amazing thing that a scientist would take time out of their busy schedule to come and help me, this eighth grader, do my science project and that’s what made me want to become a planetary scientist. And so I’m still good friends with Ken Edgett. Now we’re both over here in California and it’s just, it’s neat to be able to look back on that.
Morgan Cable: And that’s one of the things that’s inspired me to do Gravity Assist for as many early career scientists. I run a space camp to help inspire people, young kids to go into science and engineering and it’s just such a wonderful thing. And Jim, when you do a gravity assist, you actually steal a little bit of momentum from the body you’re orbiting, but hopefully Ken Edgett didn’t mind because it really was a tremendous help to me.
Jim Green Well Morgan, you have an enormous amount of momentum, so I want to thank you so much for a wonderful, delightful interview. It’s just been a pleasure talking to you about some of those ocean worlds.
Morgan Cable: Well, Jim, thanks so much. It’s been a real pleasure speaking to you and thanks for all that you do to make the missions that we get involved in possible and help expand our reaches and our senses out to the outer solar system and beyond.
Jim Green: 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.
Credits:
Lead producer: Elizabeth Landau
Audio engineer: Manny Cooper