We don't [...] even have data for an estimate, and ignorance translates to B = 0.5.

(Downvoted.) No, it doesn't: we know quite a bit about Mars and about life, surely enough to have some sort of prior probability before encountering specific data. More to the point, we're never that ignorant. If you literally had no information about a topic, then you wouldn't know enough to even phrase a question about it, or recognize an answer to such a question. By asking the question, you must have some notion in your mind of what you're asking about, and it is from that notion that we must draw our prior probabilities, rather than arbitrarily picking 0.5. "Whereof we cannot speak, we must pass over in silence."