Planets made of gases

[C D Mohatta]

Planets such as Jupiter that are made of gases don’t just evaporate. Can any member give me the reason?

[Eise]

Gravity

[Sensei]

34 minutes ago, C D Mohatta said:

Planets such as Jupiter that are made of gases don’t just evaporate. Can any member give me the reason?

They do evaporate. At the moment with slow rate. When the Sun will turn to red giant at the end of its life, outer gaseous giant planets will receive much more energy per surface area than they receive now, and evaporation will accelerate. Solid inner planets will also evaporate, much quicker and much more violently. It is just a matter of time.

Edited February 22, 2019 by Sensei

[Janus]

7 hours ago, C D Mohatta said:

Planets such as Jupiter that are made of gases don’t just evaporate. Can any member give me the reason?

Escape velocity vs. average molecular velocity.   Escape velocity is the speed something would have to be moving to prevent gravity from pulling it back to the planet.  Escape velocity for Jupiter is ~ 59.5 km/sec.   Compare this to the average speed of air molecules near the surface of the Earth at 0 degrees C. This is under 0.5 km/sec.     The average temp for Jupiter is -145 degrees C.   Lower temperature means lower molecular velocities.  Even allowing for the fact that Jupiter's atmosphere is composed of lighter gases than the Earth's, which would allow for higher molecular speeds at any given temp, they still don't ( on average) achieve escape velocity.   As Sensei pointed out, this doesn't stop the loss of atmosphere completely, as random molecules can escape if their velocity is enough over the average, but this is a slow process which will not reduce the size of the gas giants significantly up until the Sun expands. 

[swansont]

To expand on what Janus has said: The speeds will have a Maxwell-Boltzmann distribution, so a few molecules out in the long tail will have escape velocity, but they also have to not collide with another molecule (and lose speed), and have the proper direction (you aren't going to escape if you are moving toward the planet), in order to escape. So for whatever fraction of molecules has escape velocity at any point in time, the fraction of molecules actually escaping will be much smaller.

[Intrigued]

I understand the thread is addressing evaporation, but on the more general topic of atmospheric loss the solar wind is a major factor for the terrestrial planets. I was curious as to how important this might be for Jupiter and beyond, given that the greater distance (presumably reduced impact of the wind). the larger gravity and the magnetic fields of the gas and ice giants should all make for a significant reduction.  There are quite a few references to sputtering erosion on the satellites of the outer planets, but nothing on the planets themselves. Does anyone here have any info on that?