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Season 4, Episode 22: Set Sail for Europa, with Bob Pappalardo

Season 4Episode 22Dec 4, 2020

NASA’s Europa Clipper mission will give us the most detailed look yet at Jupiter’s extraordinary moon Europa.

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NASA’s Europa Clipper mission will give us the most detailed look yet at Jupiter’s extraordinary moon Europa. Smaller than our own Moon, Europa is one of the prime candidates for life beyond Earth because it has a deep ocean under its icy shell. The Europa Clipper spacecraft, named for speedy 19th century merchant ships, will map the surface, learn more about the ocean using ice-penetrating radar, and see if there are plumes of water shooting out from the cracks in the ice, among many other scientific activities. Project scientist Bob Pappalardo at NASA’s Jet Propulsion Laboratory discusses this mission as well as the possibility of life on Europa and how it would be able to survive without sunlight.

Jim Green: Ocean worlds around our giant planets are there to be discovered. Europa Clipper is being built to do just that. Let’s find out what it can do.

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. Bob Pappalardo and he is a senior scientist at JPL in the planetary science division. He’s also the project scientist for the NASA Europa Clipper mission that’s going to Jupiter’s moon Europa, a beautiful ocean world. Welcome, Bob, to Gravity Assist.Bob Pappalardo: Thank you, Jim. It’s so good to see you. And I’m so excited to be here.

Jim Green: Well, how did Jupiter get so many moons and why are they so different?Bob Pappalardo: Well, Jupiter has something like 79 moons. Most of them are little shards from collisions, or Jupiter captures objects that are passing by. Some of them go around the wrong way. So we know that those are captured, but it’s these four big ones, the Galilean moons, that are the ones that are much more like worlds. Ganymede is larger than the planet Mercury. And those four were created along with Jupiter, from the same cloud of gas and dust.

Bob Pappalardo: And the inner one, Io, lost most of its H2O, most of its water. It does not have an icy surface. The outer two, Ganymede and Callisto, have lots of rock and lots of ice and Europa is kind of in between, with a bunch of rock and then a skin of H2O, some of which is solid ice at the surface where it’s so cold and some of which is liquid water, we think, down below the surface, which makes Europa so fascinating that there’s a liquid water ocean in there today.

Bob Pappalardo: Europa’s ocean is really special because it’s pretty close to the surface about 20 kilometers — what’s that, 13 miles or so — below the surface and probably it’s in contact with rock below. So nutrients can seep into that ocean and potentially serve as a fuel for life.

Jim Green: So they were all made at the same time, and yet they look so different. So the tidal forces of Jupiter really, you know, when you look at each of the moons, have really shaped them so much.

Jim Green: What’s happening with these moons with tidal forces? What does that mean?Bob Pappalardo: Europa is going around Jupiter in a somewhat eccentric orbit, it’s not quite round. And when it’s closer to Jupiter, it stretches more because Jupiter’s gravity is pulling on it. And when it’s farther from Jupiter, it contracts a bit. And, this creates friction. Europa’s stretching and distorting as it orbits around Jupiter, every three and a half Earth days; every 85 hours. So that’s pretty quick. And it flexes by about 30 meters when it does that. So that creates heat within Europa enough to melt ice and, and keep the interior warm.

Bob Pappalardo: This is somewhat similar to Earth’s tides. Earth’s oceans have tides because as, uh, Earth rotates and the moon orbits, the, the Moon is pulling on the Earth and the Earth is pulling on the Moon.

Bob Pappalardo: If you’re standing on the right part of Europa, you’d be rising and sinking about 30 meters every three and a half Earth days. And the, the stresses would be rotating around you, the direction at which the stress is pulling would change. And this creates just bizarre, fascinating geology.

Jim Green: That’s gotta be cracking the ice, like crazy. Is some of the cracks caused by that or are there other things that caused those stresses on Europa?

Bob Pappalardo: Yeah. We think some of the cracks are related to these tidal forces. There might be longer term stresses, which create other cracks as well. If you were on Europa and put your space helmet down against the ice, it would probably be creaking like a boat.

