Why Jupiter, Saturn and the Sun have a distinctive and sharp boundary?

[russ_watters]

I don't think the gradient is significant over 100km (maybe I'm misunderstanding what you're saying).

"Gradient" means "how fast it gets thicker". The atmosphere goes from "space" to thick and opaque in 200 miles.

Also, note that the 2.5 pixels thick I calculated is an upper bound. You can't see the atmosphere at 200 miles from Jupiter's clouds - it's too thin.

 

[Keith_McClary]

how fast it gets thicker

(How fast it gets thicker)/thickness is proportional to g. This is the inverse of the scale height I mentioned.

 

[russ_watters]

(How fast it gets thicker)/thickness is proportional to g. This is the inverse of the scale height I mentioned.

Sorry, I didn't see your post on scale height and now reading it, I think it's good information, and not incompatible with what I said: I gave the end points, and scale height tells us the shape of the curve. So I don't understand What your concern is. Could you try rewording/asking again?

 

[Dragrath]

Ok, so no one bit on this, so I'll just have to give the answer: All the weather on Jupiter is in a region 31miles thick. Above that is a region 200 miles thick where the atmosphere thins out to almost nothing. So its atmosphere is only a few times thicker than Earth's (higher gravity = steeper gradient). Jupiter is 86,881 miles in diameter. So if you view a 1080p high def photo of it, where it fills the height of the screen, the entirety of that layer top will only be 2.5 pixels thick.

Technically it depends on how one defines an atmosphere for giant planets and stars that lack a well defined solid surface. until recently due to the lack of data the definition for where the atmosphere begins/ends was entirely arbitrary.

Now however the Juno mission has finally given us solid data on this via the Juno missions results from the key science mission dedicated to using Jupiter's gravitational field to parse out its internal structure and dynamics. Juno studies Jupiter's gravity field by decomposing it into a set of spherical harmonic series of gravity harmonics within Jupiter and using the resulting data to match model ensembles in order to narrow down and determine the required sructure. Key to this is the nonzero odd harmonics which indicate the extent of gravitational asymmetry resulting from internal flow dynamics within the planet. These gravity harmonnics are measured by using Juno as a test mass between the Earth Jupiter and measuring the delay signal transmission time between the probe and Earth respectively. From these gravity field results we now can reliably say that the weather layer of Jupiter extends down to at least around 3000 kilometers.

Furthermore while lacking the same precision as Juno's harmonic measurements the results from the Cassini mission's grande finale attempted the same technique and found that the Weather Layer on Saturn is probably three times deeper i.e. around 9000 kilometers deep. The difference in the Weather layer depths seems to be driven by coulomb drag induced by the extreme pressures within the planet that counteracting the differential rotation of the planet and leading to the fluids below acting as a rigid body.
For further reference read the feature article on Juno from Nature(2017)

It is important to note that inside gas giants quantum degenerate effects become increasingly important as more and more of the body becomes dominated by electron degeneracy pressure. This is why there is little size difference between gas giant planets and brown dwarf stars as the addition of more matter is counteracted by the degenerate compression of matter.

As for the question here since it seems to mostly be answered the only thing that comes to mind as add to this is the limb darkening effect within the Sun

 

[sophiecentaur]

I have a feeling that the apparent outer layer of Saturn's atmosphere is due to clouds (as it is for Earth). Clouds have a fairly well defined upper limit and this is probably what we see. This link tells us that the clouds start about 100km below the tropopause. What happens above this layer will be invisible. So it's not the gases but the clouds that we see. The exponential decrease in density isn't what counts - it the thermal environment.