T00fri's Titan @ Celestia

[t00fri]

Selden,

I strongly feel that a multi-wavelength approach should definitely go much beyond the "alternate surface" options.

Look, we have coded all this sophisticated multi-window functionality! This could be beautifully exploited for multi wavelength displays. After all, we might want to compare most of all, how an object looks if viewed with different wavelengths!

Another crucial issue already touched upon yesterday with Grant, concerns the issue of standardization of the various wavelength windows, a uniform and intuitive choice of the "false colors" to use for the displays, how to act if there are missing textures in certain bands, etc.

Bye Fridger

[selden]

Fridger,

I agree that there needs to be some kind of multispectral support in Celestia that is more useful than AltSurface textures.

However, I also think that concrete examples need to be provided so that alternatives can be criticised by those people who find it difficult to imagine how things might work. It is easy to misunderstand proposals and then argue against ideas that were never intended.

And, of course, I find it easy to try to use a hammer when a ratchet-wench is out of my reach

[maxim]

So, for brainstorming issues I throw these (unreflected) ideas into the pool:

- We could agree on certain waveband folders. As is with lores/medres/hires there could be folders for 1000-1500nm, 1500-200nm, 2000-2500nm ... this would presort the available stuff. Build into celestia it could be used like the switch for resolution change.

- The used waveband could be a graphic format header entry (most graphic formats I know have areas for custom string entries) - so we are independend from file naming conventions.

As I said: don't know how practical these ideas are.

maxim

[Evil Dr Ganymede]

One problem is that you'd be massively increasing the file space required for textures if you (for example) put several maps at different wavelengths in for each planet in the solar system.

This should very much be an optional extra, IMO, not something that comes with the default Celestia. Otherwise it'd make the file downloads much bigger.

[t00fri]

Hi all,

here are some further propositions I have for multi-wavelength displays. I think it is most important to design these displays in a

--graphically attractive,
--sufficiently flexible,
--physically most intuitive and
--operationally simple

manner.

Here is what I suggest:

Consider for example 3 solar textures, one in visible light, one in the UV band and one IR texture. Although we only have 3 discrete wavelengths, a

"wavelength alpha slider"

would be ideal as follows:

Imagine the 3 textures as three perfectly aligned layers like we have it e.g. with the main and nightlights texture. In addition, the slider would manipulate simply alpha channel masks such that a programmable manual blending function can be realized. Besides a simple default setup, individual transparency-wavelength profiles should be loadable from a respective data file!

I am sure Chris knows of appropriate OpenGl realizations of this mechanism that I have simply abstracted from my knowledge of image manipulation techniques.

Clearly with the slider that would reside e.g. in our future toolbar, one then could beautifully compare the appearance or disappearance of features according to different wave length! The actual wavelengths of the available textures and of the (continuous) momentary slider position would be displayed in the slider.
If there are no multi-wavelength images available for an object, the slider would simply be grayed out. If there is only one texture in either the IR or UV slot, the slider would work only in that direction.

Titan would be a great example of this idea: here the mapping via an alpha mask could even reflect the various narrow "windows" of haze transparency as function of the wavelength between the near IR and the IR regime! So by shifting the slider from visible (just an orange haze ball to be seen) to the near IR window of Cassini the surface features would gradually appear. Proceeding further into the IR, would restore the opaqueness until we arrive around 1.5mu. At 1.6 mu we would again be back at opaqueness!

I hope you can get a feel of the amazing possibilities of this concept both educationally and also as a scientifically quite accurate viewer...

Such a setup would allow also to gradually build up a multi-wavelength image data base without affecting other display functionalities significantly. Also this way of implementation simulates closely the actual situation in space research.

Bye Fridger

[alphap1us]

Hello everyone,

Very interesting/exciting discussion. I hope that I my thoughts are useful.

I would not worry about the extra data this intiative would require. I would be happy to make this data available from the Motherlode. Someone could always create a Celestia "scientific distro' with higher-res textures, different capabilites, etc. This would help "Wow!" more people will less effort. (I someitmes have to dl a few add-ons before i can really engage someone's interest in the program.)

