I can just imagine Elorp Munk building a space company on Mars a billion years ago to colonize the frozen and inhospitable Earth. Bringing over some germs and then failing to establish a colony… that could make for some great sci-fi.

I wonder if there'd be any good way to confirm or rule out an ancient Martian civilization. Would Martian surface structures be able to last a few billion years? Or would Martian processes be able to make all of them disappear? And how about underground structures?

I read somewhere that for geologic-scale signatures you need something that is:

   - Chemically inert
   - Recognizably artificial w/o context
   - Physically hardy
   - Planet-scale mass-produced

On Earth, apparently "panes of glass" are one of the few things that qualifies.

What if humanity oscillates between the two planets, and Elon and Elorp rise by design on each one.