# Escape velocity in the atmosphere

I have a question on escape velocity. Would a planet with 60% of Earths gravity support a relatively thick atmosphere? It would contain nitrogen, oxygen, water vapor and carbon dioxide. This planet would get the same amount of solar radiation and luminousity the earth does. Would the gases escape the atmosphere or would gravity be enough to keep them?

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mfb
Mentor
"60% of Earth's gravity" means 60% of the acceleration at its surface? With a similar density, this corresponds to 60% of its radius and 60% of the escape velocity.

While there is no known example of such a planet, I think it is possible, as long as the planet has a magnetic field. Titan can keep a thick atmosphere with just .15g (but lower temperature and solar radiation).
Venus gets twice the radiation intensity (compared to earth) and can keep its atmosphere with .9g.
Mars lost the atmosphere with .4g, the missing magnetic field is important here.

Yes and venus doesnt have a magnetic field im sure if it did it would be alot thicker than it is now. It does make sense that a planet with 60% Earths gravity, escape velocity would keep a thick atmosphere. I would imagine it wouldnt be as thick as Earths? I would think it depends on the density of the chemical properties though.

...the missing magnetic field is important here.
Could you please substantiate that? It's not from something like the movie "The core", is it?

mfb
Mentor
As you can see in the solar system, the potential to have an atmosphere is different from the real atmospheres. I would expect that earth could support an atmosphere similar to the one of Venus, and of course earth could have an atmosphere as thin as Mars.

@Andre: Does the movie "the core" has anything to do with mars? I do not know the movie. Magnetic fields deflect the solar wind and "protect" the atmosphere.

No I am wondering where it is postulated why magnetic field would be required to retain (some of/most) the atmosphere. I don't know the movie, but I found suggestions on the net about such a speculation in that movie.

I think that the atmospheric escape wiki adequately explains and notice this:

A common erroneous belief is that the primary non-thermal escape mechanism is atmospheric stripping by a solar wind in the absence of a magnetosphere....Recent models indicate that stripping by solar wind accounts for less than 1/3 of total non-thermal loss processes.

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mfb
Mentor
I am wondering where it is postulated why magnetic field would be required to retain (some of) the atmosphere
I cannot find any notion of that in this thread. It is important, but not the only relevant thing.
By the way, the same wikipedia page lists two effects as dominant non-thermal contribution for mars/venus which involve the flow of charged particles. And there, the magnetic field (if it exists) is relevant.

Anyway the loss of atmospheric particles is obviously related to its mass. For instance, the ten fold higher Deuterium Hydrogen ratio on Venus compared to Earth suggest that molecules or ions with single proton hydrogen outgassed much more than the proton-neutron hydrogen over the billions of years. That's a pretty steep ratio suggesting that the escape of particles with more mass like CO2 (44) would be negliglible.

It is thought that the deuterium ratio of Jupiter is close to the universal ratio, Universal ratio (22 ppm), while the abundance on Earth oceans is 156 ppm, so that might hint that Earth also lost hydrogen, but f course there are other ideas.

It would seem interesting to check Earths Atmospheric He3 He4 ratio, compared to that of helium wells, but the latter are so much more enriched in He3 that suggest that other processes are predominant.

But obviously Titan, with a lot less gravity is an example of a smaller body with a denser atmosphere than Earth.

I would think the magnetic field has alot to do with the atmosphere retaining a bunch of its thickness. Without one the solar winds would just strip it away. Also considering mars and it thin atmosphere and .38 G it would be interesting to see what would happen if that was amped up to .6 G. I would think with the extra gravity the atmosphere would be much thicker than it is now.

mfb
Mentor
Helium is probably tricky - radioactive decays are a natural source of He4, some atmospheric processes are a (small) natural source of tritium and therefore He3.

Anyway the loss of atmospheric particles is obviously related to its mass. For instance, the ten fold higher Deuterium Hydrogen ratio on Venus compared to Earth suggest that molecules or ions with single proton hydrogen outgassed much more than the proton-neutron hydrogen over the billions of years.
Maybe Mars lost a major part of its atmosphere this way. After radiolysis of water hydrogen escaped and oxygen has been bound by the regolith. The most obvious oxidation product is iron oxide which gives Mars its red color but there are also other "oxygen deposits". Phoenix Mars Lander detected high concentrations of perchlorate and I expect that later missions will also find nitrate, formed by oxidation of nitrogen from the former atmosphere.

But why this does not happen to earth? As we are closer to the Sun there should be even more radiolysis. Earth is heavier than Mars but this is compensated by higher temperatures. Therefore Earth's atmosphere should have been dehydrogenised too. Maybe there was much more water from the beginning but the biosphere could also play an important role by releasing oxygen from water and soil. The resulting high oxygen level reduces the concentration of free hydrogen in the atmosphere.

Maybe because the sun you to have 70% the energy it has now in the past, thus less energy.

mfb
Mentor
The ratio of escape velocity to average velocity of particles in the atmosphere is better on earth - the lower temperature does not compensate the lower gravity.

Sun's luminosity is increasing with time, a nice overview can be seen here. It was lower in the past.

Chronos