Traces of hypothetical ancient civilizations in the solar system should be looked for only on the Moon or Mars, since on Venus or other planets or moons, they should have been destroyed by geological or cosmic processes, says the planetary scientist in an article posted in the arXiv.org electronic library
“SETI and other extraterrestrial intelligence projects are targeting them in distant stars. If we believe that traces of technologically advanced aliens exist, then we should also think about whether there are traces of past highly developed civilizations in our solar system,”says Jason Wright, planetary scientist at the University of Pennsylvania and project participant Breakthrough Listen.
About 3.5 billion years ago, three planets of the solar system could, in principle, support life - Venus, Mars and Earth. Even today they are in different parts of the so-called "zone of life", where water can exist in liquid form. After a billion years, Venus turned into a hot and acidic hell, and Mars - into a waterless and frozen desert.
In principle, nothing prevented life from originating on Mars and Venus during this time, however, only the Earth was able to get out of both greenhouse periods and eras of complete glaciation, avoiding the complete disappearance of life. Life and civilizations, if they existed on Mars, Venus or other planets of the solar system, could have disappeared long before there were people and multicellular life on Earth. In addition, aliens could simply "migrate" to Earth and die in a cataclysm.
According to Wright, the idea of the existence of such civilizations, which the public often calls "ancient astronauts" or "ancient aliens", has never been taken seriously by scientists. He decided to correct this shortcoming by calculating the likelihood that the alleged traces of such civilizations can survive to this day.
Remains of animals, plants and traces of civilizations on Earth, Wright explains, disappear rather quickly due to the action of the forces of nature, the roots of plants and the vital activity of microbes. Due to this, the remains of such "ancient aliens", if they existed on Earth or other planets, will have an extremely limited lifetime.
For example, the rocks of the Earth's crust are gradually sinking into the bowels of the planet and melted, destroying all possible traces of life and civilizations in them. Such a process usually takes about 300-500 million years, which, according to Wright, is an extreme bar for the existence of traces of "ancient aliens" on Earth. The survival of the older traces of "aliens" is highly unlikely.
Mars and the Moon in this respect are more suitable for searching for such remnants of civilizations - their bowels froze for a long time, and on the surface of the Moon or in the bowels of the red planet could exist for much longer than 500 million years. On the other hand, even here their lifespan will not be infinite - on Mars artifacts of "ancient aliens" can be destroyed by wind and other sources of erosion, and on the Moon - by micrometeorites and cosmic rays.
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Venus is least of all suitable for such searches - it turned into a giant greenhouse a billion years ago, and massive volcanic eruptions, an "acidic" atmosphere and constant renewal of its surface should have destroyed any imaginable traces of ancient civilizations. This, however, does not exclude the possibility that they could have existed on Venus in more life-friendly historical eras. The same goes for Europa, Enceladus, and other moons of the giant planets whose surfaces are not stable.
All this, as Wright emphasizes, does not mean that such "ancient aliens" existed - the chance for this is vanishingly small. Nevertheless, the further study of the surfaces of Mars and the Moon, according to the planetologist, should take into account the possibility of the presence on them not only of traces of life, but also of long-extinct civilizations.