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The Martian Sky
Bjorn Jonsson
post Oct 11 2009, 06:41 PM
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I have been attempting to make computer generated images of the Martian atmosphere, both as seen from the surface and from space. To check the results I have been looking for spacecraft images to use as ground truth. I have found lots of images - by far the best ones I have found are from UMSF in this thread: http://www.unmannedspaceflight.com/index.php?showtopic=3324

However, I'm always looking for more ;-). So if anyone knows of more and/or better images I'm interested in them. What would be best are mosaics showing the sky from the horizon (with the horizon/surface visible) and towards the zenith.

The sky varies a lot because of variable amount of dust but the general impression I get is that the sky is bright near the horizon (usually brighter than the surface) but gets much darker higher in the sky. There is probably a fairly large, bright area in the sky near the sun, possibly less reddish (lower R/B ratio) than parts of the sky farther from the sun.

I'm already getting fairly interesting results, this one has a field of view of 90 degrees:

Attached Image

(needless to say this one is 'overexposed' near the horizon; dynamic range is sometimes a problem)

The problem is that even though this may not be bad the limb currently appears far too bright as seen from space :

Attached Image


This shows that my atmospheric model is erroneous in some way - I suspect that as seen from the surface the Martian sky is darker high above the horizon than I have been assuming.
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djellison
post Oct 11 2009, 07:29 PM
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That first image is surprisingly (and pleasingly) similar to what Dreamscape ( a terrain and sky plugin for 3ds max) produces when I'm rendering my HiRISE DEM flyovers.
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ngunn
post Oct 11 2009, 09:24 PM
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I expect you've seen this paper (one of my favourite ones), but just in case not:
http://marswatch.astro.cornell.edu/Bell_etal_SkyColor_06.pdf
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Ant103
post Oct 13 2009, 08:42 AM
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The first image seems very good for me. Don't know if you know, but with Terragen you can made very good sky, with various parameters like intensity of atmospheric scattering, or dust opacity (and color).

Don't know if it's accurate but I like make "artificial" skies like here :




You can also have a look on this galery of picture from Olivier : http://www.planete-mars.com/goursac/2006/vision2.html


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mcaplinger
post Oct 13 2009, 04:48 PM
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QUOTE (Bjorn Jonsson @ Oct 11 2009, 10:41 AM) *
the general impression I get is that the sky is bright near the horizon (usually brighter than the surface) but gets much darker higher in the sky...

According to Mark Lemmon -- http://www.unmannedspaceflight.com/index.p...l=&pid=9656 -- for typical dust loadings the brightness variation from horizon to zenith is no more than a factor of two and wouldn't be very visible to the naked eye.


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Disclaimer: This post is based on public information only. Any opinions are my own.
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djellison
post Oct 13 2009, 05:15 PM
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QUOTE (Ant103 @ Oct 13 2009, 09:42 AM) *
with Terragen


I've dabbled with Terragen, but not much. It would save a LOT of rendering time to pre-render a nice sky like that in Terragen, then map that to a sky hemisphere in 3ds max rather than using Dreamscape. Do you have a save file for that Terragen render as a starting point (or a list of parameters)
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Bjorn Jonsson
post Oct 14 2009, 12:49 AM
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QUOTE (mcaplinger @ Oct 13 2009, 04:48 PM) *
According to Mark Lemmon -- http://www.unmannedspaceflight.com/index.p...l=&pid=9656 -- for typical dust loadings the brightness variation from horizon to zenith is no more than a factor of two and wouldn't be very visible to the naked eye.

Thanks - the paper mentioned by Mark Lemmon turned out to be extremely useful. Seems I might end up using a more sophisticated atmospheric model than I'm currently using. For the dust I'm currently using Mie scattering only and have been experimenting with varying the amount of scattering, optical depth and adjusting a parameter similar to the Henyey-Greenstein asymmetry parameter. This has been largely a process of trial and error.

I still suspect the relative brightness of different parts of the sky varies a lot depending on atmospheric conditions and the sun's position in the sky. For example the brightness near the horizon can be highly variable:

http://photojournal.jpl.nasa.gov/catalog/PIA06915

There are many more similar examples, including images/mosaics from the pancam (this one is from the navcam). In this one the brightness of the sky varies by a factor of more than three. Here is another one, this time from the pancam. It was taken over a period of 8 days which might mess things up a bit:

http://photojournal.jpl.nasa.gov/catalog/PIA07334

Then there are these images where the sky darkens a lot when you get higher above from the horizon:

http://photojournal.jpl.nasa.gov/catalog/PIA06765 and
http://photojournal.jpl.nasa.gov/catalog/PIA06850

I suspect the sun was fairly low in the sky which probably contributes to this effect.

