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In a future where Mars has been abandoned for several thousand years, and Earth has reverted to a somewhat-habitable state, why would aliens choose to live on Mars, rather than more habitable Earth? They are very similar to humans in almost every facet except appearance, and Mars was, before its abandonment, a thriving colony world.

Edit: For clarification, Humans have long been gone from the Solar System, and have moved on to other star systems in the galaxy. The Aliens and humans have not made contact yet, but the aliens are aware of the human's previous habitation of Earth and Mars. Aliens simply want to study the solar system up close and personal.

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    $\begingroup$ Can aliens make trouble? Do they want to make trouble? $\endgroup$ – Alexander Jan 22 at 21:07
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    $\begingroup$ Earth is more habitable to US because we evolved here, it may not be the case for an alien species. $\endgroup$ – John Jan 22 at 21:13
  • $\begingroup$ Tallest volcano in the Solar System. Largest canyon in the Solar System. An extra 37 minutes every day! 'Nuff said. Um... Science based? But what is your world building question? You seem to be looking for a plot point. $\endgroup$ – puppetsock Jan 22 at 21:24
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    $\begingroup$ Hmm, I'm temped to close this question as it's too broad. Any dissenters? $\endgroup$ – kleer001 Jan 22 at 21:28
  • $\begingroup$ @kleer001, yes I dissent. And for clarification, what happened to the humans? $\endgroup$ – Xavon_Wrentaile Jan 22 at 22:50
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Mars is much, much simpler.

This is sort of an omnibus consolidation of a lot of what's in the other answers, but the bottom line is that there are lots of reasons that Mars would make a much more desirable first foothold in the Solar System.

First: Gravity. Even if the aliens didn't originate on a low-g world, if they're showing up here without FTL drives then they've been living in low gravity for a WHILE just to get here. Even if they did, unless they're so advanced that energy costs for orbital transfer are just meaningless, it'll be a lot easier to move back and forth between their colony ships and the surface of Mars than it would be to do so on Earth. These aliens also have to account for the possibility that they're going to find something here that forces them to pack up and get out of dodge just like we humans did, and it'll be easier to do that analysis and (potentially) retreat from Mars.

Second: the Biosphere. Earth is absolutely COVERED in all kinds of crazy life, and all of that represents potential threats to newcomers who don't know the neighborhood yet. There's all sorts of biochemistry that's potentially hazardous between germs and pollen and atmospheric gasses and that's not even counting the possibility of being mauled by a hippopotamus. If I'm running a colonization effort I'm going to be just SUPER into the idea of setting up camp on Mars where the environment is stable, simple, and predictable rather than jumping right into all that crazy stuff going down on Earth.

Third: Stability This is closely related to the previous answer, but adds a whole new dimension. Mars doesn't have the gravity or the magnetic field to support an atmosphere of any significance, so even if Humans had wanted to turn Mars into a Earth-copy we probably couldn't. Certainly it would have been much easier to just bury all all structures under the martian surface which solves for the atmosphere AND radiation problems both for us, AND for the aliens that come later. Between plant life, wind, rain, and tectonic activity, whatever structures we left behind on Earth are going to be in MUCH more decrepit condition than the ones on Mars, which means that whatever xenoarcheological studies the aliens are interested in doing are going to be a LOT easier on Mars where our leftovers are mostly intact than they would be on Earth where it's all buried, eroded, flooded, and overgrown.

Basically, almost everything the aliens might want to do here is going to be easier on Mars than it would be on Earth's surface, ESPECIALLY since humans would have already done most of the hard work of creating extensive radiation-proof and easily pressurized habitations on Mars. You could easily spend a hundred years going nuts on Mars before you started running low enough on challenges and exploration that moving on to Earth started to become worth all that extra effort.

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    $\begingroup$ Mars also doesn't have active tectonics so no chance of accidentally setting down near a volcano or fault zone. $\endgroup$ – John Jan 24 at 0:28
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A good reason is that Mars has a weaker gravity than Earth.

If you need to colonize a new world from outer space, you will likely need a lot of arriving/departing* spaceships, so a lower gravity will facilitate these shippings. Moreover, Mars is farther from the Sun than Earth, which means that also travels to/from Mars will require less energy.

*Arriving could of course be more difficult than Earth: since Mars has less atmospheric drag, it will be more difficult for a ship coming from deep space to lose enough kinetic energy to land. But if - as the OP said - Mars had already been colonized then relinquished, it is safe to suppose that Mars will have a thicker atmosphere than it has now

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    $\begingroup$ They may also have evolved on a <1g planet. If a race from a .5g planet comes to Sol and has to choose between Mars and Earth they’d likely go with the one that causes fewer health problems... $\endgroup$ – Joe Bloggs Jan 22 at 22:47
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The answer lies in the thriving world. The Martian environment is remarkably stable. Whereas after thousands of years while Earth might habitable, almost everything associated with a technological civilization will be unworkable, corroded into useless lumps of matter, or entirely gone. The aliens would either have to import large amounts of tech or build from scratch.

An abandoned Martian colony world would be effectively intact and possibly all in good working order. They only need to move in, do some maintenance, but they don't have import or build from scratch everything they need. Mars has already been colonized and it will be left standing intact.

The aliens may have to adapt human technology to suit their needs better. They certainly will have to change a lot of signage and write their own instruction manuals. but all that's so less onerous then starting from effectively nothing. The aliens will be squatters moving into abandoned property.

Essentially it will be vastly easier for the aliens to settle On Mars, and if humans could live there, then the aliens can too, because the planet will be full of habitats and technology.

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The aliens have a policy which prohibits them colonizing life bearing worlds. Think of it like the Star Trek Prime Directive. They may interact with alien-to-them lifeforms, but not co-inhabit any world which has evolved its own life.

Alternatively, for a more science-based answer... They are afraid of our germs. Interstellar Life is nowhere near as diverse as we expect it to be and the microscopic life of each world can be dangerous to macroscopic life on other worlds.

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Although physically similar to humans the aliens basic biochemistry is very different to that found on Earth. The aliens considered that it would be easier to inculcate their biome into a sterile environment such as can be found on Mars rather than attempt to have their biome try to establish itself on a world already teaming with life that was very well adapted to the conditions on Earth (unlike their Biome). They will use a mass of interconnected domes and other structures initially where their alien environment can be recreated more easily.

The gravity on Mars is also more to their liking, but once established on Mars they might consider what to do with Earth.

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Mars is Red

An alien civilisation that can colonise other planets already has the technological ability to live in space - create artificial habitats of its own to its liking and traverse the galaxy with hundreds of billions of planets.

So why Mars? All other considerations aside, perhaps the aliens have odd rationales that humans cannot understand. Their priorities are not human priorities. Perhaps they are simply attracted to the colour red - they love it so much that they can choose to be picky, and choose Mars over any other planet in the solar system.

Perhaps they are like moths to a flame, or find the colour red irresistibly sexual (much like ostensibly some humans do). Perhaps they use it as a mating ground, a good holiday site seeing visit, or simply a place they like.

We must be careful to attribute human logic to an alien mindset.

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