We need to look elsewhere.
If a civilisation had developed to a level high enough for space travel then the moon would be an obvious place to look for signs of its existence, but even there geological processes may have destroyed any evidence. Volcanic activity may well have been occurring on the moon as little as a hundred million years ago.
Fortunately there is another region in the Solar-System where such evidence, even from a billion years ago, could be preserved. Geological activity is almost completely absent there. That region is the asteroid belt.
Of course, asteroids are not completely free of erosionary forces. Impacts would destroy surface artifacts over time, but unlike on Earth the results of activities beneath the surface, such as mining or habitat construction, would be preserved almost indefinitely.
This makes objects in the asteroid belt the prime target for research into pre-human civilisations.
And already, the first purpose built asteroid mission has found what could well be evidence of extensive mining activity.
CeresNASA's Dawn mission to the two largest asteroids, Vesta and Ceres, is the only significant asteroid belt mission so far. The probe is still in orbit around Ceres and will remain there indefinitely. The intriguing bright spots seen on Ceres could indeed be evidence of mining activity.
|Evidence of extensive mining activity in the Occator crater on Ceres|
There is also an unusual mountain, named Ahuna Mons, in another region of Ceres that could be a huge mound of excavated waste material, much like the slag heaps found near mines on Earth.
|A mountain of excavated material on Ceres|
Such discoveries on Ceres show that it must be explored in much more detail from the surface. A lander, rover, or even a manned mission is needed.
If one of the first asteroids to be orbited by a dedicated probe throws up such compelling evidence of a pre-human civilisation, it's highly likely that others will, too.
16 PsycheAnother very interesting body in the asteroid belt is 16 Psyche. The asteroid is unique in its composition - almost pure iron and nickel. Nothing else like it has been discovered in the Solar-System. It's possible that it once had an exterior of rock and ice, and that it was once a planet, and that that exterior was blasted away completely by an unfortunate set of collisions with other massive objects. But the likelihood of that is very small.
|16-Psyche - possibly the most heavily mined object in the Solar-System|
Another explanation must be found. It could well be that the rocky outer shell was intentionally removed, and if that's true it would probably be the most incredible evidence of mining that we could ever find. It's no surprise that NASA has recently announced it will be sending a probe to 16 Psyche, which will arrive in 2030.
If such a huge amount of material was mined it would be enough to construct thousand of starships and habitats, certainly enough to provide a refuge for millions for a civilisation that needs to evacuate its home world. 16 Psyche could be hiding the evidence of the ancient technological civilisation that lived on Earth hundreds of millions of years ago.
|Humans explore one of the last surviving tunnels on 16 Psyche|
The evidence of a civilisation that we may find in the asteroid belt may not have originated on Earth, of course. It could have been from Mars or even Venus, both of which seem to have suffered catastrophic climate changes. Such changes would have prompted any advanced civilisation on those worlds to do everything possible to preserve its species. It could have set up home on Earth (it could have formed one of the long extinct Earth-based civilisations), but at that time the Earth's atmosphere would probably have been unsuitable. What's more likely is that the species utilised the vast resources of the asteroid belt to construct interstellar starships (see my earlier article on the evacuation of Mars).
It's thrilling to think that when we do identify life-bearing planets around other stars (which I'm confident we'll do within the next few decades) the life we're observing may well have originated in our Solar-System.