Let's get two things straight. First, we don't know if there are fish on Europa, one of Jupiter's largest moons. Second, we don't know there aren't any.
What we do know is that Europa has water--perhaps a vast, globe-girdling ocean up to 100 miles deep, beneath a rind of fractured ice as little as 10 miles thick. And all that's missing to make Europa's ocean like Earth's oceans--in other words, habitable--are salt and organic compounds.
Now, thanks to new surveys conducted with the Keck II telescope in Hawaii, we know those ingredients are there, adding critical seasonings to what could, just possibly, be a soup of life.
Europa owes its watery state to its sister moons, which pluck it gravitationally as they pass, flexing its interior and heating it up. This keeps the ice in its depths melted and perhaps even warm. With that in mind, planetary scientist Mike Brown of Caltech used Keck observations to study sodium traces in the atmosphere as well as on and inside the moon. Turns out, he found, there's not only magnesium sulfate salt on Europa's surface but also likely sodium chloride--ordinary sea salt--in its ocean. What's more, venting from cracks in the ice indicates that material from below reaches the surface and material on the surface regularly flows back down.
Why is that important? Because Europa has been tattooed by many comets in its long life, and comets are known carriers of organic, or carbon-containing, compounds. Salt plus organics plus warmth plus water? "I'm not an expert on life," says Brown, "but I do know that if you dip a net in the ocean here, you're bound to pick up something."