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Mars is the dusty, airless void of death and general unpleasantness it is today because it lost its magnetosphere millions of years ago. This was because Mars's dynamo in its core shut down. Because it lost its magnetosphere, the solar wind was able to strip away Mars's atmosphere, oceans, and any unfortunate aliens.

I'm working on a setting where Mars never became uninhabitable and is still nice and earth-like. I want to make sure it's scientifically accurate, though. I was thinking about giving Mars a moon (or more). My hope is that maybe the tidal forces the moon enacts on Mars could re-start the dynamo in Mars's core. I have no idea if this would work, though, and many more things might have to change that I don't know about in order to ensure Mars stays habitable. Would this work?

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    $\begingroup$ Make it bigger. If it's close to the size of Earth, the magnetosphere likely would not shut down. (I am not a planetary scientist, though.) Also, I think you give too much credit to the lack of magnetosphere: the lesser gravity is a factor as well. $\endgroup$
    – jamesqf
    Commented Feb 7, 2018 at 2:25
  • $\begingroup$ Magnetosphere to hold atmosphere better, closer to the sun for warmth, some oxygen $\endgroup$
    – Len
    Commented Feb 7, 2018 at 15:36

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Some random protoplanetoid object hits Mars

There were probably a lot of protoplanetoid objects zooming around the Solar System. Most of them probably ended up in the Sun or in Jupiter. One of them probably hit Earth.

What you need is for another one (or more) of these objects to hit Mars and coalesce into a bigger planet. Then Mars' once active plate tectonics might not have stopped, is core might still circulate, and its atmosphere would remain.

To work, you'd want the planet to be nearly as large as Earth. After all, Venus is nearly as large as Earth and doesn't appear to have either a self-generated magnetic field or active plate tectonics. So there is more to it than just size. But size doesn't hurt. Mars is only 1/10 the mass of Earth, so whatever hit it should be several times larger than Mars, at least.

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  • $\begingroup$ Venus has a magnetosphere, but unlike Earth's it is induced by the Sun, a product of solar wind. It also has a much denser atmosphere than Earth. $\endgroup$
    – rek
    Commented Feb 7, 2018 at 3:11
  • $\begingroup$ Hitting Mars with a much bigger impactor is more likely to turn Mars into rubble than solve the problem. $\endgroup$
    – Thucydides
    Commented Feb 7, 2018 at 7:11
  • $\begingroup$ Venus also rotates retrograde, once every 243 Earth days $\endgroup$
    – ArtisticPhoenix
    Commented Feb 10, 2018 at 2:33

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