It may be impossible....It may be asymptotic.
You're familiar with asymptotes of course?
eg: y = 1/x^2
Y can never = zero...It can get darn close, but can never reach that unobtainable value.
it's an applicable allegory.
Applicable to the search for hypothetical martian microbial fossils.
The Mars meteorite ALH8001 controversy showed us that the bar may be raised too high
for "proof" to be obtained for ET microfossils.
Occam's Razor doesn't help the cause.