Jim Green: Well, you know, in our ocean at these hydrothermal vents, we see life all over the place and it doesn’t require a light from the sun. Is that a similar process that may be going on at Europa? Or do we need sunlight to have life?

Bob Pappalardo: Beneath Europa’s is icy shell, light is not going to penetrate, so it’s not life that’s depended on photosynthesis that we’re talking about. Instead life that’s dependent on chemical reactions, similar to some life on Earth that isn’t dependent on sunlight, but where the metabolism is powered by chemical reactions, chemical disequilibrium, that’s the kind of life that we’re wondering might exist in Europa’s ocean.

Bob Pappalardo: We’re not talking about fish and whales, that’d be exciting, giant squid or something, but instead, probably just, just single cell organisms down there. And that’s because complex life needs a lot of energy. And at Europa, we think probably if there’s the energy for life in the ocean, it’s probably, you know, at a low level, so maybe enough to power just single cell organisms. But that would be so exciting because if there’s life at Europa’s ocean, it would almost certainly be an independent origin of life. You can’t transfer life, right, from Earth way out to Europa, maybe between Earth and Mars, but not out to Europa, that’d be pretty tough to do.

Jim Green: Over the last several years, another set of fantastic research has been done concerning the possibility of seeing geysers coming out from cracks. When we look at Europa, we see these crack structures everywhere, it seems, and yet maybe some of them are active. So what do you think about the possibility that life gets scooped up and ends up in these plumes?Bob Pappalardo: Yeah, there’s tantalizing evidence of these plumes from the Hubble Space Telescope and other observations that that says maybe every once in a while Europa lets out a big burp of activity. When we’re there with the Europa Clipper, we might be able to fly through such plumes. If they’re there, we need to confirm them. There might be a range of sizes. We don’t know, are they consistently active or sporadic? So we need to find that out. We have some time before the spacecraft arrives, and then when it arrives, uh, to identify, are there plumes coming out and can we fly through them to, um, to directly sample the interior of Europa.

Jim Green: Well, you’re the project scientist of a fabulous mission, Europa Clipper. And, and you guys are building that right now.

Jim Green: How big is the spacecraft? What does it look like?Bob Pappalardo: The Europa Clipper is solar powered and the solar panels would stretch from one end to the other of a standard U.S. basketball court. Some said it looks a little like a scorpion. It has, it has these big solar arrays, and it has a magnetometer sticking out of it like a stinger. And it has, um, uh, the big radar antennas hanging off of the solar arrays. It’s very distinctive and cool looking spacecraft.Jim Green: And of course, it’s got this big, huge dish that is used to radio back to Earth, all that fantastic data it acquires.Bob Pappalardo: Exactly.

Jim Green: What are some of the other measurements Europa Clipper is designed to make?Bob Pappalardo: So, cameras, we’ve got a camera suite, two cameras, to map out the surface, um, completely and in stereo. So 3-D and in color. And we can get with the narrow angle camera images as good as a half a meter per pixel. So those, those pictures would see my desk if it were on Europa.

Jim Green: Wow, love it. Love it.Bob Pappalardo: We’ve got an infrared spectrometer to look at the chemical fingerprints of the surface material and try to understand what it is. And what does that say about the ocean? Are there organic materials? We have an ultraviolet spectrometer. That’s great for finding plumes and characterizing them and for seeing, uh, what the surface composition is too. We have a thermal instrument to look for hotspots, places that are warm enough, that we can see them essentially glowing in the dark. There’s an ice-penetrating radar, sends out a long wavelength radar signals that can penetrate right through cold ice, bounce off liquid water, and back to the spacecraft.

Jim Green: Wow.

Bob Pappalardo: So we’ll be able to map out the plumbing beneath Europa’s surface. Then we go to the particles, fields and particles instruments. We have a magnetometer.Bob Pappalardo: That can tell us not just that there is an ocean like Galileo data hinted, but how thick it is and how salty it is, how conductive that ocean is. And the magnetometer needs the plasma instrument. The plasma instrument tells us about the charged particle environment, and that’s needed to better understand the magnetometer data and tells us about the plasma environment, which is exciting in itself and what particles are there. And then, we have two different mass spectrometers, one to get at the dust particles and one to get at the gas particles to tell us their composition, to hunt for organics and let us know about the chemistry. Oh, and last but not least, we use the, the communication system to look for the Doppler shift of the spacecraft signal, as it flies by Europa, to get out the gravity around Europa. By flying by Europa lots and lots of times when Europa is in different places in its orbit, we can actually sense how Europa is flexing. And that’ll tell us about the properties of that ice shell and the ocean beneath. So it’s going to be an incredible mission.

