The Martian Sky Options

[hendric]

Don,
Some experiments and deconvolution software might help encourage the proper folks to consider doing that.

[mcaplinger]

An all-sky pan with the 34mm Mastcam on MSL wouldn't be that many frames (about 100? Have to think about the geometry a bit) and would only take a few minutes to acquire, assuming that the remote sensing mast can actually point straight up. I'd be interested in taking it, bandwidth permitting.

[mwolff]

--- unnecessary quoting removed ---

That would be great! As with MER, one could do some basic science (for me, scattering phase functions of dust) using just the calibrated "thumbnails", so bandwidth wouldn't be a concern. Of course, higher resolution would be interesting from the point of view of water ice cloud detection (and full resolution obtained periodically would be an excellent source for flatfield monitoring). I would suspect that Mark L. would be enthusiastic about such a thing as well...

[Bjorn Jonsson]

Finally I'll end this big message with an experimental Martian sunset animation. It has a FOV of 70°. In this animation I darkened the sky a bit compared to the renders above.
[attachment=19905:martian_...set_test.avi]

I have now done an improved version of the sunset animation. The new one has more than two times as many frames (things really happened too fast in the old one) and the sun now gets well below the horizon. There was a bug in my renderer when I made the old one that resulted in a 'contoured' sky when the sun was below the horizon so the old sunset animation ended with the sun on the horizon. In the new one the sun gets well below the horizon.

 martian_sunset_new.avi ( 726.5K ) Number of downloads: 1929


I can probably still make some improvements to my Martian sky renders but probably nothing major. The next step is to combine this Martian atmospheric model with the MRO DEMs which should result in interesting (and hopefully realistic) test renders.

Edit: The area around the sun looks somewhat 'contoured' in the animation, especially when the sky is getting dark. This contouring is not present in the original frames used to make the animation file, it's an artifact of the compression used for making the AVI file. It might be possible to fix this by using a different codec.

[scalbers]

Nice animation above by Bjorn from 2009. Here's what I get so far for an animated whole sky simulation with the sun dropping lower in the sky.


Full Resolution Animation

The animation shows just the scattered light so I'll want to add the actual sun in as well. Some increased blueness shows up at low phase angles. I'm basing this (wavelength dependent extinction, single scattering albedo and asymmetry parameter) on a Pollack et al paper and using a Henyey-Greenstein (HG) function. A second HG function is used to add some back-scattering.

Dynamic range of brightness is compressed a bit for display.

This post is a continuation of a discussion I started in Earth Observations, with Mars related results now being posted in this thread.

The latest version shown here has somewhat improved handling of multiple scattering and has the sun's disk added in.

To help with rendering, phase functions for different wavelengths (red/blue) can be checked in a 2014 paper mentioned in the post linked below. It's interesting that the phase function near 90 degrees scattering angle is about the same for red and blue implying the sky should be gray at right angles to the sun, and only turning reddish at roughly 130 degrees. The blueness in the glow around the sun is predicted to be showing up at scattering angles less than about 28 degrees.

http://www.unmannedspaceflight.com/index.p...mp;#entry212746

Perhaps though a fuller consideration of reflection from the ground would shift things more to the red at somewhat smaller phase angles.

[scalbers]

Here is a more direct link to the recent paper that explores the question of why the sunset on Mars is blue:

https://www.osapublishing.org/ao/fulltext.c...8&id=281919

This comes to mind as the recent APOD with Damia's Mars sunset mentions the reasons for the blue sunset aren't completely understood. This paper though seems to explain things pretty well, in that the blue sun is from a negative Angstrom coefficient (associated with a specific dust size near the wavelength of light), and the blue surrounding the sun is related to the Mie scattering with the dust where the blue Airy diffraction disk is more concentrated than the red one. The composition of the dust is only important (and producing red) at greater scattering angles in other parts of the sky. These colors are there to some extent at other times of the day as well.

[Deimos]

Yes, I was a bit annoyed by that APOD comment. The reasons are not understood by a great many people. But they are quite clearly understood by many on this forum and any scientist working with Martian atmospheric images professionally. Interesting link, thanks.

[scalbers]

Here's a simulation with my latest software for the Mars sky, sun is 14 degrees up:



An animation of the frames every 2 degrees of solar elevation is here (animation updated 2100UTC).

And in this directory the individual frames (8 / 16 bit) are located.

[neo56]

Amazing work Scalbers, that's a really nice product of radiative transfer!
I'll use your skies for my Curiosity panos to make full 360x180 panos, with appropriate credits of course.
One thing: when downloading your skies on your website, I noticed the 78° solar elevation is all blank.

[scalbers]

Thanks Thomas, and glad they can be used in your mosaics. I worked through a few code glitches to get the 78 degree frame to work.

I have a new version including some additional improvements, such as an expanded solar altitude range from -6 to +90 degrees animated here. The individual frames are here.

[scalbers]

Greetings,

Considering that light can reflect off the colored ground and then back off of the dust in the sky, I made an updated version that looks slightly redder away from the sun. Additional refinements might still be needed, and of course it's an interesting question how well we can objectively characterize the Martian sky color. I'm considering whether I can easily add a generic surface to the images so we can see the relative color and intensity.



http://stevealbers.net/ast/mars/sky_2016 (frames)

Below we can see how this looks using the CRISM scattering parameters from earlier in this thread (instead of the Viking ones). At first glance this looks pretty good with less of a purple cast (farther from the sun).



The CRISM frames can be found here: http://stevealbers.net/ast/mars/sky_2016_crism

[scalbers]

I can report on some progress with a Monte Carlo planetary atmosphere simulation program. The first image shows the Monte Carlo result compared with the second image made with my "regular" ray-tracing program. The sun is 51 degrees high. Aesthetically the Monte Carlo one has a bit more brownish tint.

[fredk]

I'm curious what are the differences between the techniques - I guess MC uses some random element...

[scalbers]

Indeed both techniques involve tracing of light rays from the observer and from the sun, and they both have mechanisms to speed up the code. Both are pretty similar in situations with just single scattering. The MC excels when there is multiple scattering and this is where the random scattering and redirection of the light rays really comes into play. The other version parameterizes multiple scattering by an equivalent single scattering phase function. MC should also be doing a better job of handling light reflection off the ground, though this is something I'm still working on. This is a backward MC implementation with forced scattering and local estimation.

One difference I see in the images is near the horizon at 90 degrees elongation from the sun the MC appears darker. I'm unsure whether that is correct. It could relate to the preliminary nature of the ground reflection handling.

[antipode]

If you looked up on an average dusty day in Gale Crater, would you see any stars, or would the 'butterscotch scattering' extend all the way to the zenith?

P