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?
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?
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.