The Martian Sky Options

[Bjorn Jonsson]

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:


(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 :



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.

[Ant103]

Here is the settings of the atmosphere in TG2, sets by default to have a classical blue sky. Maybe you can give me some advise to have a martian sky.
And the settings to hava "martian sky", with the result rendering (notice that the fov is 104° with a sun at 45° high in the sky).

Attached thumbnail(s)

 

[Bjorn Jonsson]

Here is the settings of the atmosphere in TG2, sets by default to have a classical blue sky. Maybe you can give me some advise to have a martian sky

I assume "exp height" is the scale height. If that is the case it is incorrect - it should be ~11 km for both haze (dust) and bluesky.

Thanks to everyone for useful information - mwolff's message was especially helpful. As I was starting to suspect, using a Henyey-Greenstein (or similar) phase function didn't work well (before seeing mwolff's message I was even considering using the sum of two or more HG-functions). I was able to get nice sunset images but in that case images with the sun high in the sky looked bad or vice versa. So I replaced it with the Tomasko et al. functions (444 nm and 671 nm) mentioned by mwolff. Actually I simply measured the function values from a graph in the Tomasko et al. paper and construced a lookup table. The function isn't perfectly smooth yet - this shows up in some of the images where the sun is visible. Dynamic range is a big problem so I processed the function in Excel to greatly reduce its dynamic range while preserving the color ratio (R/B) as a function of scattering angle. The results are very promising. I still need to do some tweaks, for example I probably need to increase the optical depth (the normal optical depth is currently ~0.18) and I'm currently assuming it doesn't vary with wavelength which I'm not sure is correct.

Some test images:

The sun as seen from the surface, the field of view (FOV) is 90 degrees. Interestingly, the sky near the sun has a bright bluish color. I don't know how accurate this is:


Similar viewing geometry but looking in roughly the opposite direction:


Sunset, the upper one has a FOV of 90 degrees and the lower one a FOV of 30 degrees. From these images I suspect I need to change some parameters because if I slightly increase the solar elevation a large area around the sun gets completely saturated. Because this didn't happen with the sun high in the sky (see above) I suspect the amount of inscattering is too high relative to absorption. From a comparison with actual images from the surface I suspect the sky is also too bright near the sun here which also implies I need to increase the optical depth (i.e. absorption).



Flying above the surface, altitude 7 km (upper one) and 607 km (lower one). FOV 40 degrees:



As previously, these are rather crude test images without antialiasing. Also to speed things up I'm using a low resolution texture map and no DEM.

I might eventually make my renderer available one day - this has been a very helpful discussion. However, a lot of work remains before that's even possible.

[vikingmars]

Bjorn, your test images and simulations are really interesting
Here are 2 color samples to show the color of the Martian sky (close to the horizon + at zenith) that is considered to be the closest to the "moderate yellowish brown" (although it's not looking so "yellowish brown" close to the horizon) as seen by the VL landers and confirmed later by MPF and MER. It's the average color, useful for computing, that is NOT taking into consideration the dust opacity variations over the Martian seasons. Enjoy