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

[fbs7]

Pardon the very naive question, but why does the atmosphere in these gas giants seem to have, from a distance, a very clear, sharp and distinctive boundary?

When one looks at Earth's atmosphere from space, it seems to have a fuzzy bluish boundary, gracefully vanishing into the black. I read somewhere that's because the pressure of a gas in a gravity field follows something like

p = p_o e^-kr

There's no sharp boundary in that curve. Now, the same law should apply to a perfect gas in Jupiter, Saturn and in the Sun, leading to a fuzzy boundary; but that's not the case, so something else is obviously at play. The few things I can think of are:

(a) temperature - above a certain point the gases are visible, below that point they are invisible, leading to the apparently sharp boundary
(b) composition - above a certain point the gases have some composition that makes them not visible
(c) telescope sensitivity - that is, if your telescope is sensitive to light at level X, and the level of light reflected by the gases becomes smaller with height, then at certain height the telescope will just not detect light any more, so it's not that the gas ends at certain height, but we're just unable to see gas above a certain height
(d) that law is invalid for high heights in Jupiter, Saturn and the Sun

What is the explanation for the sharpness of the atmosphere boundary in these planets, compared to the fuzzy boundary in the Earth and Mars? Would the same thing apply to say Venus too (although Venus is not a gas giant)?

 

[lewando]

"A very sharp and distinctive boundary" is a matter of perception. In this image (demonstrating size differences) the earth/space boundary looks pretty sharp and distinctive to me.

 

[fbs7]

Correct, but there at Earth you're looking at is mostly land and oceans; the atmosphere is very thin. So, from a distance you'll see the solid mass of land and oceans, with a very very very thin fuzzy layer, which from distance will disappear.

In Saturn the whole of what you see is gas, and every other image that I have of gases (nebula, cloud, etc...) always look fuzzy.

 

[lewando]

Here is a neat image from solarsystem.nasa.gov showing Saturn being backlit by the Sun. The glowing annulus (not the "rings") would represent a "fuzzy zone", IMO.

 

[fbs7]

a-ha! that's nice!

so I guess the fuzzy zone is just not bright enough to be seen with normal illumination, right?

that is, it's really a matter of telescope sensitivity - if the telescopes were more sensitive or had longer exposition, then we might be able to see the fuzzy zone on direct light, correct?

or, another way of saying, the fuzzy zone is too transparent to be seen on direct light?

wow - what an awesome thing! so on top of the bright-colored orange/light brown clouds there is indeed a transparent layer that we can't see from Earth on direct light! that's superb!

 

[fbs7]

Man, I'm playing now this song in honor of that breathtaking picture!

take my breath away

 

[russ_watters]

Pardon the very naive question, but why does the atmosphere in these gas giants seem to have, from a distance, a very clear, sharp and distinctive boundary?

"From a distance" is the answer. It may be useful to calculate how big a feature would need to be to see it in one of those photos.

 

[fbs7]

hmm... but a nebula is seen from an even bigger distance, and it does seem all fuzzy - and is also bound by gravity.

is my understanding incorrect that the apparent sharpness of saturn's atmosphere is due to a change from something visible (orange/light brown) to something transparent, and that transparent jibby-jabby becomes visible on backlit (like the clear atmosphere on earth, so clear that you can spot stars at night, and yet it becomes orange/red on a band on the horizon just before the sunrise)?

I counted the pixels in the backlit image, and I see 3 pixels of fuzziness over a radius of 70 pixels, meaning about 4% of radius; so say 2-5%.. on a radius of 60,000 km, that is 1,200 to 3,000 km of fuzziness, which is humongous! Earth seems to have like 50 km of fuzziness, I guess.

Or maybe all that fuzziness in the backlit photo is an illusion? Maybe the Sun's rays are diffracting on different directions, giving the impression of more fuzziness than it really has?

 

[Klystron]

The Saturn photos make an awesome example of the Op's phenomenon. The apparent sharpness of the ring edges shows even on the shadow thrown on Saturn.

 

[DaveC426913]

"From a distance" is the answer.

I saw what you did there.

 

[lewando]

In Saturn the whole of what you see is gas

Well, yes on the visible exterior. You realize things become liquid perhaps solid on the inside?

and every other image that I have of gases (nebula, cloud, etc...) always look fuzzy.

Nebulas (and clouds) are another animal. They are very spread out and generally amorphous (pretty much completely gaseous), transparent in certain areas, less transparent in other areas. Not the best comparison to a gas giant planet.

 

[Ophiolite]

Pardon the very naive question, but why does the atmosphere in these gas giants seem to have, from a distance, a very clear, sharp and distinctive boundary?

IIRC the attenuation of starlight by planetary atmospheres during an occultation provides quantitative insight into the density and character of the atmosphere. That attenuation is the fuzziness you speak of and is adequately resolved with appropriate magnification.

(For those familiar with Father Ted, its case of "large cows, far away".)

 

[davenn]

hmm... but a nebula is seen from an even bigger distance, and it does seem all fuzzy - and is also bound by gravity.

yes they are bound by gravity but to a very much smaller extent

You just cannot compare nebula to a planets atmospheric layer
Nebula would be more closely comparable to the clouds in the Earth's sky

 

[russ_watters]

hmm... but a nebula is seen from an even bigger distance, and it does seem all fuzzy - and is also bound by gravity.

No, a nebula isn't necessarily gravitationally bound and even those that are, aren't planets or stars. A gas giant is a planet about as dense as water whereas a nebula is much, much, much less dense than air. They are totally different animals.

 

[Keith_McClary]

"For planetary atmospheres, scale height is the increase in altitude for which the atmospheric pressure decreases by a factor of e. "

Approximate atmospheric scale heights for selected Solar System bodies follow.

