I've been thinking about panspermia recently. If life originated in comets, and life turned up on Earth very soon after conditions allowed it, that would suggest a planet being struck by a life-bearing comet is not nessecarily a rare event.

So wouldn't we expect to find traces of life pretty much everywhere in the solar system that can or at some point in history could support it? Both Mars and Venus apparantly used to have liquid water on their surfaces, so if life-bearing comets are quite common there ought to be fossilised microbes on the surface of both planets, yes?

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"I worry that, especially as the Millennium edges nearer, pseudo-science and superstition will seem year by year more tempting, the siren song of unreason more sonorous and attractive." - Carl Sagan, 1995