Astronomers have long suspected Jupiter’s moon Europa may have conditions underneath its surface that could support life. We’ve known for a long time that its surface is a shell of ice over an ocean of liquid water, but it’s been difficult to see that water directly. The ice shell is kilometers thick, and all the evidence we have is indirect.
It looks like that’s now changed: Astronomers have just announced the direct detection of water erupting from Europa in huge geysers!
The observations were made using the mammoth 10-meter Keck telescope. Keck can detect infrared light, far outside the colors our eyes can see, and this is critical: When water molecules are struck by sunlight, they can absorb it, vibrate (like weights on springs attached to each other) and re-emit that energy at specific wavelengths of infrared light.
The astronomers observed Europa from February 2016 to May 2017, looking for those wavelengths. Sixteen of those observations produced nothing out of the ordinary, but the 17th — on 26 April 2016 — they caught the right glow. The amount of infrared light they saw implied that about 2,100 tons of water had blasted out into space from beneath Europa’s surface. In liquid form that’s very roughly the volume of a large house. In this case, though, it was in the form of vapor, so occupied a lot more volume.
This is exciting! Europa is a little bit smaller than our own Moon, but the amount of liquid water inside Europa is more than all the oceans and rivers on Earth! The surface of the moon is clearly icy and riddled with cracks, but it’s been unclear if that water can make its way through that thick ice shell.
The culprit here is gravity. And when you’re talking about Jupiter, gravity is a big deal. Europa orbits Jupiter on a very slightly elliptical orbit, and that means sometimes it’s a little closer to Jupiter, and sometimes farther away. The gravity it feels from Jupiter changes over its 3.5-day orbit, and this winds up squeezing and compressing the moon. This generates huge amounts of friction inside Europa, which manifests as heat (like rubbing your hands together on a cold day warms them up). The heat melts the water ice, creating the subsurface ocean (this is exactly the same mechanism responsible for the geysers of water Cassini saw erupting from Saturn’s moon Enceladus).
The surface temperature is so cold, about -170° C on average, that any water there though is frozen into rock-hard ice. That’s why it’s been so hard to see the liquid water directly. Still, apparently, it can squeeze itself up through those cracks, and erupt in 200-kilometer-high geysers.
Evidence for these plumes has been building for a long time. The first observations of them, though not understood as such at the time, were from the Galileo spacecraft, which flew through a plume in 1997. Computer modeling in the day wasn’t good enough to figure out what the observations meant, but later analysis supported the idea the spacecraft had flown through a water geyser.
Then, in 2013, Hubble Space Telescope observations strongly implied water was erupting into space; it saw a molecule called OH–, which is what you get when ultraviolet light from the Sun breaks apart water molecules (that, plus a hydrogen atom). These observations looked pretty good, but OH– molecules can be formed in other ways. Water is the easiest and most likely way, so it’s pretty good evidence, but it’s still indirect.
These new observations show water directly. This happens often in science; you get a hint of something, so you keep looking, and as technology and software get better, your chances of confirming or negating that idea grow. In this case, things went in the positive direction. Hurray!
So what does this mean for life under Europa? It’s been a scientific possibility for a long time now, and to be honest, I don’t think too many scientists have any lingering doubts that the water ocean is there. The good news here is that now we have direct evidence for it. So it’s a step in the right direction.
The next step, though, is a giant leap: Europa Clipper, a NASA mission to the moon. Equipped with instruments that can take mapping images, thermal scans, ultraviolet spectra, magnetic readings, and more (a lot more), it’s a surveying mission that will orbit Europa and try to answer some of the biggest questions we have about the moon: How deep is the ocean? What other molecules are in there besides water (like, organic molecules)? What’s the geography of the surface like?
And, of course, is it possible the chemistry there is right for life to exist?
So yeah, this is all a very big deal. Clipper is being built now, and should launch to Jupiter in the middle of the next decade. It won’t be too long before we have more close-ups of this moon, and have a much better understanding of its surface… and what lies beneath.