Another aspect of the Martian sky: Interestingly the sky is bluish near the sun, at least when the sun sets:

http://photojournal.jpl.nasa.gov/catalog/PIA07997

I'm fairly happy with the sunsets I'm getting from my renderer, especially because I should be able to improve it significantly:

Attached Image


Far from perfect but not too bad. As these are experimental images they are rather crude to speed things up (for example no antialiasing). The parameters I'm using now are different from the ones I used for the images in the first message of this thread. For example I decreased the scattering and increased the optical depth.
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vikingmars
post Oct 14 2009, 10:43 PM
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smile.gif ...And here is a photo controlled mosaic showing the Martian Sky at VL2 site... with quite accurate color.
Enjoy also ! smile.gif
Attached Image
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mwolff
post Oct 15 2009, 03:32 PM
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QUOTE (Bjorn Jonsson @ Oct 13 2009, 06:49 PM) *
Thanks - the paper mentioned by Mark Lemmon turned out to be extremely useful. Seems I might end up using a more sophisticated atmospheric model than I'm currently using. For the dust I'm currently using Mie scattering only and have been experimenting with varying the amount of scattering, optical depth and adjusting a parameter similar to the Henyey-Greenstein asymmetry parameter. This has been largely a process of trial and error.

I still suspect the relative brightness of different parts of the sky varies a lot depending on atmospheric conditions and the sun's position in the sky. For example the brightness near the horizon can be highly variable:

[...snip...]


As you suggest, the phase function (or scattering angle) and optical depth, are the primary variables in explaining the array of sky brightness profiles in your list of example images. As such, the assumed form of the aerosol phase function will be very important in reproducing observations. Simple functions like the Henyey-Greenstein are notoriously poor for the particle sizes and wavelengths involved here, particularly if the range of photometric angles sampled in an image (or mosaic of images) is large. Recent analyses using MEX/OMEGA and MRO/CRISM have substantially improved aerosol scattering properties in the Pancam filter range such that you should be able to reduce the number of ad hoc parameters in your rendering. At a first approximation, the shape of the Tomasko et al. (1999) phase functions does quite a nice job...and avoid the inevitable backscatter enhancement that simple analytical (nonspherical) shapes can produce, such as the D/L=1 cylinders that i typically use. Of course, one can empirically remove this enhancement without much effort...and easily capture the behavior seen in the Tomasko et al. work.

I am curious about the level of radiative transfer that you are using in your sunset images...they look quite nice.

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djellison
post Oct 15 2009, 10:12 PM
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All those words and 'sky hemisphere' didn't get mentioned once. Good work Mike.

smile.gif
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ngunn
post Oct 15 2009, 10:39 PM
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Sorry - who's Mike? - and who mentioned 'sky hemisphere'? I'm beginning to feel like a cat in a shoe factory. What is going on?

I'm extremely interested in the colour of the Martian sky but I can't make head nor tail of this. Please be a little more explicit.
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djellison
post Oct 16 2009, 05:59 AM
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Check the bottom of http://pancam.astro.cornell.edu/pancam_instrument/team.html

The Sky Hemisphere is the unlikely-to-ever-happen idea of doing a full pan of the sky, with Pancam.
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ngunn
post Oct 16 2009, 07:38 AM
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Much appreciated, thanks Doug. I'll start agitating for a sky hemisphere now. smile.gif
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DDAVIS
post Oct 16 2009, 07:53 PM
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I wish they would place a hemispherical mirror somewhere on Martian lander/rover spacecraft within camera sight, over somewhere on the spacecraft that would be blocking the scene anyway. it should be large and close enough to provide a mini fisheye view of the sky to the camera, providing at a glance (at least early in the mission) a visual summary of brightness variations, clouds, halos, etc.
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Ant103
post Oct 16 2009, 08:26 PM
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Humů I will better prefer a camera with a fisheye lens like "8mm fisheye pelang" pointed toward zenith, with a color CCD, somewhere on the deck of the lander/rover.


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