As the years go by the chances of Europa hosting life seem to keep going down. But it's not out of contention yet.
Europa is simply full of surprises. We're talking about the second Moon of Jupiter, just a little smaller than our own moon. It sits just over three quarters of a billion kilometers from the Sun. At that distance, the Sun is less than 5% of the brightness that it is here on the Earth.
It's…cold. Desolate. Inhospitable. There's water on Europa, for sure – water is by the far the most common molecule in the entire universe, so it's not exactly hard to come by – but it's all frozen. The entire world of Europa is covered in a sheet of ice tens of kilometers thick.
But Europa has a surprise. Tidal forces from Jupiter stretch and squeeze Europa on its elliptical orbit. This heats the interior, keeping it molten. And so you have a molten interior and a frozen exterior, which means in the middle you have an ocean. A globe-spanning liquid water ocean. An ocean with more water than the Earth has. An ocean that is completely and totally alien to anything we experience or encounter on the Earth: forever blocked from sunlight, with a depth reaching for up to a hundred kilometers.
Life as we know it needs water, and Europa's got that in spades. But life needs much, much more than water, which is where we start running into trouble with this little Moon. First is an energy source. Sunlight is definitely not going to be an option here. But while the vast majority of life on Earth ultimately derives its energy from the Sun, there are other creatures that don't.
For Europa, the closest analogs we can find are the deep-sea hydrothermal vents. These are fissures ithe ocean floor where super-heated, mineral rich gases escape into the ocean. At the bottom of the food chain here are chemosynthetic microorganisms, that get their energy from inorganic compounds, like hydrogen sulfide and ammonia. From there, you can entire communities like tube worms and crabs that are totally fine living life without the Sun.
For this to happen on Europa, the rocky mantle HAS to be geologically active. You need fissures and cracks and heat to keep this process in motion. Remember, life needs an energy source. If the mantle is cold and inert, then you just have liquid water meeting sterile rock, and not much interesting chemistry, let alone biochemistry, can take place.
We know that the deep core of Europa is molten, but we don't know about the rest. It's hot – but not that hot. Any liquid hot magma needs to be big and buoyant to be able to reach the outer edge of the mantle and break through the rock barrier. Computer simulations have shown that there just isn't enough magmareaching enough of the water to classify it as active.
But these are just simulations, which – and I'm saying this as an expert in physics simulations – aren't…well let's just say they're rarely the final word on any subject.
If the mantle-ocean boundary is active, then Europa has three of the four needed-for-life boxes checked. There's water, there's energy, and there's physics. But what about nutrients? Earth ahs so many opportunities to create rich chemistry, to make a huge variety of chemicals and molecules and reactions and interactions. A lot of this is ultimately powered by the Sun. Europa…has fewer chances to create chemistry and mix around an abundance of good elements while eliminating the bad.
One trick that Europa might have up its sleeve is from its outer surface. Dust can settle on the ice. Then ultraviolet radiation from the Sun and cosmic rays from deep space and strike those dust grains, transforming them into interesting combinations. Then because the surface isn't static, some of those molecules and work their way down into the ocean, which means life may instead find a home clinging to the bottom edge of the ice sheet, not on the ocean floor.
There are a lot of open questions here. The European Space Agency's JUICE mission isenroute right now, and it will soon be joined by NASA's Europa Clipper. These spacecraft will study Europa in more detail, and hopefully splash through one of the occasional ice plumes that eject from the surface. Who knows what they'll taste?
