# Reality Check: Planet with low gravity and water vapour

Using Artefaxian's excellent spreadsheet I have created a basic idea for my planet, but in the 'Stability Checker' part my low gravity planet is unable to hold H2O.

I am wondering if it is possible to have a planet with low gravity and water vapour.

Mars apparently still has water vapour so I think it must be more than just escape velocity affecting the composition of the atmosphere.

EDIT: I'm wanting a planet with low gravity and oceans, rivers and lakes. Is this feasible or is it only possible to have traces of water on a planet with low gravity?

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– JBH
Jul 5, 2018 at 6:00
• Pluto has an atmosphere. Not much of one (with only a smidgen of water), but it has one. How much water vapor are you looking to hold against what gravity?
– JBH
Jul 5, 2018 at 6:13
• Hello and welcome to Worldbuilding Dane. :-) Just a friendly note: edits that others do to your posts are done with the best of intentions to improve your posts, usually in order to make them easier to read and understand, and — in the case of MathML — make it less computationally demanding for the site. That is not an admonition or complaint, it is simply aimed at making your post even better. Jul 5, 2018 at 7:11

The gravity of a body dictates that body escape velocity.

The combination of molecular mass and temperature of the body atmosphere dictates the velocity distribution of the molecules. The lighter the molecule, the faster it can move for the same temperature.

Those molecules having a velocity higher than the escape velocity can leave the body for outer space.

Since the velocity is statistically distributed, you will always have a tail of slow moving molecules, so you will keep traces of any gas (Earth has traces of Helium, though on paper it should not be massive enough for it to happen).

If you lower the temperature you can shift the tail even further, keeping more water vapor with the same gravity.

You can refer to this handy chart

• Thanks for the great answer, would it be possible to have oceans with an earthlike temperature or would it have to be on the lower end of the scale?
– Dane
Jul 5, 2018 at 7:04
• @Dane, check the chart I added in my edit
– L.Dutch
Jul 5, 2018 at 7:32

The secret here is the magnetosphere. if you have a planet with a hefty protection against solar winds, a low gravity planet as Mars can retain its water vapor and actually have a working weather cycle.

• A magnetosphere is useful because solar winds will contribute to blow away gases from the high atmosphere, but if the water vapor can achieve escape velocity it will vent it up, only slower - how much slowly depends on gravity, temperature, intensity of the solar wind and strength of the magnetosphere. Jul 5, 2018 at 7:34
• "...a low gravity planet as Mars can retain its water vapor..." how about oceans?
– Len
Apr 28, 2021 at 20:48