The Martian Sky |
The Martian Sky |
Jan 31 2018, 04:27 PM
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#61
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Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
Great shot of Phobos, mcaplinger! (I assume that's not Deimos, simply because Phobos would be the easier shot.) Phobos should be visible every day, then.
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Jan 31 2018, 07:39 PM
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#62
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Senior Member Group: Members Posts: 1621 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
Actually, there was a Mastcam mosaic of the sky, near and on the zenith conducted by Curiosity on Sol 101. Interesting to see this Damia - thanks for posting. Would you happen to recall the geometry or map projection of your mosaic? -------------------- Steve [ my home page and planetary maps page ]
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Jan 31 2018, 08:29 PM
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#63
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Senior Member Group: Members Posts: 1078 Joined: 19-February 05 From: Close to Meudon Observatory in France Member No.: 172 |
Actually, there was a Mastcam mosaic of the sky, near and on the zenith conducted by Curiosity on Sol 101. It was quite a challenge to stitch it because of the lack of control points usable, but I used imagery metadatas to place them exactly in their good position. Anyway, this shows us that the zenith is not black, but more brownish. But, it could be very dark because I don't have the information about the exposure length of this set of images. Congratulations Ant 103 for this superb work of yours. As a reminder, here are the sky pics from VL1... Please, note that the color saturation of the sky changes with the dust opacity and with the Martian seasons. Enjoy |
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Jan 31 2018, 09:11 PM
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#64
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Senior Member Group: Members Posts: 2504 Joined: 13-September 05 Member No.: 497 |
Actually, there was a Mastcam mosaic of the sky, near and on the zenith conducted by Curiosity on Sol 101. Looks like there was one survey done on sol 100 at around noon, and then another on sol 101 in the late afternoon. All of these images were autoexposed but the exposure times can be pulled out of the PDS index file https://pds-imaging.jpl.nasa.gov/data/msl/M...EX/EDRINDEX.TAB . (Second column below.) Of course, to be really quantitative one would have to linearize the square-root images. CODE "0100ML0004900050102961E01_XXXX.LBL" 1.0 "0 " "Sol-00100M11:58:10.077 "
"0100ML0004930000102962E01_XXXX.LBL" 4.0 "0 " "Sol-00100M11:59:11.392 " "0100ML0004930010102963E01_XXXX.LBL" 5.0 "0 " "Sol-00100M11:59:25.017 " "0100ML0004930020102964E01_XXXX.LBL" 5.0 "0 " "Sol-00100M11:59:38.643 " "0100ML0004930030102965E01_XXXX.LBL" 2.5 "0 " "Sol-00100M12:00:10.760 " "0100ML0004930040102966E01_XXXX.LBL" 2.8 "0 " "Sol-00100M12:01:16.941 " "0100ML0004930050102967E01_XXXX.LBL" 5.5 "0 " "Sol-00100M12:01:35.433 " "0100ML0004930060102968E01_XXXX.LBL" 6.9 "0 " "Sol-00100M12:01:51.978 " "0100ML0004930070102969E01_XXXX.LBL" 6.9 "0 " "Sol-00100M12:02:05.604 " "0100ML0004930080102970E01_XXXX.LBL" 5.6 "0 " "Sol-00100M12:02:21.176 " "0100ML0004930090102971E01_XXXX.LBL" 7.4 "0 " "Sol-00100M12:02:38.695 " "0101ML0006740000102999E01_XXXX.LBL" 6.4 "0 " "Sol-00101M15:40:03.764 " "0101ML0006740010103000E01_XXXX.LBL" 3.5 "0 " "Sol-00101M15:40:55.347 " "0101ML0006740020103001E01_XXXX.LBL" 5.7 "0 " "Sol-00101M15:41:15.785 " "0101ML0006740030103002E01_XXXX.LBL" 7.6 "0 " "Sol-00101M15:41:31.357 " "0101ML0006740040103003E01_XXXX.LBL" 9.0 "0 " "Sol-00101M15:41:47.902 " "0101ML0006740050103004E01_XXXX.LBL" 10.1 "0 " "Sol-00101M15:42:04.447 " "0101ML0006740060103005E01_XXXX.LBL" 10.1 "0 " "Sol-00101M15:42:19.046 " "0101ML0006740070103006E01_XXXX.LBL" 10.1 "0 " "Sol-00101M15:42:34.618 " "0101ML0006740080103007E01_XXXX.LBL" 11.3 "0 " "Sol-00101M15:42:50.190 " "0101ML0006740090103008E01_XXXX.LBL" 13.6 "0 " "Sol-00101M15:43:05.762 " "0101ML0006740100103009E01_XXXX.LBL" 15.9 "0 " "Sol-00101M15:43:21.335 " "0101ML0006740110103010E01_XXXX.LBL" 17.7 "0 " "Sol-00101M15:43:36.907 " "0101ML0006740120103011E01_XXXX.LBL" 17.7 "0 " "Sol-00101M15:43:51.505 " "0101ML0006740130103012E01_XXXX.LBL" 13.9 "0 " "Sol-00101M15:44:15.837 " "0101ML0006740140103013E01_XXXX.LBL" 13.9 "0 " "Sol-00101M15:44:29.462 " "0101ML0006740150103014E01_XXXX.LBL" 13.9 "0 " "Sol-00101M15:44:43.088 " "0101ML0006740160103015E01_XXXX.LBL" 11.1 "0 " "Sol-00101M15:44:58.660 " "0101ML0006740170103016E01_XXXX.LBL" 19.7 "0 " "Sol-00101M15:45:19.098 " "0101ML0006740180103017E01_XXXX.LBL" 13.6 "0 " "Sol-00101M15:45:48.296 " "0101ML0006740190103018E01_XXXX.LBL" 16.2 "0 " "Sol-00101M15:46:13.600 " "0101ML0006740200103019E01_XXXX.LBL" 16.2 "0 " "Sol-00101M15:46:27.226 " "0101ML0006740210103020E01_XXXX.LBL" 16.2 "0 " "Sol-00101M15:46:43.771 " "0101ML0006740220103021E01_XXXX.LBL" 14.0 "0 " "Sol-00101M15:46:59.343 " "0101MR0004910040104491E01_XXXX.LBL" 0.5 "0 " "Sol-00101M15:49:01.000 " "0101ML0004910050103023E01_XXXX.LBL" 1.0 "0 " "Sol-00101M15:49:33.117 " -------------------- Disclaimer: This post is based on public information only. Any opinions are my own.
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Jan 31 2018, 11:35 PM
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#65
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Martian Photographer Group: Members Posts: 352 Joined: 3-March 05 Member No.: 183 |
Navcam images of Phobos just before & after sunset: https://photojournal.jpl.nasa.gov/catalog/PIA17270. Look in PDS for the lossless version of the image mcaplinger posted; and see http://www.planetary.org/blogs/emily-lakda...phobos-msl.html.
Overall, daytime visibility of Phobos is very seasonal. Omitting non-rover sites, and the advantages trained observers have, especially with positional cues, it is unlikely any other stars would be easily visible in most conditions. The Mastcam mosaics (sol 100, 101) were taken in a moderate dust storm. Even so, the sky never clears, and there are many images of zenith that show this (see Navcam zenith movies and many Mastcam images on PDS, where exposure times can be found). But, Phobos is bright. In less dusty skies, it seems not too hard to see it, at least with the Sun low. With very dusty skies, no way. There is also an approximate true color 'sky' release at http://www.planetary.org/blogs/emily-lakda...n-analemma.html. The sky part of the image was taken with the Sun nearly at zenith, and it was dusty, but it's another view (the sol 100 Mastcam survey also had a high Sun). Note that the brightness peak near the Sun is typically supplemented by another near the horizon (high scattering path length), leaving a darkest region either partway down (if the Sun is high) or medium-low and opposite Sun if the Sun is medium-low and moving up toward zenith as the Sun sinks. |
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Feb 1 2018, 12:33 AM
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#66
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Senior Member Group: Members Posts: 1621 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
Indeed it's kind of similar to this earlier simulated movie with a rising sun. I'm getting a bit more confidence in the Monte Carlo code so I hope to make a movie of that version before too long.
Pretty neat Opportunity panorama too, giving a good idea of the relative brightness of the sky and land. This will help check things like the land reflection of the random light rays as the horizon sky brightness may presently be underdone (e.g. with the high sun images). -------------------- Steve [ my home page and planetary maps page ]
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Feb 4 2018, 12:54 AM
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#67
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Senior Member Group: Members Posts: 1621 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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Mar 31 2018, 12:19 AM
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#68
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Member Group: Members Posts: 923 Joined: 10-November 15 Member No.: 7837 |
This is wonderful work Steve.... can you please check the 16bit images? They are all appearing dark, the 8bit images are fine.
