Previously in this class, Mike has discussed the habitability, past and possibly present of, of Mars and what the context for life on Mars is. Today we're going to go a little bit further out in the solar system, and talk about some places where liquid water oceans. Might exist today. So shown here, is a portrait of the ocean worlds of our solar system. At the center, of course, is the Earth. And around it are these ice-covered moons of the outer solar system, beneath which we have good reason to believe that at least in some cases. Vast bodies of liquid water. Possible oceans exist. And these moons are really transforming our understanding of habitability. In the early days of astronomy and planetary science, there was this kind of Goldilocks Model for habitability. In order to be a, a habitable world, you had to be at just the right distance from your parent star, such that liquid water could exist on the surface, in contact with a nice atmosphere. If you were too close like Venus, then you're too hot and your ocean boiled away. If you're like, if you're too far away like Mars, and you're too cold, and your water froze out. But if you're like the Earth, and at one astronomical unit from your parent star, then you've got just the right amount of energy to maintain liquid water and contact with with an atmosphere on the surface. Now, that's a vastly oversimplified version of the traditional habitable zone. But what's important to appreciate is that. That idea of being at the right distance from your parent star, so as to maintain a liquid water on ocean on the surface, that idea is kind of out of date. It's an old, Goldilocks, and there's a new Goldilocks in town, it's a, it's a Goldilocks of habitability that is maintained by tidal energy dissipation. So instead of being at the right distance from you parent star, now we're talking about moons orbiting massive primaries, like Jupiter, and getting tugged and pulled, and creating a lot of mechanical energy. Such that, that mechanical energy helps maintain liquid water oceans beneath icy shells. Now, no place better exemplifies this tidal energy dynamic, than Jupiter's moon Io. Io, we don't think ha, has an ocean, but Io is the most volcanically active body in the solar system. Shown here. In Io's northern hemisphere, is a volcanic eruption, a plume. Io is more volcanically active than the Earth. And it is so volcanically active, because it's orbiting Jupiter, which is some 318 times as massive as the Earth. And it's got a slightly excentric orbit, so it's getting squeezed and pulled as it goes around Jupiter. So, in this kind of New Goldilocks scenario for having ability, Io kind of like Venus. You can see here that Io's covered in this white, and yellow, and reds. Those are all different colors of sulfur compounds that are erupting from Io's volcanos. So, Io's got too much tidal energy activity. If it ever did have some water, it lost that long ago, because of, of this, this energy dynamic where it's just erupting away a lot of vol, volatile. On the further out end. the, the most distant of Jupiter's large moons is Callisto. Now, Callisto, as I showed at the beginning, we do think has a subsurface ocean, but it turns out that the tidal energy dynamic is, is somewhat minimal on Callisto, and, and much of its liquid water ocean is likely might, maintained through radiogenic decay. And it's likely trapped under a very thick ice shell. Thick and old ice shell, and we know it's old because you can look at it and see those little pockmarks all over it's surface. Those are little impact craters and to a planetary scientist. Impact craters mean old age, kind of like ski tracks in the snow. In the center, we've got Europa and Ganymede. And Europa in particular, might inhabit this kind of, or occupy this, this new sweet spot of this New Goldilocks zone. Where Europa's got just the right amount of tidal energy. Such that our current understanding that Europa's got this global, salty, liquid water ocean. Maintained under a relatively thin ice shell. An ice shell is maybe a few, to as much as ten or so kilometers in thickness. And then coupled with that, we think that the ocean is underlain by a rocky silicate sea floor and so that's important because you've got a water-rock interaction. And, and, we'll get into some of the energy dynamics of that later. And just to emphasize the point here of course part of why these ocean worlds of the outer solar system are so intriguing from a habitability standpoint is that, if life on Earth has taught us anything from life in extreme environments, hydrothermal vents, hot springs in, in Africa, the coldest regions of Antarctica. From life in extreme environments to a life of extreme lifestyles. All life on Earth requires liquid water. So let's focus in a little bit here on Europa. Europa is about 3000 kilometers in diameter. It's about the size of our own moon, and as you can see from this image, there are a lot of fractures and barely any craters. So something is happening on Europa's surface, to re-pave and, and has kind of hit the reset button geologically, to wipe out those craters, and to create these fractures. As of now, we don't have any smoking gun for definitive