The Martian Sky Options

[scalbers]

Can we trust the colour of the disk in un-ND-filtered frames? Maybe one or more channels are saturated, even near sunrise/set?

There is some consensus from other sources as well - and I'd be interested in a more precise comparison of the sun's disk chromaticity. It turns out that with the Monte Carlo simulation one seems to need a blue solar disk to get a sufficiently bluish color in the scattered light around the sun when it is low. Here is one paper about that as well. Looking at chromaticity values, perhaps there it too much blue in my example, so I'll next try an intermediate solution.

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

In this paper they are at least looking very close to the sun with chromaticity when it is low.

https://trs.jpl.nasa.gov/bitstream/handle/2....pdf?sequence=1

[Deimos]

The Sun was saturated in the recent M2020 sunset image.

The Sun is in fact slightly blue when low in the sky, but only slightly. That is mentioned here, which showed the dust to be several percent more transparent to blue light compared to slightly infrared.

The blue around the Sun is almost entirely a diffraction effect, where the width of the diffraction lobe is directly proportional to wavelength--the blue is more concentrated. The particle size distribution of the dust is numerically dominated by sub-micron particles (in the math, since they are not very impactful) but area and mass are dominated by larger particles (within about 50% of 1.5 microns and larger than 1.5 microns, respectively).

The sky brightness 90 degrees from the Sun is not consistent with Mie scattering, which has been known since Viking--this is not troublesome, since there is no known process to get spherical dust. It is small enough and compact enough to have Mie scattering work pretty well--especially for diffraction--but Mie still misses the details.

[Deimos]

Holy Moly! That Applied Optics paper modeled the press release image?!?! I could have pointed them to the data. In the data, the Sun was saturated and not viewed through the same atmospheric path in each image since it was 3 sequential filters (for example). In the press release image, the Sun was the color I told it to be, which was taken straight from the Tomasko et al. model. I guess I'm glad someone noticed it was blue...

[scalbers]

Yes we could still provide the authors with that data as I've recently been in touch with one of them

Thanks much for the links. I agree the diffraction effect is the principal reason the solar aureole is blue, especially when the sun is high and things are simpler with more single scattering. I suspect it may be more nuanced when the sun is close to the horizon since when I have near zero Angstrom Exponent much of the aureole stays somewhat redder near sunset.

My post above #74 had a very large negative Angstrom Exponent of around -1.8 between 615nm and 450nm so I'll try tests of -0.25 and -0.15 to check the sensitivity of the images.

[scalbers]

Here's a animated version with -0.15 Angstrom exponent:

 mars_cyl.mp4 ( 1.88MB ) Number of downloads: 446

8-bit and 16-bit individual frames are here:

http://stevealbers.net/ast/mars/mc/

[fredk]

This MSL navcam image:

https://mars.nasa.gov/msl-raw-images/proj/m...NCAM00558M_.JPG

was taken at around 20:24 LMST, when the sun was around 23 deg below the horizon according to http://www.greuti.ch/msl/clock_and_filenames.htm.

You can still see some twilight glow in the sky, as this low-pass filtered (and half-sized) version shows:

This view is towards azimuth 140 deg, and the sun was at azimuth 242 deg, so the brightening of the sky towards the right of the frame is consistent with a twilight glow.

On earth, twilight would be completely over well before this, when the sun is around 18 deg below the horizon. Do we expect martian twilight to last longer than earth's, given the dominance of dust scattering? The length of twilight will presumably depend on the atmospheric tau, though judging from the daylight images tau was fairly average on that sol.

[climber]

Any chance it could be zodiacal light instead ?

[nprev]

Likely has at least something to do with the scale height of Mars' atmosphere being a few km higher than that of Earth. Presumably that means that in addition to dust there are CO2 & H2O crystals at high altitude that would increase scattering & thus prolong twilight.

[djellison]

Any chance it could be zodiacal light instead ?

Over a period of ~20mins from the first to last observation in that group, it fades to almost nothing.

So I presume it's a twilight phenomenon and not anything else.

[scalbers]

As mentioned a longer twilight on Mars relates to two things that I would characterize as follows: A larger scale height compared with the radius of Mars. Also more dust relative to gas in the atmosphere (including the upper atmosphere). To get a good picture of all this it would be useful to make a plot of both gas and aerosol extinction coefficient for typical Earth and Mars atmospheres. These can be plotted with a Y-axis of planetary radii.

[Bill Harris]

"Do we expect martian twilight to last longer than earth's, given the dominance of dust scattering?" (Fred)

I'd suspect more particulates (dust + CO2/H20) with a larger scale height (dustier and fluffier).

Uh, yes. What they said.

--Bill

[scalbers]

Earth sky analogy to Mars:

https://www.scmp.com/news/china/science/art...ijing-mars-blue

[scalbers]

For convenience is there a handy reference to find a solar ephemeris to list elevation angle and azimuth as seen from the various spacecraft on the surface of Mars? Thanks.

[fredk]

The filename sites for MER and MSL are still up:

http://www.greuti.ch/oppy/html/filenames_ltst.htm
http://www.greuti.ch/msl/clock_and_filenames.htm

[mcaplinger]

https://www.giss.nasa.gov/tools/mars24/