I would also agree that some alternate surface texture for viewing planets and moons in other wavelengths be created now, before we have the slick version that t00fri is suggesting. This way, everyone will see the benefit of having these other views. Then it will be relatively little work to incorporate these textures into the slider function, once it is ready.

Cheers,
Joe

[Spaceman Spiff]

t00fri,

could you add to your slider idea that both centre frequency and bandwidth are adjustable in some sort of widget? If it's done well, I'd hope to be able see a band that represented the whole of the EM spectrum from gamma rays to radio waves. Initially, a rectangle or maybe an isoceles 'weighting' triangle sits over the visible part, bracketing the UV and IR ends. The triangle slopes represent the degree of alpha blending at each wavelength. Gaussian or Hamming, etc might be too much computation for not much gain.

The pointer can grab the rectangle/triangle to change the centre frequency, or either rectangle/triangle end to change the bandwidth. This way, you can slide a 'narrowband' down the whole EM spectrum and see what you described for example with Titan's atmosphere. On the other hand, I would like to widen the bandwidth so that something could be blended from X-ray to Radio.

It would be best to have a fairly regularly spaced set of monochrome, narrowband filter images in greyscale. The three RGB colour all sit with a 2:1 ratio, but maybe root(2) or 2 scaling would be OK. The weighting triangle would determine alpha and 'colourisation' of each greyscale image. The wider the bandwidth, the more colourised images are blended (by averaging alpha-weighted RGB values). The limit to how many is computer performance related. Colour composite images (even in IR from 2MASS, say) should not be converted to greyscale if its composites can be found. Gaps mean that a 'badly placed' triangle may mean monochrome or no image. We'll have to live with that.

I'd assume everyone would agree that the gamma end is colourised blue/violet and the radio end is colourised red. There could be an option as to whether the violet-red colourisation is kept within ends of a narrowband triangle, or kept across the entire EM spectrum.

The success of this might depend on choosing one single object first and doing it well. I was going to suggest the sun, but now I think a galaxy (Milky Way centre? Centaurus A?) might be better covered.

I wasn't keen on this multi-spectral feature at first, because of the chaos with which astronomical data covers the EM spectrum, but somehow your slider idea should bring order to it.

I'd agree with Evil Dr Ganymede that the core of Celestia is about the visual representation as if I were seeing things with my own eyes. For this reason, I gamma factored my Venus and Titan cloud textures so that Venus appears as a pure white ball and Titan is a pale smokey orange ball. However, your Titan ideas and piccies were very interesting and if this were done right, it would be quite something. Maybe this should be restricted to a 'special edition' of Celestia, since you are keen to stress it's scientific (technical) merits. I can imagine future forum posts from people who don't understand it, such as (I'm teasing slightly here ):

> Q. Why can't I see the whatever nebula in X-ray? It goes all dark. Is there something wrong with Celestia?
> Q. Why is the sun green in X-ray?
> Q. Why are Earth's cloud's black in infra red? (Think about it... ).

Spiff.

P.s., some loose ends when I re-reviewed this topic:

... I read a number of more or less scientific reports about Titan and its atmosphere. One repeating statement is that the light at the surface is about 1/1000th of the light on earth.

I'm amazed! Saturn and Titan are 30 times the distance from the sun as Earth. By the inverse square law, sunlight is then 900 times weaker. This repeating statement is implying all the light diffuses through to Titan's surface. If so, I'd consider that the light level imediately after sunset - a bright twilight. Or, maybe it meant at the upper atmosphere. Personally (and I've posted this before) I think there are very little actual clouds in Titan's atmosphere away from the poles - it's just smog everywhere else. If that's the case, I reckon the sun would be detectable as a diffuse bright patch in the sky, maybe even as a small faint disc. At night, Saturn would also be a large disc, also faint and diffuse. I reckon you could see both of them from Titan's surface.

Well, IIRC, Titan's atmosphere is actually supposed to be about 400km high...

The resulting X-ray atmosphere thickness comes out as 880+-60 km!
...
Typically my best estimate is that above 250 km the atmosphere should be blue, while the hydrocarbon cloud layer extends up to 60-70 km.