evidence of plumes or new fractures being created. But, we're, we are very data limited. The Voyager spacecraft sent back just a few images, and the Galileo spacecraft sent back. A lot of images, but it was also hampered by the, the limitations of the high-gain antenna. If we take a closer look at Europa here. Or put it in context as I mentioned, it's about the size of our moon. But of course it orbits Jupiter, and that creates that tidal dynamic that maintains that subsurface liquid water ocean. To put Europa's ocean in a relative size comparison to our own ocean, take a look at this graphic. Here's the earth. And this is a An illustration by some colleagues at Woods Hole, where what they did is, took all of the water on earth, rolled it up into that little blue of marble that you see, and that's the Earth's water. All of the oceans, all of the lakes, all of the ice sheets. And so that's. One Earth unit of water. If you do the same thing for Europa, you get a sphere as shown here. That has a volume of water that's two to three times the volume of all liquid water on Earth. How can that be? How can it be that Europa contains two to three times the volume of all liquid water on Earth? Well, the first thing to appreciate, of course, is that on Earth, we're talking about a an ocean that covers only 70 to 80% of the surface. On average, our ocean is four kilometers in depth, and at it's greatest depth, it's 11 kilometers. Whereas at Europa, we're talking about a global ocean that, that is in the range of 100 kilometers deep. So you do the math, turns out that Europa has two to three times the volume of all liquid water on earth. And our current model for what Europa looks like. As shown here. Where if you were to cut away and, and look at the interior. We think, that Europa's got an iron, iron sulfur core, then a rocky silicate mantle, then this blue layer, the global liquid water ocean, and then on top of that, the ice shell. And that right here is one of the. Highest resolution images we have of the surface of Europa, from the Galileo spacecraft. Everything in white that you see there is, is water, ice, H2O. And then the, the blacks and grays, we, we have good reason to believe that some of that is salt, some of it is sulfuric acid. But beyond that, we just don't have that much data about whether or not some of that material is coming up from below, or if some of it's being produced by the radiation processing on the surface. And we'll get into the, in subsequent lectures. Okay, so, how do we think we know that Europa actually has this ocean? As I mentioned, there's, there's not really a smoking gun for for plumes, or, or new cracks. And our understanding of the existence of Europa's ocean, really kind of folds into some subtle, but beautiful physics. And I like to break it into three easy pieces. First, is you gotta find a rainbow connection. A rainbow is a spectrum, and spectroscopy. It's kind of a fancy scientist word for studying rainbows. This first step in the puzzle towards figuring out that Europa has a liquid water ocean, involves determining that the surface is water ice. And Vaishali Moroz and Kuiper did the first ground based spectroscopy, of the Galilean moons, back in the late 50s, and early 1960s. And if you look here at Moroz's spectrum. That stepwise function going from about one micron to two and a half microns. That is highly diagnostic of water, ice, and as you can see here, Moroz says, spectrum of Europa, average of four scans on October 1st 1964. so, working from that, astronomers like Kuiper and then later Johnson and McCord and Pilcher, were really able to show that Europa, along with Ganymede and Callisto had surfaces of water ice. And then, we sent the Voyager spacecraft and the Galileo spacecraft, to actually image these surfaces. And I'll just show you a few. Kind of beautiful shots of how this ice is modified by the geological fracturing and double ridges. You can even see over here a little bit of strike, slip, faulting and displacement there. Some of the chaos regions where some form of melt through is occurring. We'll zoom in now and, show you some of what looks like ice rafts, these look like icebergs that are kind of. Drifting off into a, a slushier medium, but it's far too cold for these to actual, actually be bergs migrating. the, the surface of Europa is about 100 Kelvin. We'll zoom in on this guy here, and you can see. In the, the close up image here, some of the ice [UNKNOWN] those blocks in the highest resolution image are in the range of about five to ten meters in diameter. So step, one is to find that rainbow connection and determine that the, the surface of Europa was, was water ice. Step two. Brings us from the surface into the interior. And, I like to make the analogy here to babysitting a spacecraft. The babysitter in this scenario, is the deep space network which was keeping very close track of the Galileo spacecraft shown here on the right. And I love this picture of the Galileo spacecraft. It's actually not flying by Europa in that particular image. Of course, we don't have cameras to take that picture, but the reason that we have that picture at all, is because Galileo was launched from the space shuttle, and