The Martian Sky |
The Martian Sky |
Oct 11 2009, 06:41 PM
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IMG to PNG GOD Group: Moderator Posts: 2254 Joined: 19-February 04 From: Near fire and ice Member No.: 38 |
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. |
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Feb 4 2018, 12:54 AM
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Senior Member Group: Members Posts: 1669 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
Here is an attempt to generate a similar polar (fisheye) projection of the Opportunity sky link two posts up. The sun is at the zenith and a generic land surface shows up in the corners.
This is simulated with the ray-tracing (non MC) code. It's a good case to test improvements in the handling of multiple scattering, so the minimum brightness now shows better midway up in the sky. To get the color reasonably close, the aerosol optical properties were adjusted quite a bit. As often happens I'm bumping up against the question of whether the color/contrast has any enhancement in the actual mosaic. One way to help make color more objective is to specify the xy chromaticity of various points in the sky. This is done for various missions here where I was led by the Opportunity image release. Typical sky values far from the sun are x=.40, y=.38. In the simulation the xy values so far look reasonable near the horizon. As we get closer to the sun the colors are less saturated, though it doesn't tilt to all the way to bluish next to the sun as one might anticipate. With some phase function and single scattering albedo adjustments we have a second version below: Near the horizon this has a chromaticity of x=.39, y=.38 making it a bit less orange compared to the typical values in the paper, yet a bit more tan compared with the Opportunity mosaic. A related factor is where the white point is set (for conversion to RGB) and I'm using 5800K that I also set my monitor to. This version does get slightly bluish right next to the sun. It is white here due to saturation though the blue tint would be visible in a 16-bit image. This is a good exercise for checking various aspects of color processing. Based on this a new set of animated and individual 8/16 bit frames at various solar elevation angles (in 2 degree steps) is taking shape. Ten degrees of land surface shows up at the bottom, with some variations in brightness depicted. The frame-to-frame brightness is a bit inconsistent, though the full brightness range is accessible via the 16-bit images. An overall Mars sky website is here. -------------------- Steve [ my home page and planetary maps page ]
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