If a civilization like ours know they are dying and want to leave a legacy that a future generation can find in 50 million years, when the Planet renews itself - what would that be? What structure, gadget etc will last that long? 50 million - 100 million - 200 million -
Of particular concern in nuclear waste management are two long-lived fission products, Tc-99 (half-life 220,000 years) and I-129 (half-life 17 million years), which dominate spent fuel radioactivity after a few thousand years. The most troublesome transuranic elements in spent fuel are Np-237 (half-life two million years) and Pu-239 (half life 24,000 years). This will be our legacy for those in the future to become contaminated with because by then they won't know what radioactive waste is. Nice of us to leave this crap for them isn't it. Please Register or Log in to view the hidden image!
If there is an Extinction Level Event, there may not be new natural growth for 100 million years. By then, man made radioactivity may not be an issue. At the same time, I think most metals, plastics and other man made material would have long oxidized and would have gone. So would most man made structures and gadgets....
The only way would be to leave monuments on the moon. Little we do on earth would survive resembling anything we wish. Even orbital leftovers would have drifted away or fallen into the atmosphere by then. The moon (or some other atmosphere-less body) is the only realistic hope for leaving something that would last so long. ~String
Heh! And I'm sure you realize how much that sounds like Arthur C. Clarke's novel/movie 2001, don't you? Please Register or Log in to view the hidden image!
Billions of years old rocks found on earth (http://en.wikipedia.org/wiki/Oldest_rock); again billions of years old fossilized micro organisms (http://en.wikipedia.org/wiki/Oldest_fossils#Earliest_fossiliferous_sites); so finding a suitable material would not be a big problem. Problem is the selection of location. Because earth is an active planet; everything is moving constantly, and it is difficult to guess where this "message" would end up in 50 million years time. Second issue is the receiver of this message. They must be thinking at certain level in order to differentiate our message from other complexities and weird features of nature. Humanoids separated from chimpanzees around 6 million years, and common ancestor of modern humans around 200 thousands years ago. So it is difficult to guess what will happen in 50 million years.
Under the ice cap or in the ice cap and yet designed to float for when the ice caps melt. The ice caps/water world might be a quite durable combination. If Earth becomes Water World that might work for awhile? Perhaps we consider the various earth changes and provide multiple objects in various geologic settings and with different features like some float, some sink, some roll, etc.
If doomed civilization notices that there is lack of Moon in their era, long ago and far away, then doomed civilization could create/acquire sizable satellite in expectation that even clueless future planet husbands would recognize that perfect juxtaposition of apparent size of primary star and only satellite during eclipse would be so mathematically unlikely as to be virtually impossible by pure luck. Such perfect match of apparent size would be practically undeniable signal of deliberate engineering. Even apes much less clued in than Moon Watcher would notice something unnatural.