so astronauts were able to de, to image the, the craft as it's as it left the shuttle. But the key thing here is that as the Galileo spacecraft. Flew by Europa, the DSN, the Deep Space Network, shown on the left here, was able to very closely track deviations in the trajectory of the Galileo spacecraft, as the gravity, as the interior of mass distribution of Europa. Cause that trajectory to vary slightly from what you would expect with a uniform point source of, of mass, and that then helps you figure out the interior structure. And, what the babysitting in the, of the spacecraft, what the careful tracking of Galileo revealed, is that there needs to be at least in a three, three layer model. An iron core, or a high-density core, iron or iron sulfur, with the density of say, 5000, 8000 kgs per cubic meter. Then a slightly lower density, silicate layer, of roughly 3000 kilograms per cubic meter. And then, to best fit the moment of inertia, you need a low density. Roughly 1,000 Kgs for cubic meter. Low Density. Outer layer of about 80 to a 170 kilometers. Now, what fits that kind of low density bin. Well, a water layer, of roughly 80 to a 170 kilometers in thickness, fits that data quite well. At the end of the day, you would end up with this kind of three layer model where you got the iron. core, the rocky mantle, and then water in either liquid or solid phase, in that outermost layer. The gravity data and the moment of inertia are, are not sufficient for resolving the subtle difference between liquid water and solid water. To figure out that there's actually liquid water ,beneath the surface of Europa's icy shell, we have to advance to, to step three. And this is an analogy that I like to make. To airport security. You've gotta adhere to airport security. So, I'm showing you here a picture this is a bit of a fuzzy picture I apologize, this is from JFK Airport, and I was trying not to get arrested, but what happens when you walk through this little doorway at an airport? What you're doing is walking through a time varying magnetic field, and if you've got a conductor in your pocket. That time varying magnetic field, will induce electric currents in that, in this conductor. And those, electric currents, will create what's called an induced magnetic field. And then there are the little magnetic field detectors in the doorway that are searching for induced magnetic fields, and if they detect one, the alarm goes off. So what's actually happening at Europa? Well, in the upper left here, I've drawn Jupiter, and then Jupiter's magnetic field which, for the most part can be approximated as a dipole that is slightly offset from the center point and then tilted. About 9.6 degrees. Jupiter is whipping around, and its magnetic field is sweeping past Europa, if we now zoom in on the lower left here, what I've drawn is the magnetic field experience by Europa, so. At one point in time, the magnetic field is coming in, in this direction. These red lines. Roughly five and a half hours later, it's coming in, in this direction. And if you decompose the vector, you of course see that the z component is roughly constant. But you get this dB/dt in the equatorial plane, in the, in the sort of x, y plane. So, Europa is experiencing. This time varying magnetic field. And, what the Galileo spacecraft observed, was that Europa does not have it's own intrinsic field, the way, say, the Earth does. Instead, what the Galileo spacecraft magnetometer measured. Is that Europa's field, was rising and falling, in direct response to the variations in the primary field of Jupiter. In other words, Jupiter was in, or is inducing a magnetic field in, within Europa. So, that Faraday says, well that's, that's fine, but in order to do that, you need a conducting layer. well, we knew from step one and step two, that Europa's got this iron core. Iron's conductive. How about that? Turns out that the core is too far away to explain the, the amplitude of the induced magnetic field. Okay, fine. What about the mantel? Turns out that rocky silicates just really aren't that conductive, and, and therefore cannot explain the induced magnetic field signature. Okay, that leaves us with the water outer shell. What about ice? What about salty ice? Neither of those can create the induced magnetic field signature levels observed. The best explanation for the induced magnetic field signature is a salty. Liquid water ocean, that is experiencing the dB/dt of Jupiter's primary field, and as that field changes over the course of that 11, two, 0.2 hour synodic period with Europa, you get the electric currents within the ocean, giving rise. To this induced magnetic field and the alarm went off. And that's the final piece of the puzzle of detecting Europa's subsurface ocean. It's subtle but very beautiful physics. The ocean is not leaping out at us with plumes and new cracks or have not yet been observed, but are geophysical underpinning for understanding that the ocean is there. Is really quite solid when you couple the surface spectroscopy and imagery, with the gravity data about the interior structure. And then, the need for this near surface conducting layer that is best explained by a salty liquid water ocean.
