An often repeated statement about any extraterrestrial object is: “if it has liquid water it might suport life”. On this assumption a lot of space probes, robots and rovers include the sensors and the instruments the search for traces of past life. This has had high priority in many missions to Mars and it will have high priority also in future missions to the satellites of Jupiter.
Now the thought came to my mind that the ability to support life might not be enough. Life on Earth exists in the most inhospitable places, even in lakes that formed below the polar caps. But the theory is that life evolved in the primordial soup, which was a very favourable environment, only later it spread to inhospitable environments.
To repeat myself, what I am saying is that the ability to support life and the ability to support the birth of life might be two different things. How much different is the question. If the answer is that the difference is strong and life needs a cosy environment in order to arise the assumption it had liquid water therefore it might have had life is moot.
So, how strong is the difference? Is just some liquid water in unknown conditions enough to let life arise, even if it might support existing life?
This hope comes from the fact that we don’t fully understand how life originated on earth but we do know that it pretty much happened as soon as conditions allowed (geologically speaking).
So although it’s a big unknown, it seems to suggest the barrier to first life might be on the lower side. So it’s worth exploring anywhere that’s had water (prerequisite for lots of interesting chemistry) and tectonic activity (zones of varied temperature).
Having a certain mass is also an indicator as this increases the likelihood of comet and asteroid collisions in the past, which as far as earth goes it’s thought these might have carried useful molecules for life that had generated as a result of cosmic rays acting on more basic compounds on the comets surface. (NASA has found some amino acids and other pre-organic compounds on comets)
Also worth pointing out that the expectation is usually that if there are signs of life it’ll be long dead. It’s possible there’s a window in the geological evolution of some planet/moons that makes basic replicating life not just possible but likely. That would at least offer an explanation of why earth’s life arose so early. Places like Mars and Europa may have briefly supported life in the past before conditions became unsustainable. It’s worth checking just in case it’s still going!
Can you imagine if we found stromatolites on Mars, but hidden away in some pocket that has better conditions for life?
I hope to live to see the day