Since a few imprecise observations of Phobos, there’s been an elaborate theory that this tiny little moon was really not as natural as it may seem at first glance, and it was made by an alien species visiting Mars while making its way through our solar system, or an artifact of a lost civilization that might have once existed on the surface of the Red Planet and built the temple complex at Cydonia. Of course a little reading shows that the observations were corrected decades ago and that while Phobos is porous and is most likely a rubble pile of a moon, it’s very much a natural object as far as we can tell. But on the other hand, thinking about Phobos as some sort of massive, generation ship suddenly made me realize that hollowing out an asteroid and giving it some powerful engines was actually an really neat idea for a quick assembly of a massive spacecraft. Carve out some tunnels deep into the rock, reinforce them with extremely strong plastics and alloys, fill them with a branching network of inflatable bases, hook up a few reactors to power massive ion engines, and voila! You have a massive craft ready to explore an attractive exo-planet, or possibly get away from your dying one…
Even the craft construction methods aren’t necessarily all that complex. You just need to find an asteroid, and you’ll have plenty to choose from in any solar system, and using rockets, slip it into orbit around a planet from which you’ll launch spacecraft carrying all the equipment to be placed into it. An asteroid riddled with caves is pretty much ideal, providing plenty of places to anchor and inflate living modules which could then be hooked up to power generators like self-sustaining nuclear reactors and RTGs hidden behind tons of rock and heavy shielding with relatively little effort. The hardest part would be assembling big enough ion engines to embed them deep into whatever you want to be the back of the asteroid spaceship. After the habitats are inflated and life support systems are running just fine, crew your craft, turn on the engines, and slowly accelerate over the next several years into deep space. No need to run enormous factories for years on end to crank out tons and tons of metal or kevlar sheeting to be assembled into vast, aircraft carrier sized spaceships when you already have an asteroid the size of a small suburb at your disposal. Extremely low gravity will obviously be a problem but you’ll have protection from cosmic debris and the brunt of the radiation which fills the universe.
Having established that an asteroid turned into a relatively cheap spaceship is not an impossible concept in the least and might actually be a great shortcut for a species eager to expose deep space but not keen on a commitment of massive manufacturing capacities to the task, let’s get back to the idea of Phobos being such a craft and who could’ve built it. Maybe the Martians fled as their air got thinner and more toxic, and their seas dried out into barren red deserts? Seems very unlikely since early Martian oceans would’ve been hypersaline and hospitable only to microbial life. Even if these microbes had the potential to evolve into an intelligent and complex species, it’s overwhelmingly unlikely that they would’ve hit on the right combination of mutations and selective pressures to produce a species which could build civilizations and eventually launch itself into deep space to escape the dying world quickly enough. Evolution is a messy process and timelines could vary from environment to environment, producing major changes in just a few generations, but considering that out of a wide variety of life capable of intelligence here on Earth only one species has ever built spacecraft during the entire 3.5 billion year history of living things, it’s hard to declare that on another world, a billion years would be plenty to go from basic photosynthesis to spacecraft. We have no evidence that this could happen.
And if it did, what would it imply about us? Mars dried out billions of years ago and even if we are generous in assuming that maybe there was underground water than an intelligent species could’ve somehow utilized, a geologic shutdown of the planet over a billion years ago would’ve signaled a time to leave. So let’s say, that’s when they packed up and left Phobos behind for one reason or another as they readied their space arks. Did they go to settle our idyllic blue planet? Hard to say. At the time, Earth was a desert with green oceans which were saturated with iron and volcanic dust. It would’ve hardly seemed very appealing since its air was thick with noxious gases, oxygen was almost nonexistent outside the seas, and the entire world would’ve looked like a dusty, brownish spheroid punctuated by green splotches of liquid. Not exactly the most attractive world for an intelligent species looking for another place to live. Maybe it would’ve sufficed for them but they either left for a more appealing place or went extinct here since we haven’t exactly been finding billion year old cities filled by countless Martians. So if Phobos is so unlikely to be a Martian relic, that only leaves us with another species that could’ve built it and that species had to have come from another solar system, perhaps in the very distant past when Mars was still an attractive, habitable world, the life-friendly jewel of the solar system.
But there are problems with that theory as well. Detecting a Mars sized planet from another solar system is a task not to be taken lightly and require some serendipity and a lot of patient scanning for a transit so an alien species could detect that it may have been habitable. And if they hollowed out an asteroid to get here, we can estimate that they must’ve come from a room just down the hall from us, astronomically speaking. The trip in such a craft would take thousands and thousands of years otherwise, if they even managed to notice Mars in the first place. Though we could just put all this guesswork in proper perspective and note that from what we have seen so far, there’s no indication that there’s anything artificial about Phobos and that it’s just a battered, porous piece of rock orbiting Mars, and which will eventually fall to the surface and create a long, big crater as its orbit becomes unstable in the relatively near future. But hey, all the guessing about alien civilizations and a possible legacy of Martians in our solar system was fun, huh? And the concept isn’t even close to impossible when we consider the technical details, right? Alas, we can’t simply state that because something is possible it must have happened. We can only go by the evidence, and the only evidence we have says that Phobos is a moon and nothing more than that. And unless that evidence changes, that’s what it should remain.