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If we ever try to make Mars outdoors habitable, won't the air just keep flying off into the outer space because of low gravity?

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Q: If we ever try to make Mars outdoors habitable, won't the air just keep flying off into the outer space because of low gravity?

Yes, but very, very slowly, so slowly, that it would not matter for terraformers.

Remember that Mars had quite a substantial atmosphere in the past! Evidence for running water and oceans is overwhelming. Of course Mars has enough gravity to hold a breathable atmosphere.

However, Mars' magnetic field is now gone, so the atmosphere escaped very, very slowly due to interaction with the solar wind.

This is why there are several question in this site about "reactivating" Mars's magnetic field, or adding a magnetic deflector at the Sun-Mars L1 point to reduce the flux of solar wind. Answers there point out that these are unnecessary because the lifetime of a terraformed atmosphere would be plenty long enough to make it useful. The science is not developed sufficiently to say exactly how long, but it's going to be something like a million years, rather than a hundred years.

From the Wikipedia article on Atmosphere of Mars:

However, early in its history Mars may have had conditions more conducive to retaining liquid water at the surface. In 2013, scientists published that Mars once had "oxygen-rich" atmosphere billions of years ago.

Possible causes for the depletion of a previously thicker Martian atmosphere include:

  • Gradual erosion of the atmosphere by solar wind. On 5 November 2015, NASA announced that data from MAVEN shows that the erosion of Mars' atmosphere increases significantly during solar storms. This shift took place between about 4.2 to 3.7 billion years ago, as the shielding effect of the global magnetic field was lost when the planet's internal dynamo cooled

  • Catastrophic collision by a body large enough to blow away a significant percentage of the atmosphere

  • Mars’ low gravity allowing the atmosphere to "blow off" into space by Jeans escape.

The last item is a contributing factor, but the timescale would be hundreds of millions of years, not really relevant for a question about terraforming

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Not for the long term, although it might last for a while. We can see what level of gas can stay based on the gravity and size of an object, as seen in the below chart from Wikipedia.

enter image description here

Bottom line, Mars can probably hold on to oxygen and nitrogen barely, but will have difficulty with water vapor, which is required. It does, however, do a great job of holding on to carbon dioxide. If the temperature was increased a bit, its ability to hold on to the atmosphere would be even further reduced.

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  • $\begingroup$ So, warming Mars to Earth-like temperatures will actually make the possibility of a steady atmosphere even more unlikely? $\endgroup$ – Doug May 30 '18 at 13:06
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    $\begingroup$ Yes. Warmer molecules move faster, and might move faster then the escape velocity of Mars. It seems one way to solve the Venus problem might actually be to heat the planet up even more. Hmmm.... $\endgroup$ – PearsonArtPhoto May 30 '18 at 13:13
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    $\begingroup$ OTOH I wonder how deep would you need to dig to allow for livable atmosphere on the bottom. Humans made some impressive strip mines on Earth; with enough dedication a hole a couple kilometers deep and wide should be doable. $\endgroup$ – SF. May 30 '18 at 18:50
  • $\begingroup$ @SF: Use the hydrostatic law to find out... $\endgroup$ – AtmosphericPrisonEscape May 30 '18 at 21:07
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    $\begingroup$ @PearsonArtPhoto: "Solve venus problem". Seesh, that's so geocentric of you. $\endgroup$ – SF. May 30 '18 at 23:01

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