B Atmosphere on moons

[zuz]

Why don't other large moons in our solar system have atmospheres like Titan?

 

[DaveC426913]

I think you will find an intersection of criteria between moon diameter, its orbital distance and the nature its parent body that explains a lot of the absences, though maybe not all.

Many of the larger moons are around Jupiter, which has very strong magentic interations with its moons and tends to strip away the atmosphere. Many have a thin tenuous atmosphere clinging to them.

Titan is a Saturnian moon, and so is less affected by its parent (orbit 1.2M mi).
Triton is a Neptunian moon, so has managed to retain an atmo (orbit 220,000mi).

Below a certain size, most minor moon will have virtually no atmo simply because they lack the gravity to hang into it.

This is just a rough description - a more detailed correlation between moon radius, orbital distance and parent body should put most of them into place.

Of course, Earth's Moon remains an outlier in this explanation , as it is orbiting a small, rocky body - not giant, active Jupiter - and I don't think it's close enough to have an atmo interact with Earth (at least in modern times). I suspect here there is a fourth effect in play - our Moon could not hold onto its atmo like other outer system moons because of its proximity to the sun in the inner system.


Here's a list of moons by size:
https://thesolarsystem.fandom.com/wiki/List_of_natural_satellites_by_size

If my understanding is correct, you could go down this list one-by-one and strike off any moon that meets any of these criteria:
- Jovian
- in close orbit near one of the other gas giants
- near the sun (inner system)
- less than some diameter (only the first dozen or so moons are > 1,000km).

If I'm lucky, you'll be left with a very short list moons that do still have an atmo.

I welcome correction by our more well-informed astro members.

 

[phinds]

Why don't other large moons in our solar system have atmospheres like Titan?

what research have you done on this? What have you found so far?

 

[Hornbein]

How about them neutron stars? They have an atmosphere of superheated carbon plasma that is 2 millimeters thick.

 

[Baluncore]

The surface of a moon is not a good place to live. It is not exactly windswept, more likely it is UV sterilised. Maybe drill a hole down several kilometres, then fill the hole with a blend of xenon and oxygen. That should retain a subsurface atmosphere, with sufficient pressure and oxygen that you can breathe.

 

[Filip Larsen]

Below a certain size, most minor moon will have virtually no atmo simply because they lack the gravity to hang into it.

This would then depend on a combination of gravity and (local) surface temperature.

For each potential gas species (i.e. from its molecular weight) one can compare its Maxwell-Boltzmann speed distribution for the ambient surface temperature and compare it with the escape speed of the moon in question to see how large a fraction of the gas will have escape speed. This out-flux will lead to an exponentially decaying amount of surface gas, with bigger escape speed fractions meaning faster decay. Over time nearly all gasses are going to "boil off" most moons unless the flux is balanced by an influx of new gas, e.g. due to out-gassing from surfaces rocks or from within.

Also, I assume that gas influx on a moon in principle can also be due to out-gassing from the parent planet (or a close sibling moon), but I am not aware if this in practice is a significant or even observable mechanism for any of the solar system moons.

 

[zuz]

Thank you.

 

[Filip Larsen]

Regarding comparing escape speed and surface temperature for various gasses I found this nice diagram:
https://en.wikipedia.org/wiki/Atmosphere#Compositions

(Click diagram for big version)

Note that color bands indicate what kind of gas the planet or moon in question can expect to gravitationally hold on to and not that the gas necessarily is present. For example, Titan has no atmospheric oxygen (it has 94% nitrogen), but since it gravitationally could retain some oxygen it then must mean that over time there has been no significant gaseous oxygen producing process on Titan.

Also, the sharp edges of the color bands in the diagram does not indicate there is a sharp limit where on one side the gas can be fully retained and on the other it all escape, but as mentioned earlier, it more indicates that as you go down or to the right on the graph the rate of loss for that particular gas increase. So well below the color band of a gas, only trace amounts of the gas should be expected to be present (if at all).

 

[phinds]

but since it gravitationally could retain some oxygen it then must mean that over time there has been no significant gaseous oxygen producing process on Titan.

I agree it's unlikely but given how reactive oxygen is, that isn't necessarily true. It could be a modest amount that is reacting to something.

 

[Filip Larsen]

I agree it's unlikely but given how reactive oxygen is, that isn't necessarily true. It could be a modest amount that is reacting to something.

True, I should probably have said net production of oxygen.

My point was just to emphasize to the OP that just because, say, oxygen could potentially be gravitationally bound it doesn't mean there will be any oxygen present unless there over time has been a process or a set of processes with the net result that oxygen was produced or released. For example, as I understand it, on Titan the high amount of atmospheric N₂ is likely to have originated from the NH₃ ammonia compounds abundantly present, but at the same time the also abundantly present amount of (solid/liquid) H₂O has apparently not given rise to any significant amount of atmospheric O₂.

 

[zuz]

Regarding comparing escape speed and surface temperature for various gasses I found this nice diagram:
https://en.wikipedia.org/wiki/Atmosphere#Compositions

(Click diagram for big version)

Note that color bands indicate what kind of gas the planet or moon in question can expect to gravitationally hold on to and not that the gas necessarily is present. For example, Titan has no atmospheric oxygen (it has 94% nitrogen), but since it gravitationally could retain some oxygen it then must mean that over time there has been no significant gaseous oxygen producing process on Titan.

Also, the sharp edges of the color bands in the diagram does not indicate there is a sharp limit where on one side the gas can be fully retained and on the other it all escape, but as mentioned earlier, it more indicates that as you go down or to the right on the graph the rate of loss for that particular gas increase. So well below the color band of a gas, only trace amounts of the gas should be expected to be present (if at all).

Thanks, That helps explain a lot.