Venus: 15.9 km[5]
Earth: 8.5 km[6]
Mars: 11.1 km[7]
Jupiter: 27 km[8]
Saturn: 59.5 km[9]
Titan: 21 km[10]
Uranus: 27.7 km[11]
Neptune: 19.1–20.3 km[12]
Pluto: ~60 km[13]

The height of the fuzzy boundary is proportional to the scale height. Relative to radius (compared to Earth), the fuzz is 3 times smaller for Jupiter and a bit smaller for Saturn.

Rank Name Equatorial Radius (kilometer)
1 Jupiter 71492
2 Saturn 60268
3 Uranus 25559
4 Neptune 24764
5 Earth 6378.1
6 Venus 6051.8
7 Mars 3396.2
8 Mercury 2439.7
9 Moon 1738.1
10 Pluto 1195

 

[Keith_McClary]

The scale height might not be the whole story. I think the smaller curvature of the large planets would increase this effect. Someone would have to do the math. This must have been studied because as

IIRC the attenuation of starlight by planetary atmospheres during an occultation provides quantitative insight into the density and character of the atmosphere.

 

[256bits]

Venus: 15.9 km[5]
Earth: 8.5 km[6]

I was looking into that - scale height -
Interesting that all are greater than earth's, even with less or more gravity of the planet.

 

[fbs7]

No, a nebula isn't necessarily gravitationally bound and even those that are, aren't planets or stars. A gas giant is a planet about as dense as water whereas a nebula is much, much, much less dense than air. They are totally different animals.

OHHH... I see... maybe the apparent sharpness of that boundary is due to much higher gas densities in the gas planet! Got it! thank you!

 

[fbs7]

The height of the fuzzy boundary is proportional to the scale height. Relative to radius (compared to Earth), the fuzz is 3 times smaller for Jupiter and a bit smaller for Saturn.

OHHH... I see this too!... So, as saturn's height scale/radius is 1/3 as that of Earth, that means that, in distance to planet / radius of the planet one would have to be 3x closer to the planet (in planet radius terms) in order to see the same fuzzy boundary as one sees from Earth!

The Earth's fuzzy boundary is very clear from low orbit (less so in high orbit), so one would have to be (relative to the planet radius) in a 1/3x lower orbit in order to see the same fuzzy boundary!

Therefore geometry/gravity alone mostly explain this! What an awesome thought!

 

[fbs7]

Thanks all for the wonderful insights in this thread - that was most excellent information - I learned a lot from it!

 

[Keith_McClary]

Saturn being backlit by the Sun

I tried to find a comparable image of Earth. I can find Pluto and Titan backlit by the Sun, but not Earth.

 

[Klystron]

I tried to find an online reproduction of a backlit Earth photographed from a lunar orbiter I had at NASA Ames years ago. I found this image from a Google search but have no knowledge of authenticity, ownership or how it was enhanced. If at all authentic, the Sun should be roughly at bottom left, so not back light but interesting.

 

[hutchphd]

It is my understanding that the optical surface of Jupiter is probably thick ammonia and water vapor clouds in the atmosphere and so we are not seeing a solid planetary surface. Then the question is how fuzzy is the atmosphere above this layer...Anyone know more about this?

 

[sophiecentaur]

yes they are bound by gravity but to a very much smaller extent

You just cannot compare nebula to a planets atmospheric layer
Nebula would be more closely comparable to the clouds in the Earth's sky

The density of a nebula is extremely low compared with the atmosphere of gas giant planet. I think the
p = p_o e^-kr formula only applies accurately for the atmosphere around a 'rocky' body but the density / field inside a nebula is very different and unbelievable much lower (Shell theorem for field inside a sphere). So that makes a very significant difference between Jupiter and a planetary nebula, for a start as the nebula density would be varying much less around the periphery.

I would have to do some image searches to find a good example but there are actually some pretty sharp features in some nebulae. This would be where we see sideways through a flat region.

Basically, I think the answer to this could be that, basically, we cannot trust what we think we are seeing. That applies to most Astronomy observations.

 

[russ_watters]

"From a distance" is the answer. It may be useful to calculate how big a feature would need to be to see it in one of those photos.

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.

 

[glappkaeft]

I tried to find an online reproduction of a backlit Earth photographed from a lunar orbiter I had at NASA Ames years ago. I found this image from a Google search but have no knowledge of authenticity, ownership or how it was enhanced. If at all authentic, the Sun should be roughly at bottom left, so not back light but interesting.

View attachment 248357

Looks completely made up in photoshop. A real image of the Earth from the moon can be found here https://earthobservatory.nasa.gov/images/82693/earthrise-revisited

 

[sophiecentaur]

the entirety of that layer top will only be 2.5 pixels thick

A useful figure. Consider that photo's - particluarly Astrophotography - are processed and they are all 'sharpened'. Sharpening nearly always involves at least 3 pixels, or you can hardly see its effects. So that will have the effect of changing / lessening / even eliminating the fuzziness that @russ_watters calculated.

 

[Klystron]

Looks completely made up in photoshop. A real image of the Earth from the moon can be found here https://earthobservatory.nasa.gov/images/82693/earthrise-revisited

Thanks for locating the intended image. I understand even the photo sold at NASA gift shops was a composite retouched image from multiple data sources. This space artist inspired many an amateur astronomer back in the day.

 

[Keith_McClary]

higher gravity = steeper gradient

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

 

[sophiecentaur]

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

This wiki article is 'in depth' and shows that the k in the p = p_o e^-kr formula is a function of g. So the slope of the exponential formula is proportional to g. The fact that g hardly varies at all in a planetary atmosphere is not the relevant thing; it's the differential wot does it.