Where Did the Martians Go?

Billions of years ago, Mars may have been habitable, with oceans and an atmosphere containing sufficient oxygen to support human life. The planet would have had a magnetosphere that protected the atmosphere from the ravages of the solar wind. As the magnetosphere weakened however, the atmosphere thinned and eventually could no longer support human life.

This would have been a gradual process and, assuming the Martians were technologically advanced, they would have had plenty of time to prepare as the strength of the planet's magnetosphere waned. What would they have done to survive the looming catastrophe? Earth was not like it is today and so, even if the Martians were capable of space travel, there was no where in the solar system to go. There only hope was to burrow beneath the surface.

Mars has never had tectonic plates and so there was no danger of serious earthquakes. When you consider that current Earthlings can tunnel under the English Channel and link England with France, the idea of burrowing down is not that unreasonable. Such habitation would be immune from the bombardment of cosmic and solar radiation. As on Earth, there would be abundant water deep beneath the surface but the main challenge would be creating breathable air.

The Martians may have learned how to harness the energy of nuclear fusion but even if they had not, geothermal energy would would have been sufficient to supply their energy needs. This energy could be harnessed to produce breathable air. How far underground would they needed to go? Probably not very far, a kilometre or two would have been sufficient. However, bombardment by space rocks would have been more frequent as the atmosphere was stripped away and so they may have chosen to go deeper in order to be better able to withstand rocks that on Earth would be deemed planet killers. 

Martian pyramid created using Wonder (AI) app

There would be no need to revisit the surface. The dangers would have been too high. The Martians would have become totally subterranean and no trace of their above ground habitation would survive the onslaught of time. Would they have left beacons on the surface to alert the inhabitants of future planets that there was life underground? Probably not. Why risk attracting attention and possible colonisation? They would have maintained remotely monitored observation posts to watch and listen to what was happening elsewhere in the solar system.

They would have begun picking up radio signals from Earth well over the century ago and they would certainly be aware of our current exploration of their planet. What would have happened to the physical appearance of the Martians over billions of years? They would probably have gotten smaller as there would be no advantage in remaining large. They may have become quite tiny, making them all the more wary of us relatively massive Earthlings. They would strive to remain invisible to us and the outside Universe.

Perhaps they recreated, underground, the life that they once enjoyed above ground. With unlimited energy from the interior of the planet or from nuclear fusion, they could have made their underground world as large as they wanted and supportive of a massive population, possibly in the trillions. Alternatively, they may have decided to strictly control their population size as the global elite are trying to do on Earth where a figure of 500 million is regarded as ideal.

It's far easier to control a population of half a billion rather than eight billion which is what Earth will be supporting by the end of 2022. I think they might have inclined to maintaining a population below one billion. Assuming the Martians were not destroyed at the hands of their own technology, they would have access to some impressive defensive resources and fearful offensive weapons. Our current exploration of the planet might be making the Martians increasingly nervous and predisposed to disabling our capability for interplanetary travel or even eliminating us altogether.

The Martians however, would have been aware that Earth had become an oasis just as Mars had been all those billions of years ago. Perhaps at some point they decided to abandon their underground world and colonise the Earth. Perhaps we are all Martians and thus to answer the question "where did the Martians ago", it might be the case that they simply become Earthlings. It's not impossible. Several science fiction stories and movies have relied on less plausible narratives.