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Posted by: nprev Feb 14 2007, 07:30 AM

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1084/1P224433531ESF79MNP2632L8M1.HTML from Oppy made me wonder about Phobos' & Deimos' visual aspects for a hypothetical surface observer using Mark I eyeballs. The image seems to show at least a faint lens flare on the right side from the main image; how bright are the moons from the surface? Also, are at least some surface features on Phobos like Stickney visible?

Probably old questions, probably old territory...but still interesting to contemplate from the rather unique viewpoint of the MERs.

Posted by: edstrick Feb 14 2007, 10:31 AM

Pictures from early in the mission or the early extended mission showed (as I recall) rather vague details on Phobos that reasonably matched models. The 3-X higher resolution (I think) of the MSL cameras should show some real details and probably vague details on Deimos.

Posted by: AndyG Feb 14 2007, 10:47 AM

In terms of the brightness...


Baseline Phobos (observer at 70 deg north latitude, Mars at aphelion, Half Phase) = -7.6

Take away 0.4 for Mars at perihelion
Take away 0.8 for 0 deg (equatorial) viewers
Take away 0.8 for Full Phobos
Max brightness of Phobos = -9.6


Baseline Deimos (same conditions) = -4.3

Take away 0.4 for Mars at perihelion
Take away 0.3 for 0 deg (equatorial) viewers
Take away 0.7 for Full Deimos
Max brightness of Deimos = -5.7


(Give or take a tenth or so.)

Since Venus (max mag -4.4) can cast shadows, both moons also will throughout most of their phases.** Phobos, at best, is about 17 times dimmer than a full moon on Earth - that's still a lot of light for the human eye, but no chance of colour vision: Mars' cats will also look grey at night.

As to details, I can convince myself that the Moon's Mare Crisium is viewable without aid - but that's a dark maria over lighter rock. Phobos has less variety in albedo, and seen from Mars is around a third the apparent size of the Moon. However, I think it would be possible to see shadows in Stickney depending on the angle of illumination - there must be times when the near and far rims of that crater will catch sunlight, leaving a pool of black in between.

Andy

** I'd like a photo of the shadows of the pancam mast during a good full Phobos/Deimos night - just how unearthly would that look...

Posted by: djellison Feb 14 2007, 11:21 AM

Problem is, it'd be fairly hard to see - and require an exposure long enough to probably smear such a shadow to a greater ammount than its own width

Doug

Posted by: AndyG Feb 14 2007, 03:25 PM

Hmmm...I'm not so sure. Here's my reasoning:

Previous sixty-second pancam exposures of the night sky resulted in stars of around the 7th magnitude being seen. Thinking empirically, Deimos's casting of a shadow - on a par with Venus's and at the limit of human eye perception, should be just catchable in an exposure that roughly mimicks that of the human eye. Since we have a limiting magnitude of around 6, that would suggest a Pancam exposure of around 25 seconds would be the minimum required.

Since Phobos is "up" for around five and a half hours at the MERs latitude, and Deimos even longer the angles of shadows won't change much at all over (even more shadow-catching) 1 to 2 minute exposures. Being near the equator doesn't help much in terms of getting good, long shadows when the moons are near their zenith, but this means short shadows won't move much either.

As Phobos does about 1860m/s over the ground, at an altitude of some 5830 km, the end of the shadow of a 1.4m-tall pancam mast is only going to move at just over an inch in a minute.

All that said, if I see a problem at all, it's with the nearly forty-to-one contrast ratio between the two light sources: I'm not sure the pancam could detect a washed-out Deimos shadow at that level.

Andy

Posted by: JRehling Feb 14 2007, 06:20 PM

Misc. thoughts:

If any of this is practical, it would be interesting to take a double moonlight image with Phobos and Deimos casting double shadows of objects. I'm not sure if the original mention of a Phobos/Deimos night meant "or" or "and", but "and" is the more interesting one.

I would think eclipses would be a frequent impediment to getting a full Phobos. Summer solstice would be the best way to avoid that and still have enough juice in the batteries for night photography.

Posted by: ustrax Feb 16 2007, 03:49 PM

BBC http://news.bbc.co.uk/2/hi/science/nature/6324923.stm.