What I see is that Celestia makes atmospheres out of a flat ring around the limb of planets. It has two linear colour gradients - one from lower to upper, and one from upper to black. The black occurs exactly at the Celestia Atmosphere height. This is physically incorrect, but easy to define.

What are all these 'atmosphere heights'? An atmosphere has a scale height and a ground pressure. If you could assume it's isothermal, the atmospheric pressure drops to 1/e per scale height. For Earth, ground pressure = 1,013mB, scale height = 8km. For Titan, IIRC ground pressure = 1,500mB, scale height = 64km (I can't remember Titan's surface gravity but I'm taking 1/8th of Earth here: 8?8km = 64km). The same atmospheric pressure at 100km ("edge of space for the X-prize!") over Earth then maps to 800km over Titan, so that's OK.

I propose atmospheres are best defined by ground pressure alone (ignoring colours for the moment - that's mostly Raleigh scattering). Scale height derives from surface gravity. How the atmosphere appears to fade away depends on density and opacity. Opacity is also what governs visibility through smoggy atmospheres. ...Ah if I had the time to dig up all that physics...

[t00fri]

t00fri,

could you add to your slider idea that both centre frequency and bandwidth are adjustable in some sort of widget? If it's done well, I'd hope to be able see a band that represented the whole of the EM spectrum from gamma rays to radio waves. Initially, a rectangle or maybe an isoceles 'weighting' triangle sits over the visible part, bracketing the UV and IR ends. The triangle slopes represent the degree of alpha blending at each wavelength. Gaussian or Hamming, etc might be too much computation for not much gain.

The pointer can grab the rectangle/triangle to change the centre frequency, or either rectangle/triangle end to change the bandwidth. This way, you can slide a 'narrowband' down the whole EM spectrum and see what you described for example with Titan's atmosphere. On the other hand, I would like to widen the bandwidth so that something could be blended from X-ray to Radio.

It would be best to have a fairly regularly spaced set of monochrome, narrowband filter images in greyscale. The three RGB colour all sit with a 2:1 ratio, but maybe root(2) or 2 scaling would be OK. The weighting triangle would determine alpha and 'colourisation' of each greyscale image. The wider the bandwidth, the more colourised images are blended (by averaging alpha-weighted RGB values). The limit to how many is computer performance related. Colour composite images (even in IR from 2MASS, say) should not be converted to greyscale if its composites can be found. Gaps mean that a 'badly placed' triangle may mean monochrome or no image. We'll have to live with that.

I'd assume everyone would agree that the gamma end is colourised blue/violet and the radio end is colourised red. There could be an option as to whether the violet-red colourisation is kept within ends of a narrowband triangle, or kept across the entire EM spectrum.

The success of this might depend on choosing one single object first and doing it well. I was going to suggest the sun, but now I think a galaxy (Milky Way centre? Centaurus A?) might be better covered.

I wasn't keen on this multi-spectral feature at first, because of the chaos with which astronomical data covers the EM spectrum, but somehow your slider idea should bring order to it.

Spaceman Spiff,

I am impressed. You did not only understand fully what I have in mind, but in addition you added quite a few great ideas yourself. I go perfectly along with what you wrote.

I'd agree with Evil Dr Ganymede that the core of Celestia is about the visual representation as if I were seeing things with my own eyes. For this reason, I gamma factored my Venus and Titan cloud textures so that Venus appears as a pure white ball and Titan is a pale smokey orange ball. However, your Titan ideas and piccies were very interesting and if this were done right, it would be quite something. Maybe this should be restricted to a 'special edition' of Celestia, since you are keen to stress it's scientific (technical) merits.

If I had more time available that's probably what I wanted to push. The point is that according to my perspective, there are so much more exciting and notably topical astrophysical features to incorporate into the Celestia (simulation) engine.

But as it seems, toolbars, key shortcuts and other graphical enhancements are more important for now.

