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The Martian Sky
djellison
post Jan 29 2018, 09:28 PM
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Do you mean at night?

If so - yes - https://www.jpl.nasa.gov/news/news.php?feature=4121
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mcaplinger
post Jan 29 2018, 09:56 PM
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QUOTE (djellison @ Jan 29 2018, 01:28 PM) *
Do you mean at night?

I presume the OP meant in the daytime.

With all due respect to pre-spaceflight artist's conceptions, I don't think you can see stars in the daytime on Mars any more easily than you can on Earth.


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elakdawalla
post Jan 29 2018, 11:46 PM
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Would Jupiter be visible to the naked eye in the daytime?


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Explorer1
post Jan 30 2018, 12:02 AM
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A quick Google search shows that some folks have seen Jupiter from Earth unaided before sunset/sunrise. It apparently helped to have the Moon nearby as a guide. I suppose it should be a cinch from Mars, being a few AU closer, especially if Phobos or Deimos have a conjunction with the giant planet.
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scalbers
post Jan 30 2018, 12:50 AM
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I've seen Jupiter at 10:20am from a high desert location, right next to the moon (as documented in an ancient issue of Sky and Telescope). There is a case to be made that Sirius would be visible from a clean air location when it is at the zenith and I recall some sightings have been made (I can dig out a paper on this).

For Mars it seems that daytime stars might be similar to Earth. The atmospheric scattering (and optical depth) is quite a bit greater and more than compensates for the dimmer sun. Yet aerosols have a greater angular dependence than gases, so away from the sun the sky brightness would tone down a fair amount. Extinction is more of a factor on Mars.

Mars being closer to Jupiter mostly applies near opposition. Daytime signtings would be closer to quadrature when there can be both a distance and a phase angle disadvantage impacting the apparent magnitude. Based on a review of the above sky simulations (assuming an optical depth of 0.5) I may be able to calculate the zenithal limiting magnitude for various solar elevation angles. The best part of the sky to look would be somewhat lower than the zenith and opposite the sun in azimuth.


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Explorer1
post Jan 30 2018, 01:17 AM
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QUOTE (scalbers @ Jan 29 2018, 07:50 PM) *
The best part of the sky to look would be somewhat lower than the zenith and opposite the sun in azimuth.


And the best place to look from is probably the summit of Olympus Mons! One of the only spots, along with the other volcanoes, where not even the global dust storms can reach. 12% the atmospheric pressure, right? Not even Mauna Kea gets close to that in terms of ideal observing locations...
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antipode
post Jan 30 2018, 05:34 AM
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Hi all, OP here (at least in terms of Mars' sky at the zenith question).

Yes, I did mean during the daytime. This question was prompted by 2 things.

1. All the pre Viking paintings of Mars' surface that showed a thin blue band around the horizon.
This came before we knew the amount of fines suspended in the atmosphere, but I always assumed that at 7mb, the sky directly above the viewer would be dark enough
(assuming the Sun wasn't close in the sky) to see stars. Or would it be like the Moon, where glare and contrast extremes apparently made it near impossible for the astronauts to see stars?

2. Now add the fines we know are always there to some extent. Take a nice relatively dust free day at Gale (for instance) and look straight up. With the caveat about the Sun still standing,
would you see butterscotch all the way to the zenith, with just the color slowly darkening - or would the sky be effectively black? And if it was black, what would be the chance of seeing say, a magnitude 1 star?

P
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ngunn
post Jan 30 2018, 09:57 AM
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I seem to recall one of the rovers acquired at least one vertical sky panorama. Maybe someone can link back to that and any related discussion from that time.
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scalbers
post Jan 30 2018, 02:36 PM
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QUOTE (antipode @ Jan 30 2018, 05:34 AM) *
with just the color slowly darkening - or would the sky be effectively black? And if it was black, what would be the chance of seeing say, a magnitude 1 star?

