But I guess my question is this: we're going to spend $XB to send a probe to a distant moon. Once it lands, we get to ask exactly two questions. Question one is confirm/deny organic life (the microscope). Why is question two's ability to provide a meaningful answer so contingent upon a very specific outcome of question one?

Is this a combo of a) the cost of the second question (i.e. the weight and size of sending the DNA sequencer) is fairly small, b) we're confident enough about question one that putting resources toward question two is worth the opportunity cost of asking a different question, or c) we have no other questions that we can reasonably ask for the same amount of resources?