Say in the near future, a fatal virus brings the human race into extinction overnight. Then, at some point, aliens finally find their way onto Earth. Will they still be able to find traces of human existence? How long before all evidence of our being were to vanish?
First, massive structures, such as power plants and canals, will disintegrate quickly mainly because they depend on humans for maintenance. However, come to think of Stonehenge, which is at least 5,000 years old yet visible today. While most buildings aren’t that strong, their traces, even just the steel bars, are most likely to be visible at least for 10,000 years.
Of course, nuclear waste will stay long. Earth’s atmosphere is brimmed with immense levels of plutonium-239, which resulted from the nuclear weapons testings performed amid the Cold War. Naturally, this isotope is only present in a relatively small volume but will be observable as a pollutant for nearly 250,000 years.
Other radiation hotspots can be detectable further in time. Though it can be regarded as natural elements in case traces of our technology be fossilized, and eventually, all remains of human existence melt into the mantle once tectonic plates subdue.
Truth to be told, humans have dealt ineradicable impact on Earth, and we will be discernable to extraterrestrial beings for millions of years. Human and animal fossils will be present, seemingly how we discovered dinosaurs fossils or even that of their ancestors. Moreover, indications in the planet’s chemistry will be enough to tip off that former beings brought change to climate, the planet’s surface, and used its raw materials to create various things.
With that, the remains of human existence may dally on Earth infinitely. While growing vegetation, fires, frost, rust, storms, and tremors may erase noticeable traces, concrete structures will be there to stay for a long time.
Then, other pollution records made by humans can also stay for millions of years. If in case, other beings from space are knowledgeable enough to discern geochemical results, it wi be easy for them to detect our existence.
However, the longest enduring hints of human civilization would be deep mines in hard rock, like lead mines in Australia and the gold mines in South Africa. If aliens visit these places, they can witness signs of our existence for billions of years. These mine tunnels will be brimmed with sediments as they are washed away by rainwater, producing vast amounts of industrial fossils.
Still, in Australia, there is a crust thriving for around 4.4 billion years. So, any mine tunnel humans built in the crust can remain as long if it isn’t affected or subdued by earthquakes.
With that, traces of human existence can vanish from the planet’s surface relatively quicker, but indications of human life will be buried deep on Earth as long as it survives.
But, not only on Earth will signs of our existence stay as technology allowed us to reach other bodies within the solar system, more significantly the Moon and our neighboring-planet, Mars. While not visible, another strong indication of human life is the electromagnetic communications signatures, which stretches into space at light speed in a continuously expanding medium. Who knows, it can even contain information about human’s ultimate fate if we happen to transmit these details before our doom.