T00fri's Titan @ Celestia

[selden]

I was thinking about this earlier - maybe I'm being utterly dense here, I dunno....

What I don't get is that if you need a near-IR filter (at about 900 nm) to see through Titan's haze, and the view is obscured at visible wavelengths, then does that mean that no visible light can get through to the surface?! Surely not.

Yes, that's what it means. (well, maybe not "no", but certainly very, very little.)

Why does this idea cause you problems?

My understanding is that only specific infra-red frequencies can make it through the smog. Everything else is highly attenuated. It's something like the visibility through a cloud of smoke here on earth.

I don't know of any EM filters that pass one frequency in one direction and a different frequency in the other direction, which is what you seem to be suggesting might be the case for Titan. Is that what you mean? Do you know of any filters like that?

Could it be that the "greenhouse effect" is causing some conceptual confusion for you? Remember that glass is transparent to optical light in both directions. What's blocked going out (and in) is IR.

[Evil Dr Ganymede]

Well, it was more that I figured that would mean that if you were standing on Titan's surface you wouldn't be able to see anything since no visible light was coming through the atmosphere to illuminate the surroundings. Yet the artists impressions I've seen don't seem to indicate this is the case - granted, they may not be accurate though.

Is this really the case there? Or am I being dense?

[selden]

Oh Evil One,

Indeed, the surface of Titan is sure to be pitch black in visual frequencies. You wouldn't be able to see your hand before your face.

There's no reason to expect an artist's interpretation would be any more or less accurate than Fridger's surface texture images. You have to find out what their intent was when they created the artwork. A black square wouldn't be particularly useful or entertaining, after all

[t00fri]

Oh Evil One,

Indeed, the surface of Titan is sure to be pitch black in visual frequencies. You wouldn't be able to see your hand before your face.

There's no reason to expect an artist's interpretation would be any more or less accurate than Fridger's surface texture images. You have to find out what their intent was when they created the artwork. A black square wouldn't be particularly useful or entertaining, after all

Selden & friends,

during the past few days 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. We should not forget that despite the thickness of the atmosphere diffuse light can possibly penetrate quite deeply via multiple scattering, as long as there is no strong absorption at each scattering stage. Also the atmosphere might effectively act as a big light collector-diffusor this way.

So, I would rather go for a /dim/ diffuse lighting on the ground with very low visibility and lots of raining (of liquid hydrocarbons).

The large /very bright/ (high albedo) features that are very conspicuous (e.g. below the dark "lying H" feature ), represent presumably continental areas covered with /deep frozen water ice/.

The presence of a high amount of water ice is beyond doubt, I think.


Bye Fridger

[Rassilon]

It all looks good to me Fridger, Keep up the good work....

[t00fri]

Hi all,

I just downloaded today's "authoritative" view of Titan in (RGB) true color of visible light from the Cassini site.



In my Titan texture package I shall also include such an opaque haze cover for people to chose according to taste. Personally, I am still undecided as to what I am going to choose for myself;-)

Bye Fridger

[granthutchison]

Well, it was more that I figured that would mean that if you were standing on Titan's surface you wouldn't be able to see anything since no visible light was coming through the atmosphere to illuminate the surroundings.

As Fridger says, some predictions are that the illumination at Titan's surface is like a dense overcast day on Earth.
There's no paradox here - visible light going through the dense haze layer is strongly scattered, so that each photon will have multiple interactions before it comes out the other side (or is scattered back the way it came). For illumination of Titan's surface this means that the whole sky would glow dully, with nothing more than perhaps a patch of brightness to indicate where the Sun is - so you could see easily by indirect illumination. But light coming back out through the haze gets bounced around everywhere, too, so there is no coherent detail to see by the time it's back out into space - the view of Titan's surface is effectively randomized and mixed in with the (much brighter) light reflecting from the top of the haze layer. So you can't see the surface in any meaningful way. Same principle applies on Venus, and on an overcast day on Earth.

The IR wavelengths chosen for surface imaging aren't scattered in this way, and so go straight in and straight out again, producing an image we can interpret.

Grant

[t00fri]

Well, it was more that I figured that would mean that if you were standing on Titan's surface you wouldn't be able to see anything since no visible light was coming through the atmosphere to illuminate the surroundings.

As Fridger says, some predictions are that the illumination at Titan's surface is like a dense overcast day on Earth.
There's no paradox here - visible light going through the dense haze layer is strongly scattered, so that each photon will have multiple interactions before it comes out the other side (or is scattered back the way it came). For illumination of Titan's surface this means that the whole sky would glow dully, with nothing more than perhaps a patch of brightness to indicate where the Sun is - so you could see easily by indirect illumination. But light coming back out through the haze gets bounced around everywhere, too, so there is no coherent detail to see by the time it's back out into space - the view of Titan's surface is effectively randomized and mixed in with the (much brighter) light reflecting from the top of the haze layer. So you can't see the surface in any meaningful way. Same principle applies on Venus, and on an overcast day on Earth.

The IR wavelengths chosen for surface imaging aren't scattered in this way, and so go straight in and straight out again, producing an image we can interpret.

Grant

People --like Grant or myself-- who happen to live in an environment where the sky is heavily overcast throughout large portions of the year and the light is dim because of our northerly latitude, develop a "natural" sense for what the visibility should be on Titan's surface...

Bye Fridger

[t00fri]

charming...have a click
http://saturn.jpl.nasa.gov/multimedia/videos/movies/terrainhi.mpg

Bye Fridger

[fsgregs]

Boy, now I'm really getting confused. In my Educational Activities, I try to take my students to places and show them what it really looks like there. From the comments above, Titan is a big orange ball visually, with no real surface texture apparent anywhere. If one descends to the surface, there is either virtually no visible light reaching it and it is black, or it is overcast but lit. The NASA Cassini Huygens video Fridger posted above shows the surface to be a cratered surface bathed in a bright, thick red haze with a visibility of at least a mile or two.