I've opened the 16bit images in Photoshop & Irfanview with the same result, very dark with a pixelated sun. Maybe I'm opening wrong? -------------------- |
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Mar 31 2018, 12:27 AM
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#69
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Senior Member Group: Members Posts: 1621 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
Hi Sean - thanks. Good question - the intent (as before) with the 16-bit images is to have the image reside in the middle of the logarithmic brightness space so there's extra room on both the dark and bright ends. Thus to convert it to an equivalent 8-bit image one could try dividing the image by 16 and truncating at 255. How does this look? Depending on the viewer multiplying by 16 can look OK, though you'd be clipping next to the sun a bit. The actual numerical data in the images look basically OK. Interestingly the sun doesn't seem go to the max at 65535 counts. Sky areas that are 128 counts in the 8-bit images should be 2048 counts in the 16-bit ones.
Looking with a 16-bit image viewer with some type of sliding brightness scale seems like an ideal way to view the entire dynamic range. I wonder if browsers might someday support something like this. On a Mac "Photos" works to do a quick image stretch, though perhaps there's another viewer out there with better brightness controls. I looked at the numerical values by using ImageMagick to make a .ppm image with "convert -compress none". I could then see the image with a plain text editor. -------------------- Steve [ my home page and planetary maps page ]
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Apr 2 2018, 04:24 AM
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#70
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Member Group: Members Posts: 890 Joined: 18-November 08 Member No.: 4489 |
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Sep 4 2021, 06:31 PM
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#71
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Senior Member Group: Members Posts: 1621 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
Thanks JohnVV. Revisiting this a bit I made a sequence of frames with a Monte Carlo ray-tracing algorithm located here:
https://stevealbers.net/ast/mars/mc/ An HTML5 viewer to see an example 16-bit frame with various brightness settings can be found here: http://stevealbers.net/ast/mars/mc/briloop...ghtness_36.html It still needs some work on the blue sunrise. In fact the blue color surrounding the sun should actually be present all day long in addition to sunrise/set. Exactly how strong the blue color saturation is appears open to some interpretation. Figure 10 of this paper gives a modestly blue chromaticity near sunset. Another more recent paper has some interesting discussion. It's a bit unclear how to translate this into the optical properties I'm looking for, so it's a bit of trial and error to simulate close to observed chromaticities. -------------------- Steve [ my home page and planetary maps page ]
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Dec 5 2021, 09:19 PM
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#72
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Senior Member Group: Members Posts: 1621 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
Relating to the last linked paper above, I was able to set up some aerosol optical properties based on personal communication with the authors. The latest Monte Carlo update can be found on my website here:
http://stevealbers.net/albers/allsky/mars.html I would note this is based on modeling the dusty atmosphere to have just coarse mode aerosols around 1.7 microns in effective radius. This gives a strong brightness difference near the sun vs farther away. It's possible (though a bit debatable) there are some finer aerosols present as well and this would reduce that contrast somewhat. One thing to consider when combining these sky simulations with rover mosaics of the surface is that a better match can be had by considering the "BRDF" of the land surface. In other words one should account for the actual land reflectance to be brighter opposite the sun (fully lit objects plus opposition effect) and darker toward the sun (e.g. with shadows present). If a mosaic has been equalized to show uniform brightness vs azimuth it will be more challenging to match up to the simulated sky. -------------------- Steve [ my home page and planetary maps page ]
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Dec 27 2021, 06:21 PM
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#73
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Senior Member Group: Members Posts: 1621 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
After looking at some additional papers and doing a few more experiments, I made a further Monte Carlo revision (on Jan 1 - Happy New Year) as can be seen here:
https://stevealbers.net/albers/allsky/mars.html#mc The overall Monte Carlo algorithm has been improved in accuracy and with noise reduction. Some of the dust optical properties are described on the website, and may continue to be refined further. -------------------- Steve [ my home page and planetary maps page ]
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Jan 23 2022, 09:09 PM
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#74
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Senior Member Group: Members Posts: 1621 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
Thanks to Damia's post in a different thread (an excellent sunset panorama) I was thinking about getting the sky simulations to better show an additional amount of blue sky near the sun when close to sunset/sunrise. A clue is that the sun's disk also then turns somewhat blue. This points to a specific optical property of the dust, variation of extinction with wavelength. I made such a modification and below is a new animation of the sky. Hopefully the overall set of optical properties is starting to converge better on reality. One question I still have is whether smaller aerosols <1 um in diameter are more present to make the sky brightness a bit more uniform as a function of azimuth.
mars_cyl.mp4 ( 1.87MB ) Number of downloads: 144 https://stevealbers.net/ast/mars/mc/mars_cyl.mp4 -------------------- Steve [ my home page and planetary maps page ]
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Jan 23 2022, 10:37 PM
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#75
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Senior Member Group: Members Posts: 4246 Joined: 17-January 05 Member No.: 152 |
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