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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$\begingroup$ That's called Atmospheric escape $\endgroup$– smciJan 7, 2020 at 9:26
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$\begingroup$ It's a weaker gravity field, but remember, Mars once had a thick atmosphere similar to Earth's, and it held onto that atmosphere for millions and millions of years with the exact same gravity field. The problem with Mars is this: it doesn't have a magnetic field to shield the atmosphere from the energetic solar wind, which can then easily break down water molecules into oxygen and hydrogen ions... and hydrogen ions, being electrically charged and super light, are then easily carried away from Mars' gravity field with the similarly electrically charged solar wind. $\endgroup$– user39728Jun 3, 2021 at 0:32
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$\begingroup$ I think. I'm not an actual scientist. I just like to speak like I know stuff. Please don't quote me. $\endgroup$– user39728Jun 3, 2021 at 0:33
2 Answers
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.
- Is a magnetic field really needed to terraform Mars?
- How would it be possible to kick start Mars's magnetic field?
- How strong a magnetic field does Mars need to contain Earth-like atmosphere?
- Could a powerful enough laser or maser heat the core of Mars?
- How could an induced magnetosphere be created at Mars for terraforming?
- Would it be possible to deploy a man-made magnetic field in Mars orbit for terraforming?
- Place a satellite at Sun-Mars L1 to shield Mars from Sun radiation
- Artificial magnetic shielding for Mars feasible?
- How long would a magnetic shield placed at the Mars L1 point take to replenish Mars's atmosphere?
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
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.
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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1$\begingroup$ So, warming Mars to Earth-like temperatures will actually make the possibility of a steady atmosphere even more unlikely? $\endgroup$– DougMay 30, 2018 at 13:06
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1$\begingroup$ 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, 2018 at 13:13
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2$\begingroup$ 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, 2018 at 18:50
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4$\begingroup$ @PearsonArtPhoto: "Solve venus problem". Seesh, that's so geocentric of you. $\endgroup$– SF.May 30, 2018 at 23:01
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1$\begingroup$ It would be nice if this answers clarified what "long term" and "a while" meant. Are we talking hours or centuries? $\endgroup$– HarabeckJun 3, 2021 at 16:12