When we think of Earth-like life, we tend to associate that with full on planets, like earth. But that’s not the only option. There is also the possibility of Earth-like exomoons. Examples from science fiction being Endor from Star Wars and Pandora from Avatar. And it may well be the case that Earth-like moons could make up a significant portion of habitable worlds in our galaxy.
We already know from our own solar system that ice shell moons like Europa and Enceladus could be habitable for some kind of life. While questions abound as to whether life can arise easily, or if it’s a very difficult or even a fluke process, it seems to have emerged here on earth at the earliest time it could. If simple microbes are a natural result of certain organic chemistry, then there could very well be simple microbes in the oceans of Europa. But anything past the microbial world is an open question. There is some thinking that it may be hard in an ice shell environment for life to move past the simplest forms to more complicated organisms. But no one can say for sure, for all we know there could be space octopuses living there. To answer the question, we need to go to these moons and study them.
But ice shell moons aside, there is also the possibility of Earth-like moons with liquid water oceans. It’s possible that within the habitable zones of suitable stars, gas giants might orbit that have rocky moons. These moons can be as big as planets, and there is a reason to think based on our own solar system that natural satellites outnumber planets in the universe, potentially offering more possibility of Earth-like moons orbiting something else, rather than stand-alone planets like earth.
There are currently no confirmed exomoons, though there are candidates, in fact, what may be a Neptune-sized one orbiting a huge gas giant was announced just a few days ago. They are hard to see and push our limits on detection, but as equipment improves, so will our ability to detect these kinds of worlds. They would also be subject to specific issues inherent to gas giants, if Jupiter is any indicator moons can be located in compelling radiation environments, limiting the possibilities of life, such as with volcanic and radiation-bathed IO.
In the end, there seems to be no reason to say that exomoons couldn’t be habitable, even in an earth-like way. Perhaps even civilizations can arise on them, with gas giants always gracing their skies. But what of our solar system when the sun becomes a red giant, and its habitable zone moves outward? Most of our moons can’t hold atmospheres, so it’s probably not a possibility for most of them. But there is one. Of particular interest here is Saturn’s moon Titan.
Titan is already a candidate for habitability alternative to that of the earth for two reasons. One is that it’s thought to have an ammonia rich liquid water ocean beneath its surface, though this has not been confirmed. In principle, some form of life could exist there, though the high ammonia content would make it different from Earth life. The other possibility is shallow temperature life at the surface. Titan is the only other body in the solar system with at least a significant amount of fluid present on its surface, in the form of liquid hydrocarbon lakes which in principle could act as a solvent for life. But it’s hypothetical; we’ve never seen life like that here on earth.
Current possibilities aside, there does seem to be a third chance for life on Titan in the far future. In a paper by Ralph Lorenz and colleagues, they detail that when the sun enters its red giant phase, there could be a period of several hundred million years where Titan’s usually very hazy atmosphere will change as the sun’s ultraviolet light levels drop. As the haze drops, which has the opposite of a greenhouse effect and keeps things cold, then the methane in Titan’s atmosphere could come into play, which is a greenhouse gas, and warm things up. This could, in turn, allow for a thawing of Titan and a surface liquid water and ammonia ocean to develop. Given the abundant organics present on Titan, that would hypothetically give life both the time it needs to arise and the materials. It’s believed that this period of several hundred million years of habitability was enough for life to appear on earth, though the presence of ammonia may slow that process down, so it’s also possible that Titan won’t have enough time for native life to arise.
If it’s already arisen, however, or appears before the sun’s red giant phase this newly habitable world could allow that life to adapt and evolve further, at least for a time. Or in this case, habitability as we generally see it could be an extinction event for anything that might be present on Titan. For the life that evolved in icy conditions or in a subsurface ocean, warm weather and surface oceans could be deadly.
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