Exobiology was the original term given to the sciences central to the question of life-in-the-Universe. It’s now been largely replaced by Astrobiology, but I’ll stick with the original. To investigate life-in-the-Universe perhaps we should start a bit closer to home and look to our own solar system. Apart from Planet Earth, we have no absolute proof positive to date that any other life, albeit relatively simple forms compared to terrestrial life forms such as ourselves and associated companion animals and plants, exists within the confines of our solar system. However, I conclude in this overall summary the odds are fairly high, almost certain in fact; those other, albeit relatively simple extraterrestrial life forms do exist in our immediate cosmic neighbourhood.
Life is ultimately nothing more than an ongoing series of complex biochemical reactions. These sort of reactions tend to be optimal at warmer (quasi Earth-like) temperatures. Too cold and the necessary chemical reactions are too sluggish, if they proceed at all. Too hot, complex organic molecules rapidly break down. So, Titan (moon of Saturn) is probably way too chilly even though it has a thick atmosphere and lots of organic molecules on the surface, even organic liquids forming large lakes if not exactly oceans. The surface of Venus in contrast is way too hot, but it has a thick atmosphere that’s a lot more temperate at higher altitudes. Some real estate is both too hot and too cold at the same time – Mercury and Luna (our Moon) for example and lacking any real atmosphere and liquid water aren’t top of the pops for finding life, however simple.
Mars, long associated with intelligent extraterrestrial life, is still a near certainty for much housing far simpler extraterrestrial life forms, like microbes. There are many independent chains of observational evidence that support that conclusion. Proof positive however, still waits.
Europa (moon of Jupiter) has in all probability a liquid water ocean beneath a global ice cap. Water plus a geothermal/tidal energy heat source opens up a good plausibility for going ice fishing to our descendents several generations down the track. There are several other moons with ice cap surfaces covering possible liquid oceans that might rival Europa as an eventual fishing destination.
My favourite solar system locations for (probably) tough-as-nails microbial life, Mars apart, are the upper atmospheres of the Jovian (gas giant) planets (Jupiter and Saturn; maybe Uranus and Neptune). Their atmospheres are rich in organics and no doubt water vapour. The gas giants, Uranus excepted, radiate more heat energy than they receive from the Sun. There will be regions in their upper atmospheres that have Earth-like temperatures; there will be a lot of atmospheric mixing (useful for bringing different chemicals together); and of course these planets will also have been seeded with organics and water from space via comets, meteors, cosmic dust, etc., if not in fact seeded directly with microbial life forms via the Panspermia hypothesis.
The problem in finding out for sure is going to revolve around sending appropriate instrumentation into those planetary atmospheres that will be able to detect actual life forms, as opposed to just measuring just purely physical parameters and chemical constituents. So, don’t expect definite answers anytime soon.
Lastly, we can’t rule out comets and meteors from housing dormant extraterrestrial microbial life forms.
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