Please, ignoring what Titan may look like in IR, visually is it a big orange ball with perpetual darkness at the surface or is it a big orange ball with a red haze surface, or is it something else (to the best of everyone's estimates)?

Thanks folks.

Frank

[chris]

Please, ignoring what Titan may look like in IR, visually is it a big orange ball with perpetual darkness at the surface or is it a big orange ball with a red haze surface, or is it something else (to the best of everyone's estimates)?

Thanks folks.

Fridger's and Grant's explanations are correct--light travelling through Titan's atmosphere is diffused not absorbed, so the surface wouldn't appear completely dark. An interesting question is what the visibility on the surface is like. Could you see features one kilometer distance? Five kilometers? At what range does the haze obscure things completely? And how does the visibility vary with altitude?

--Chris

[Evil Dr Ganymede]

Ah, right. I knew I was forgetting something - the diffusion of the light through the atmosphere.

I hope they remembered to put a spotlight on Huygens!

[chris]

Ah, right. I knew I was forgetting something - the diffusion of the light through the atmosphere.

I hope they remembered to put a spotlight on Huygens!

There is in fact a spotlight: a 20 watt lamp is switched on within a few hundred meteres of the surface, I believe.

--Chris

[HankR]

NASA has released a new map of Titan based on recent Cassini imaging:

http://photojournal.jpl.nasa.gov/jpeg/PIA06086.jpg

Fridger, please do your stuff!

- Hank

[t00fri]

NASA has released a new map of Titan based on recent Cassini imaging:

http://photojournal.jpl.nasa.gov/jpeg/PIA06086.jpg

Fridger, please do your stuff!

- Hank

Great Hank,

I'll start working on it right away;-)

It will also be interesting to compare/combine these data with the earthbound ones!

Bye Fridger

[ElPelado]

Here you have(just in case someone missed it). They say that in the flyby of july 2 the quality will be at least 17 times better...

[t00fri]

Hi all,

I now have made a 1k texture from the new cassini data about Titan,
http://photojournal.jpl.nasa.gov/jpeg/PIA06086.jpg

It was quite a bit more work than I had anticipated, since
these guys from the Cassini imaging team apparently
were under time pressure to release something.

The 16 images of Titan were joined "carelessly" to say
the least. In order to make this less obvious they
decreased the brightness on the lhs side of the texture.
That in turn resulted in highly nonmatching
intensity levels on the lh-end and the rh-end of the Titan
raw texture.

Anyhow, this is the new 1k texture:


The next sequence shows Titan with the orange/yellow
clouds. I have still made it marginally transparent such
that some surface markings can just be made out...



Finally, Saturn as seen from Titan's surface now is barely
visible, matching many illustrations from NASA...



Let me know any suggestions for improvement.

I am going out now for my usual Saturday night "Dim Sam" dinner. After returning, I shall package the stuff. Probably I shall include also an orange structureless, untransparent cloud texture ( for purists like Grant and others;-))

Bye Fridger

[t00fri]

Hi all,

after a great "Dim Sam" dinner, its now time for my next
round of Titan textures. Compared to the above "before
dinner" colors, there were still some changes. Since the
surface colors are essentially unconstrained, I chose
them such as to optimize fine details...

I have finally decided to provide an untransparent
yellow-orange haze cloud texture, matched to the Cassini
true-color photo in visual light.

Here is the package for download:

http://www.shatters.net/~t00fri/images/titan1k-2.zip

and here are a few shots of the final set:




Enjoy,
Bye Fridger

[t00fri]

I have worked a bit further on Titan's
clouds/atmosphere. Below you find for download an
addendum-archive, containing two alternative
cloud textures (along with an 'titan-clouds.ssc' file).

1) titan-clouds-opaque.png: color matched
carefully to the latest Cassini truecolor image, but no
alpha channel, i.e. entirely opaque!

2) titan-clouds-alpha.png: RGB color as in 1) but
in addition an alpha channel providing just a hint of
visibility of surface features as well as allowing to
make out Saturn barely from Titan's surface, as many
illustrations from NASA/ESA indicate.

e.g. here in an official ESA-illustration of Titan's
anticipated atmosphere close to the south pole including
also a large , dark Methane cloud:



Here is the URL of my titan-clouds package:

http://www.shatters.net/~t00fri/images/titan1k-clouds.zip

Next as described a composite image, illustrating the
close color matching of my cloud textures (notably the
opaque one), the faint visibility e.g. of the "lying H"
feature and of Saturn from Titans surface in case of the
2nd cloud texture with an alpha channel.



I am still not decided which of the two textures I prefer
personally...

Enjoy
Bye Fridger

[granthutchison]

e.g. here in an official ESA-illustration of Titan's
anticipated atmosphere close to the south pole including
also a large , dark Methane cloud

Unfortunately, even the "official" illustrations generated by space agencies concentrate more on catching the eye than on accuracy. Saturn's rings are never so open when seen from Titan (its orbit is inclined at only a third of a degree to the ring plane), and the ring plane can never be so tilted relative to the horizon when viewed from "near the south pole" (since Titan rotates more or less in the plane of its orbit). Also, if this is a portrayal of the real Huygens mission, the season on Saturn (as shown by the ring shadows) is in error by quarter of a Saturn-year.
So I'm just guessing that they maybe didn't spend a hell of a lot of time trying to get the atmospheric transparency right ...

Grant