Should we be more surprised to find some ancient evidence of living organisms on Mars or not to find such evidence? There has been a lot of debate about the proability of life ocurring on any given planet in the universe. Although this probabilitiy (call it P0) has not been satisfactorily determined, “Amir Aczel”: estimates the probability of life existing on a planet orbiting at least one other star in the universe as 1, to a reasonable approximation. His hypothesis has been “criticized”: but the criticisms, which relate principally to his estimate of the P0 variable —do not seem to make much difference to his result. The universe is so vast that the immense size of the “sample space” swamps all other variables in the probability expression. What, however, is the probability of life occurring more than once in a solar-system (e.g. ours ). The probablility must be large in comparison to P0 because many of fundamental pre-conditions for life have already been met (e.g. age, size and type of the star). And what, if anything, does the discovery of this life, occurring in a cosmic neighborhood where life has already occurred once, tell us about the value of P0? Nothing, I suggest. The Mars probe budget needs some other justification.

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