As a first approximation the sky brightness relative to the sun's brightness at the zenith is proportional to the aerosol optical depth (AOD) times the phase function that applies for the scattering angle at that part of the sky. Other parts of the sky are brighter based on the cosine of the zenith angle. Thus the sky brightness could be a little darker than Earth in some spots, though not really black. The depth of the phase function minimum (near 140 degrees scattering angle) would be important. The AOD (+gas) is about a factor of 2 disadvantage typically on Mars vs Earth and the phase function gives an advantage of about 6 times in the darkest portion of the sky.

With the sun 51 degrees high the non-MC method (I sometimes call this method Simulated Weather Imagery or SWIM, as shown in the post #42 upper image), gives a zenith limiting magnitude of about -3.2. This phase function is roughly 10 times smaller as we get farther from the sun, though reflection from the ground smooths out the variation. So far I obtain that the theoretical best place to look is 55 deg high opposite the sun's azimuth with a limiting magnitude of -2.6. Jupiter in this situation may be invisible as it would be a fainter magnitude when at that elongation from the sun. A lower sun would of course improve things. Nudging Jupiter underneath the best limiting magnitude spot would increase its (top of atmosphere) brightness as well, since it would then have a larger elongation.


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mcaplinger
post Jan 30 2018, 03:55 PM
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QUOTE (ngunn @ Jan 30 2018, 01:57 AM) *
I seem to recall one of the rovers acquired at least one vertical sky panorama.

See MSL sol 1637 (I think) and sol 1817.

I'm pretty sure it's never dark enough at the zenith to see stars from elevations anywhere near the datum. Top of Olympus Mons, I don't know.

Without doing the math in detail, I think the following is true: if you could see Jupiter in the daytime, it would be like seeing Venus in the daytime on Earth -- possible if you know just where to look, but not anything like obvious.


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scalbers
post Jan 30 2018, 04:45 PM
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Olympus Mons should be pretty good as I'm getting limiting magnitudes of about zero, using the earlier mentioned 12% of the atmosphere and recalling the similar scale heights for gas and dust.


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JRehling
post Jan 30 2018, 07:29 PM
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I'm not sure how much the martian sky varies, but the terrestrial sky, needless to say, varies enormously, even when cloudless. I've heard of lifelong earthlings who are unaware that the Moon is visible in the daytime sky, and indeed, it's a lot less visible in low-transparency, high-humidity conditions (and climates) than otherwise.

In what must have been profoundly unusual circumstances, I saw satellites orbiting overhead in Phoenix during the daytime. Not a flare, which can exceed Venus in brightness, but during typical illumination.

Those who have tried report that it's easier to see redder stars through a telescope in the daytime sky than whiter ones. The reason is obvious: greater contrast against the blue sky. So I would imagine that Sirius would be the easiest to see on Mars, whereas Arcturus is possibly easiest on Earth.

Of course, the elephants in the room for Mars are Phobos and Deimos. How visible are they in a daytime sky?

And Venus and Earth ought to be interesting daytime objects to consider. Blue Earth ought to contrast nicely with the orange sky.
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mcaplinger
post Jan 30 2018, 08:22 PM
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QUOTE (JRehling @ Jan 30 2018, 11:29 AM) *
Of course, the elephants in the room for Mars are Phobos and Deimos. How visible are they in a daytime sky?

https://mars.nasa.gov/msl/multimedia/raw/?r...1_DXXX&s=45 was taken at 17:27 LMST, which I think was somewhat before sunset.


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Ant103
post Jan 30 2018, 11:33 PM
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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.



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fredk
post Jan 31 2018, 12:20 AM
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QUOTE (mcaplinger @ Jan 30 2018, 09:22 PM) *
https://mars.nasa.gov/msl/multimedia/raw/?r...1_DXXX&s=45 was taken at 17:27 LMST, which I think was somewhat before sunset.

Given its field of view, the M-100 image might correspond more closely to a binocular view than a naked eye view, so Phobos will be harder naked eye. But I think very likely still visible during the day.

For viewing through an optical system, aperture helps with point sources against a bright sky even more than extended sources, since the brightness of the point source will be enhanced by about the ratio of aperture to pupil area, while the surface brightness of the sky will never exceed that seen by naked eye. On Earth brighter stars are easily visible through telescopes in daylight, most notably around lunar transits.
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