Robert wrote:
Like Levin's logic chain, Benton's too has factors at opposite ends of the process that work against each other. I noted that larger fragments (boulders >1m) enhance the odds both of complete ejection and survival of microorganisms during transit; while the greater impact velocities at the other end of larger objects tends to decrease the odds of surviving the impact. You lose some, you win some, you lose some again.

Alex writes:
Since this is a probability equation (i.e. the parameters when multiplied together cannot exceed 1), then it is not surprising to see individual terms constrained (or modelled) in such a manner, especially given the unknowns involved in planetary ejection processes. The real problem is that most of the terms can only be crudely guessed at and might have several orders of magnitude of range differential. In other words, some parameters are simply a WAG. That said, however, like the Drake Equation Clark's work is an attempt to at least think of the problem in an orderly fashion while recognizing the practical difficulties (not to mention that actual utility of such an equation) in getting a *precise number*.

Alex R. Blackwell
University of Hawaii