Neil deGrasse Tyson: Life on Europa, Jupiter's Moons, Ice Fishing and Racket Sports | Big Think

Nobody doesn't love Europa. Let me back up. When we think of places you might find life, we typically think of the Goldilocks zone around a star where water would be liquid in its natural state. If you get a little too close to the star, heat would evaporate the water, and you don't have it anymore; it's gone. If you go too far away, it would freeze, and neither of those states of H2O are useful to life as we know it. We need liquid water.

So, you can establish this green zone, this habitable zone, this Goldilocks zone where, if you find a planet orbiting there, hey, there's a good chance it could have liquid water. Let's look there first for life as we know it. Now, it turns out that this source of heat, of course, is traceable to the sun, and if you go farther out, everything would or should be frozen, all other things being equal. But Europa, a moon of Jupiter sitting well outside of the Goldilocks zone, is kept warm. Not from energy sources traceable to the sun, but from what we call the tidal forces of Jupiter itself.

So, Jupiter and surrounding moons are actually pumping energy into Europa. And how does it do that? As Europa orbits Jupiter, its shape changes. It's not fundamentally different from tides rising and falling on Earth. The shape of the water system of the Earth is responding to tidal forces of the moon, and when you do that to a solid object, the solid object is stressing, and because of this, a consequence of this is that you are pumping energy into the object.

It is no different from when you say, anyone who's familiar with racquet sports, indoor racquet sports, it could be racquetball or squash: you say, “let's warm up the ball before we start playing.” You want to hit it around a few times. You are literally warming up the ball. It's not just simply “let's get loose”; you are literally warming up the ball. How? You are distorting it every time you smack it, and then the resilience of the ball pops it back into shape, and every time you do that, every smack, you're pumping energy into the ball. That's not fundamentally different from what's going on in orbit around Jupiter.

So, you have this frozen world, Europa, completely frozen on its surface, but you look at the surface, and there are cracks in the ice. There are ridges in the ice where there's a crack, and it's shifted and then re-froze. So this ridge has a discontinuity in the crack, and it continues in another place. So what this tells you is that Europa cannot be completely frozen because if it were, nothing would be moving. You look at the surface of Europa, the frozen surface, and there are ice chunks that are shifted and refrozen and shifted again. It looks just like if you fly over the Arctic Ocean.

Fly over the Arctic Ocean in the winter; these are ice sheets that are breaking and refreezing all the time. It's the same signature as that. So all of us are convinced that beneath this icy surface is an ocean of liquid water, and there's no reason to think it wouldn’t have been liquid for billions of years. On Earth, where we find liquid water, we find life. Wherever we find liquid water, we find life. Even places like the Dead Sea—why did anybody call it a Dead Sea? Because the microscope hadn't yet been invented.

Sure, maybe there's no macroscopic vertebrate fishes, but microbes have no problems thriving in practically any condition under which you would find liquid water. So what this means is not only do we have a source of heat outside of the Goldilocks zone, we have conditions under which life could be thriving, and knowing that this is possible has completely broadened the net that we are casting in search for life in the universe. No longer is it “let's find a 72-degree tidal pond and see life forming there.” No, life is pretty hearty, and if it takes liquid water, that may be the other place in the solar system that is teeming with some form or another of life.

So, yeah, we all want to go to Europa, period. We’ve got to go ice fishing, though, because you've got to cut a hole or melt a hole; you’ve got to get there somehow because the sheet of ice, by some estimates, are several kilometers thick. It will take some effort to get through that, which we haven't really fully figured out yet. But it's an engineering problem, not a science problem. So a clever engineer should be able to figure that one out.

One of the challenges of mounting a mission to Europa is that we don't yet really know how to dig through the ice. Yet, there are other missions to the outer planets and their moons that we know how to do, and yet we haven't ever done before. One of the challenges we face in my field and surely other sciences is there's a mission you want to mount, an experiment you want to conduct, but it's expensive, and you don't really know how to do it yet. It's tantalizing, whereas there are other experiments where you know how to conduct the experiment, and you've never done it before, so there's a higher sort of return on that investment and a more immediate return.

So you're always jockeying these two ways you might be spending limited resources. We all want to go to Europa, but sensibly, it might not be the first or second or even third of the missions to the moons of the outer planets, because it poses special challenges to us, not only to our science but especially to our engineering. And by the way, Europa is not the only one of these moons in the outer solar system that is kept warm by these sort of tidal stress forces; there are other moons that feel the same influx of energy.

For example, Io, that's the innermost moon of Jupiter, suffers from this phenomenon even more. And that moon is so hot there are volcanoes erupting from within. It is rendered molten, whatever solid parts of that moon there are, so, in fact, the most volcanically active place in the solar system is Io, one of the moons of Jupiter. We don't know how to sustain life under temperatures that hot, but it's a reminder that if you're looking for sources of energy, we no longer need to be anchored to a host star in our search for life in the universe.