Europa is the second-innermost big moon of Jupiter. It is currently thought to be the most likely place in our solar system that extraterrestrial life might be found. I’ll review the question, and end up with (maybe) a somewhat different catch on it.
The big moons of Jupiter go: Io, Europa, Ganymede, Callisto. They are all roughly the size of our own moon. The larger ones are composed of both ice and rock, while our moon is mostly rock, so their surface gravity is a bit weaker that our moon’s — but still — comparable. Each of them is quite distinct and interesting in its own right. Io especially is the most volcanic object in the solar system, with dozens of volcanos actively erupting at any given time, visibly to space probes. It is hot because of “tidal forces” brought on by the relative motions of Jupiter and the other three big moons. And it really is hot, despite being so far from the Sun.
The next moon is Europa. The visible distinction of this moon is that only a handful of meteor craters is visible on its surface. That seems to mean its surface is geologically very young. Furthermore, precise measurements of its motion by several planetary probes indicate that its surface moves like a crust floating on liquid. Various models of the data indicate that the liquid layer is tens of kilometers deep. This liquid is most likely water. The heat required to keep the water liquid can be explained the same way as the heat that makes its neighbor Io hot: tidal energy (just less of it).
So: water, relative warmth. Life?
Objections might be: the Earth has life because it’s so big, or because it’s much warmer, or because it has sunlight, or (if you’re not thinking hard) it is so old.
The last objection first: All the big bodies in the solar system formed around the same time, they say 5.5 or so billion years ago: Europa first formed around the same time the Earth did — the two bodies are the same age.
And the Earth itself was no nice place the first billion or so of those years. But after things settled down, BANG there was life. The oldest fossils that appear to be cells are about 4.3 billion years old. The point is, as soon as organic life became feasible, BANG, it happened. (That is BANG, in the geological sense — tens or hundreds of millions of years. A matter of patience.)
Is the Earth really so much warmer? The land areas, except at the poles, probably are. Then again, we don’t really know how warm Europa might be deep down. Warm enough to melt ice, evidently. But you know, the temparature of most of Earth’s oceans, except for just the top layers, is below 5°C, just above the melting temparature of ice. And that’s most of the biosphere of the planet. And that’s where most organisms on Earth live.
As to the size, it is true that the simple surface area of the Europa-ball is only about 6% of the Earth-ball’s. But then again, the models show an ocean on Europa much deeper than the Earth’s — some models predict a total volume of water on Europa greater than that of Earth’s oceans. But let’s say, the volumes are comparable.
It’s true that on Earth, most organic energy derives ultimately from sunlight, via the food chain, from photosynthesis to top predators; the Sun is what keeps the water from mostly freezing solid. But in the past few decades, places have been discovered in the ocean where creatures have long lived off geothermal energy. And that is like the tidal energy that keeps Europa from freezing solid.
So on Europa there is an amount of liquid water comparable to Earth’s, and a source of energy, and plenty of time.
To me, it seems the question of life on Europa is reversed: if there isn’t life there, why not?