Once more we seem to have reached the point, where one has to realize that Celestia cannot possibly serve everyone's interest. Despite attempts of modular design, respective decisions are unavoidable soon or later.
If one wants to make museum/planetarium people, scientists, amateur astronomers and laser gunners happy at the same time a clash will arise for sure...

I can imagine future forum posts from people who don't understand it, such as (I'm teasing slightly here ):

> Q. Why can't I see the whatever nebula in X-ray? It goes all dark. Is there something wrong with Celestia?
> Q. Why is the sun green in X-ray?
> Q. Why are Earth's cloud's black in infra red? (Think about it... ).

Spiff.

P.s., some loose ends when I re-reviewed this topic:

... I read a number of more or less scientific reports about Titan and its atmosphere. One repeating statement is that the light at the surface is about 1/1000th of the light on earth.

I'm amazed! Saturn and Titan are 30 times the distance from the sun as Earth. By the inverse square law, sunlight is then 900 times weaker. This repeating statement is implying all the light diffuses through to Titan's surface. If so, I'd consider that the light level imediately after sunset - a bright twilight. Or, maybe it meant at the upper atmosphere. Personally (and I've posted this before) I think there are very little actual clouds in Titan's atmosphere away from the poles - it's just smog everywhere else. If that's the case, I reckon the sun would be detectable as a diffuse bright patch in the sky, maybe even as a small faint disc. At night, Saturn would also be a large disc, also faint and diffuse. I reckon you could see both of them from Titan's surface.

In agreement with what I wrote somewhere in this long thread!

Well, IIRC, Titan's atmosphere is actually supposed to be about 400km high...

The resulting X-ray atmosphere thickness comes out as 880+-60 km!
...
Typically my best estimate is that above 250 km the atmosphere should be blue, while the hydrocarbon cloud layer extends up to 60-70 km.

What I see is that Celestia makes atmospheres out of a flat ring around the limb of planets. It has two linear colour gradients - one from lower to upper, and one from upper to black. The black occurs exactly at the Celestia Atmosphere height. This is physically incorrect, but easy to define.

What are all these 'atmosphere heights'? An atmosphere has a scale height and a ground pressure. If you could assume it's isothermal, the atmospheric pressure drops to 1/e per scale height. For Earth, ground pressure = 1,013mB, scale height = 8km. For Titan, IIRC ground pressure = 1,500mB, scale height = 64km (I can't remember Titan's surface gravity but I'm taking 1/8th of Earth here: 8?8km = 64km). The same atmospheric pressure at 100km ("edge of space for the X-prize!") over Earth then maps to 800km over Titan, so that's OK.

I propose atmospheres are best defined by ground pressure alone (ignoring colours for the moment - that's mostly Raleigh scattering). Scale height derives from surface gravity. How the atmosphere appears to fade away depends on density and opacity. Opacity is also what governs visibility through smoggy atmospheres. ...Ah if I had the time to dig up all that physics...

I think atmosphere revision is also on Chris shopping list. Clearly, putting in 250Km of atmosphere height into solarsys.ssc clearly exposes a number of weaknesses upon inspection of the result...

Thanks for your creative thoughts,
Bye Fridger

[Evil Dr Ganymede]

I'm amazed! Saturn and Titan are 30 times the distance from the sun as Earth. By the inverse square law, sunlight is then 900 times weaker.

Er, no.

30 times the distance from the sun as Earth is 30 AU, which is roughly where Neptune is.

Saturn is 10 AU from Sol. So actually it's getting 1/100th of the light that Earth gets, since it's 10 times further away.

[t00fri]

I'm amazed! Saturn and Titan are 30 times the distance from the sun as Earth. By the inverse square law, sunlight is then 900 times weaker.

Er, no.

30 times the distance from the sun as Earth is 30 AU, which is roughly where Neptune is.

Saturn is 10 AU from Sol. So actually it's getting 1/100th of the light that Earth gets, since it's 10 times further away.

Evil Dr.

