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Author Topic: How visible are eclipsed Martian moons?  (Read 1245 times)

Offline Atomic-S

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How visible are eclipsed Martian moons?
« on: 28/09/2015 04:55:57 »
In view of the recent eclipse of the moon, which did not fully extinguish it even in totality, the following question suggests itself: On Mars, when its moons are eclipsed, can they be seen at all? Phobos flies much closer to the planet, indicating a much more intense shadow. Does Deimos ever get close enough to experience a meaningful eclipse?


 

Offline evan_au

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Re: How visible are eclipsed Martian moons?
« Reply #1 on: 28/09/2015 11:02:29 »
Another factor is that Mars has very little atmosphere (about 1% of Earth's atmospheric pressure, at the surface), so less light will be refracted around Mars.

The "Blood Moon" effect will be much less apparent on Mars.
 

Offline evan_au

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Re: How visible are eclipsed Martian moons?
« Reply #2 on: 28/09/2015 11:56:26 »
A "back of the envelope" calculation suggests:
  • The Sun's disk is around 0.2 across in the sky of Mars (and its moons).
  • Mars has a diameter of around 7000km, which means that the zone of totality extends about 1.7 million km beyond Mars.
  • Phobos and Deimos both orbit quite close to the equator of Mars (within 1 degree), so they pass roughly through the middle of this zone.
  • Phobos is 20km in diameter, and orbits at a radius of 9,300km, so it is well within the zone that will give a total eclipse.
  • Deimos is 6km in  diameter, and orbits at a radius of 23,500km, so it is also well within the zone of totality.

Another way of looking at this is:
  • From Phobos, Mars fills about 40 of the sky, which will totally block the tiny disk of the Sun (0.2 across).
  • From Deimos, Mars fills about 16 of the sky, which will totally block the tiny solar disk. 
 

Online Bored chemist

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Re: How visible are eclipsed Martian moons?
« Reply #3 on: 28/09/2015 15:20:26 »
Also, the moon has a higher albedo than Phobos or deimos so, even if it were as well lit (and it won't be) it will look darker.
However, as far as I can tell, the point is moot.
From earth, neither of them is visible anyway (unless you use a telescope- and then the question becomes "how good a telescope do you have?").
If, on the other hand you were on one of Mars' moons I think you would still be able to se it during an eclipse by simply looking down near your feet- at the very least, you could see it by starlight.

Another interesting point would be to look (with a big, space-based telescope) at the light reflected from one of those moons during and after a (local) eclipse.
In one case it would be reflecting sunlight and in the other sunlight filtered (and scattered) through the atmosphere of Mars.
If, for example, there were differences in the IR corresponding to water vapour, then you would know there was water vapour in the atmosphere of Mars without having to go there.
 

Offline Atomic-S

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Re: How visible are eclipsed Martian moons?
« Reply #4 on: 01/10/2015 05:04:23 »
On Earth, the dark side of the moon can be seen along with the light side when the moon is crescent, due to reflections from the Earth. On Mars, this effect will certainly be present, undoubtedly much stronger in the case of Phobos because 0f the closeness of Phobos to Mars; however as Phobos slips around behind Mars, the portion of Mars that will reflect upon it should decrease rapidly as it approaches Mars' shadow. So Phobos will be illuminated by the Sun and Mars when it is generally on the sunlit side of Mars, and we should see the effects of both, with, however, the effect from the reflections off of Mars decreasing rapidly as Phobos approaches eclipse. Once it hits Mars' shadow, it should dim out very quickly with respect to both sources of light, and, as evan_au pointed out, Mars does not have much of an atmosphere to refract any more, and even what is refracted ought to drop quickly as it progresses behind the planet on account of the rather considerable angle between Phobos, the Martian horizon as seen from it, and the direction of sunlight. In view of this, it is undoubtedly correct that there would be very littl light on Phobos, unlike Earth's moon when eclipsed, so that the question arises whether it could even be seen at all.  There would be starlight, but would that be enough to view it with the unaided eye? Deimos is of course further out, but many of these same considerations seem to apply to it also. Of course, being smaller and more distant from Mars' surface, it would be much harder to see in any case.
 

Online Bored chemist

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Re: How visible are eclipsed Martian moons?
« Reply #5 on: 01/10/2015 12:19:23 »
There would be starlight, but would that be enough to view it with the unaided eye?
It would be if you were standing on it.
Starlight on Mars is just the same as starlight on Earth.
 

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Re: How visible are eclipsed Martian moons?
« Reply #5 on: 01/10/2015 12:19:23 »

 

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