Bob Pappalardo: We’ll actually be orbiting Jupiter and making flybys of Europa. We’re in an orbit that brings us by Europa about every four weeks. And we’re also looking into the option of maybe flying by every six weeks too. So we’ll see where we land with that. And, um, so, so every several weeks we’re going to have new data just pouring in.

Jim Green: Yeah. What I really like about that whole concept of orbiting Jupiter, and then getting into the belts, flying by Europa and then radioing the, the, the data back is you have time to analyze it. You have time to really pore through it, figure out what’s happening and fine tune those measurements that you want to make.

Jim Green: So, with all that it does, is there a possibility that it may be able to find life?Bob Pappalardo: The Europa Clipper mission isn’t designed to search for life itself. What we’re trying to understand is habitability. Is Europa a potentially habitable environment? Does it have the ingredients for life water, the right chemical elements and, and the chemical disequilibrium that could power life?Jim Green: So we may have to wait ’til our next mission where we get down to the surface and go into one of these cracks.Bob Pappalardo: Exactly. By sending a lander down to Europa’s surface someday we could scoop up some of that dark reddish stuff and examine whether they’re organic materials in there, or perhaps some signs of life in there. So the Europa Clipper mission will also scout out places where we might want to send a future lander.

Jim Green: So Bob, when will Europa Clipper be on its way?Bob Pappalardo: We’re scheduled to launch sometime in the mid 2020s. And then depending on the launch vehicle we take, it’ll take a few years, or up to maybe six years, to get out to Jupiter and we might go on a direct path, or we might need a gravity assist. In this case, we’re looking at the possibility of a gravity assist by Mars and then to swing back in past the Earth and out to Jupiter and Europa.

Bob Pappalardo: You know, Jim, I was so lucky to have been able to work, even though just for a little while, with Carl Sagan and to audit a couple of his courses and to do a project under the tutelage of his postdoc, the late Reid Thompson. And, you know, I first saw Carl when he did the broadcast on TV of the Viking landings back in 1976 on PBS live, waiting for that first picture of the foot of the lander. And then he would appear on the Johnny Carson show and communicate science, on that late-night talk show. And, and then of course in 1980, the original cosmos series and the reason I investigated Cornell as a possible school to go to as an undergraduate was, well, I knew Carl Sagan was there. They must have some good stuff. And I went there thinking I was going to do astronomy.Bob Pappalardo: But then I moved to, to the geology side of things. And then I found out, wait, there’s something called planetary geology, like do both. And, I really learned about that because there was, uh, the course catalog, “Ices and Oceans in the Outer Solar System” taught by Carl Sagan, where I first learned about Europa and about Titan and Triton.

Bob Pappalardo: And I’m kind of still doing the same thing today. So, Carl was great, even when I went back to visit Cornell after I finished up and moved to grad school at Arizona State University. And I remember running into him in the hallway and he’s like, ‘Bob, come on in the office, tell me how it’s going.’ And it was, it was just so nice that he remembered my name, that he cared enough to take a few minutes, to help a new grad student along and, and, and push them along in their careers. It was, it was quite inspiring.

Jim Green: Well, I, I never had the privilege to meet Carl, but I, all the stories I hear about Carl was just such a fantastic scientist, but also as you say, really cared for his students really cared for what they were doing.

Jim Green: So, Bob, thanks so much for joining me in discussing this fantastic topic.Bob Pappalardo: You bet, Jim. What a blast. It’s been a lot of fun and, we’re all looking forward to Europa, getting there and all this great data that’s going to pour in.Jim Green: Absolutely. All right. Europa Clipper onward.

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.


Lead producer: Elizabeth Landau

Audio engineer: Manny Cooper