re: Andromeda Strained
I agree with your opinion regarding probable lack of other extant life in this solar system, however there is a slim possibility that we might pull a T.S. Elliot and find Earth microbes on Mars, Europa, Enceladus or Titan. S&T had an article on Earth panspermia via incidents like the Dino impact.
I also think that any putative Martian microbes would have arrived here long ago and repeatedly via an even easier impact scenario on Mars.