Let me remind everyone that the figure of 1/1000th I quoted (from NASA!) referred to the light fraction on the SURFACE of Titan. I can see nowhere in your estimate what amount of absorption within the haze you have used?;-)

Bye Fridger

[granthutchison]

Let me remind everyone that the figure of 1/1000th I quoted (from NASA!) referred to the light fraction on the SURFACE of Titan. I can see nowhere in your estimate what amount of absorption within the haze you have used

Fridger, the Evil Doctor is simply pointing out the error in Spaceman Spiff's calculation of the insolation of Titan above the atmosphere. Combined with your figure, it implies that only a tenth of the light gets to the surface, rather than Spiff's erroneous estimate of almost all of it.

Grant

[t00fri]

Let me remind everyone that the figure of 1/1000th I quoted (from NASA!) referred to the light fraction on the SURFACE of Titan. I can see nowhere in your estimate what amount of absorption within the haze you have used

Fridger, the Evil Doctor is simply pointing out the error in Spaceman Spiff's calculation of the insolation of Titan above the atmosphere. Combined with your figure, it implies that only a tenth of the light gets to the surface, rather than Spiff's erroneous estimate of almost all of it.

Grant

Yes, Grant, I also realized that after I had written my post already(those amazing 30 AU;-)). Then I thought I don't erase it, since it can't do much damage either;-).
Perhaps it would even help avoiding further buggies...

Bye Fridger

[granthutchison]

I gamma factored my Venus and Titan cloud textures so that Venus appears as a pure white ball and Titan is a pale smokey orange ball.

But there are faint albedo variations in Venus's clouds at visible wavelengths. This was recorded by Earth-based observers long before spacecraft reached the planet, and has been confirmed by Galileo imaging in visible light.

I think there are very little actual clouds in Titan's atmosphere away from the poles - it's just smog everywhere else. If that's the case, I reckon the sun would be detectable as a diffuse bright patch in the sky, maybe even as a small faint disc. At night, Saturn would also be a large disc, also faint and diffuse. I reckon you could see both of them from Titan's surface.

A search on the optical depth of Titan's smog in visible light turns up recent estimates of around 5 - that would count as "opaque". (200km of sub-micron particles is hell of a lot of scattering.) We can't actually see down to the methane cloud layer in visible light, only in infrared - so I'm thinking of Fridger's "clouds" simply as a way of indicating a dense limit to visibility (which is why I think the cloud definition should place this layer above the highest likely level of methane clouds, rather than at their lowest likely extent). It's by no means perfect, but of course if we were to simulate the real situation with pure haze, we'd have no way (in Celestia's current configuration) of turning it off to see Fridger's nice surface texture!
So I guess I'm not convinced you would see much of Saturn. It doesn't take much mist (read, optical depth) to reduce the Moon to a diffuse glow and then extinguish it entirely (trust me, I had a very unpleasant experience during a moonlight hike in the Scottish mountains), and Saturn has a surface brightness less than 5% of the full Moon. An ill-defined patch of slightly higher luminance? OK. But a banded giant with razor-edge rings, so beloved of NASA illustrators? Nah.

Grant

[t00fri]

I gamma factored my Venus and Titan cloud textures so that Venus appears as a pure white ball and Titan is a pale smokey orange ball.

But there are faint albedo variations in Venus's clouds at visible wavelengths. This was recorded by Earth-based observers long before spacecraft reached the planet, and has been confirmed by Galileo imaging in visible light.

I think there are very little actual clouds in Titan's atmosphere away from the poles - it's just smog everywhere else. If that's the case, I reckon the sun would be detectable as a diffuse bright patch in the sky, maybe even as a small faint disc. At night, Saturn would also be a large disc, also faint and diffuse. I reckon you could see both of them from Titan's surface.

