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?