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The Martian Sky
JRehling
post Jan 31 2018, 04:27 PM
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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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scalbers
post Jan 31 2018, 07:39 PM
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QUOTE (Ant103 @ Jan 30 2018, 11:33 PM) *
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?


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vikingmars
post Jan 31 2018, 08:29 PM
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QUOTE (Ant103 @ Jan 31 2018, 12:33 AM) *
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 smile.gif

Attached Image
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mcaplinger
post Jan 31 2018, 09:11 PM
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QUOTE (Ant103 @ Jan 30 2018, 03:33 PM) *
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 "


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Disclaimer: This post is based on public information only. Any opinions are my own.
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Deimos
post Jan 31 2018, 11:35 PM
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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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scalbers
post Feb 1 2018, 12:33 AM
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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).


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scalbers
post Feb 4 2018, 12:54 AM
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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.

Attached Image


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:

Attached Image


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.

Attached Image


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.


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