A search on the optical depth of Titan's smog in visible light turns up recent estimates of around 5 - that would count as "opaque". (200km of sub-micron particles is hell of a lot of scattering.) We can't actually see down to the methane cloud layer in visible light, only in infrared - so I'm thinking of Fridger's "clouds" simply as a way of indicating a dense limit to visibility (which is why I think the cloud definition should place this layer above the highest likely level of methane clouds, rather than at their lowest likely extent). It's by no means perfect, but of course if we were to simulate the real situation with pure haze, we'd have no way (in Celestia's current configuration) of turning it off to see Fridger's nice surface texture!
So I guess I'm not convinced you would see much of Saturn. It doesn't take much mist (read, optical depth) to reduce the Moon to a diffuse glow and then extinguish it entirely (trust me, I had a very unpleasant experience during a moonlight hike in the Scottish mountains), and Saturn has a surface brightness less than 5% of the full Moon. An ill-defined patch of slightly higher luminance? OK. But a banded giant with razor-edge rings, so beloved of NASA illustrators? Nah.

Grant

Grant,

did you take into account that the statement of 'opaqueness' is not necessarily independent of the viewing direction in case of Titan? when I look towards Titan from a distance, the haze will clearly appear opaque and no surface markings are seen. But on Titan's surface, there are also no bright light sources (I suppose so at least ). On the contrary, when I am standing on Titan's surface, looking at the hazy sky: both the sun and Saturn might be just bright enough sources perhaps to be seen through the haze.

In any case I could imagine well such a situation that I cannot see the surface from far away yet see the sun /from/ the surface.

Did I forget anything here?

Bye Fridger

[granthutchison]

Did you take into account that the statement of 'opaqueness' is not necessarily independent of the viewing direction in case of Titan?

There's certainly an interesting thing about that, which is worth mentioning - if it takes 200 vertical kilometres of aerosol to produce an opaque mixture, then it's likely that horizontal visibility on Titan is good enough to see the few kilometres to the horizon without difficulty. Especially since there is supposed to be low-level "rain-out" of the aerosols, so that the local environment would consist of clear atmosphere and large droplets, a much less opaque mixture.

Did I forget anything here?

Have you heard of the "nude in the shower" effect? A diffusing "screen" obscures detail of an object in proportion to that object's distance behind the screen. You can convince yourself of this with a little bit of sketching of light rays, or by observing the naked person of your choice through a shower screen. If the person places his/her hand against the screen, it is clearly discernable, whereas his/her body (more distance) appears as simply a blur. Even if you walk up to the screen and press your eye against it, you won't get a better view of the person on the other side.
By analogy, Titan:
Viewed from space, the surface of Titan is the hand pressed against the back of the diffusing screen (= the haze) - and yet we can't make out any detail at all.
Viewed from the surface, the Sun or Saturn is the body far beyond the shower screen, with your eye pressed close to the obscuring screen - we can expect to have poorer resolution than when we looked at the surface from space.
As to brightness, the optical depth of clouds that completely obscure the Sun on Earth is around 10. So I'd expect a very diffuse patch of luminance to be associated with the Sun, if those optical depths for Titan's haze are approximately correct - I believe I've already mentioned this image (though without the underpinning argument) elsewhere on this thread. But for Saturn, with only 5% of the surface brightness of the Moon, it's difficult to imagine it either standing out particularly or showing any detail at all.

Grant

PS: After some reflection, it's best to point out that some of the above hinges on their being something to see on Titan's surface at visible wavelengths. If for some reason Titan is very uniform in visible light, then our difficulty in seeing detail might relate to very low contrast (in line with Fridger's comment about the absence of bright lights on the surface).
But that still leaves us with estimated optical depths that are known to be in the same ballpark as those that obscure the Sun on Earth.

[Evil Dr Ganymede]

Let me remind everyone that the figure of 1/1000th I quoted (from NASA!) referred to the light fraction on the SURFACE of Titan. I can see nowhere in your estimate what amount of absorption within the haze you have used

Fridger, the Evil Doctor is simply pointing out the error in Spaceman Spiff's calculation of the insolation of Titan above the atmosphere. Combined with your figure, it implies that only a tenth of the light gets to the surface, rather than Spiff's erroneous estimate of almost all of it.

I was more pointing out that Saturn wasn't at 30 AU

[granthutchison]

I was more pointing out that Saturn wasn't at 30 AU

The rest was, of course, implicit.

Grant

[t00fri]

Have you heard of the "nude in the shower" effect?

Heard of? My God...

...after an entire morning of "nude in the shower"
experiments , forgetting even to go to work etc..the solution of Titan's haze visibility still remains unclear to me.

During our exciting experiments, we substituted 2 kinds of glass in our shower cabinet:


a) opal glass
b) a clear but very dark glass

--------------------------------
These characterize 2 limiting situations:

(a) high diffusion, low absorption.
--------------------------------------------
-- The nude in the shower can easily locate the soap
-- The observer outside is clearly frustrated ...


(b) low diffusion, high absorption
--------------------------------------------
-- The nude is frustrated since the soap is hard to find
-- The observer is also frustrated. His frustration strongly
decreases, however, as soon as the nude switches on the light in the
cabinet (to look for the soap...)


What did we learn from all this?

Hmm...Perhaps that various combinations of the two crucial parameters, absorption and diffusion, in Titan's haze atmosphere can lead to a rich variety of phenomena in connection with visibility.

We might better be careful in making predictions before knowing both pretty well.

Bye Fridger

[Guest]

I'm amazed! Saturn and Titan are 30 times the distance from the sun as Earth. By the inverse square law, sunlight is then 900 times weaker.

Er, no.

30 times the distance from the sun as Earth is 30 AU, which is roughly where Neptune is.

Saturn is 10 AU from Sol. So actually it's getting 1/100th of the light that Earth gets, since it's 10 times further away.

Oh, the shame of it! I used to know the distance of all the planets in the solar system to 2 decimal places in AU. That's a worse mistake than my 'the moon's apparent magnitude is -18' (it's -12). How did I do that? I'll tell you how I did that. I read all 6 pages of this thread to make sure I really followed it, and filtered out the 2 or 3 sub-topics along the way, then spent ages editing my thoughts. I was so tired, that when I tried to recall Saturn's distance, the number 29.5 popped into my head. Now I realise, I confused Saturn's year with its distance. Please forgive me!

I gamma factored my Venus and Titan cloud textures so that Venus appears as a pure white ball and Titan is a pale smokey orange ball.

But there are faint albedo variations in Venus's clouds at visible wavelengths. This was recorded by Earth-based observers long before spacecraft reached the planet, and has been confirmed by Galileo imaging in visible light.

True, but I was rushing what I meant: I gamma factored, not replaced with RGB=255,255,255. Actually, Venus appears as a pure white ball on first glance in my Celestia set up, but after a moment I can recover the faint shadings from the original texture. However, I am sure my monitor does not have the dynamic range to do justice to the subtleties of Venus' clouds in visible light. I looked at that Galileo link, and sorry, but I would never consider that a realistic 'visual' portrait of Venus. When I see Venus through a 'scope, it is pure, blazing white. To get that Galileo shot, you would have to have heavy polaroids, like t00fri would wear! And it seems to have the colour Titan's smog should have .

Have you heard of the "nude in the shower" effect? A diffusing "screen" obscures detail of an object in proportion to that object's distance behind the screen.

Meanwhile, I'm much happier now with the repeating NASA statement that (implicitly) only 10% of sunlight filters down to Titan's surface. However, I would side with t00fri here that it's not necessarily a foregone conclusion that this means the sun and Saturn are totally blurred out as per Grant's shower scene analogy. A heavy layer of overcast cloud on earth will mean we can't tell where the sun is shining from, but I've seen the sun through hazy or low cloud, even fog, and although diffusion clearly occurs, I can see a faint (that is low contrast but sharp! solar disk through the fog. I'd guess overcast cloud has an attenuation of more than order of magnitude 10 if I recall photographic exposure correctly: 1/60 at f/32 for bright sunny day, 1/30 at f/8 for overcast (hope I'm not too wrong this time!)? It's not the ambient dimness that matters, but the contrast between light source and ambient brightness. People may look blurred through shower screen, but a torch in their hand could still shine with a sharp edge but low contrast against ambient. The possibility that Titan's atmosphere mostly has uniform smog, not discrete cloud layers, is what might make things different from what we're familiar with. Yup, absorption and diffusion - two different things.

Spiff.

[Spaceman Spiff]

Guest? But I was logged in!