Uranus System Imaging Options

[machi]

I finished my first good image of Uranus.
Planet is colorized from three filtered images (orange, green, blue).
Slightly brownish color of rings is entirely artificial.

Attached thumbnail(s)

 

[Phil Stooke]

Since this is the 20th anniversary of the discovery of planetshine on the Uranian satellites (reported at LPSC in March 2001, abstract attached here), it's good to get back to it again.

This link:
https://www.planetary.org/articles/1362
discusses the situation in more detail with some of Ted Stryk's processed images.

As Voyager 2 approached the Uranian system the moons were spread out around it and viewed at closest approach from different directions. If you get lucky the part of the moon lit by planetshine is going to be visible at closest approach, and that was the case for Ariel. For the others, either the part illuminated by Uranus was facing the other way or it was only seen as a very thin crescent and at lower resolution. Remember that the idea of viewing the 'night' side in reflected light had not been considered by the Voyager science team (or maybe considered but rejected), so imaging was not undertaken with a view to capturing these scenes and anything we got was serendipitous.

Cassini took full advantage of this discovery and we got spectacular results that significantly increased coverage of many moons.

Phil
 stooke_lpsc_32_satellites_1074.pdf ( 130.24K ) Number of downloads: 272

[JRehling]

Thanks, Phil. I found a diagram showing how the uranian system was oriented at the time of the V2 flyby, but it's hard to read a 4-dimensional reality as rendered in 2 dimensions.

For each moon, at the time of V2, there was effectively a division of the moon into quadrants. Two were in sunlight. One was in uranusshine. One was in neither. Uranusshine imagery would depend upon whether or not V2 ever could see that uranusshine quadrant and furthermore took an image at that time.

Saturn and Neptune in certain ways are each more favorable for this – you can get the night sides of the moons receiving a full planetary shine rather than ~1/4 brightness. Also, Saturn is even more huge in the skies of its moons than Uranus is for its moons.

[Explorer1]

Saturn and Neptune in certain ways are each more favorable for this – you can get the night sides of the moons receiving a full planetary shine rather than ~1/4 brightness.

Unless you catch the system during one of the two equinoxes per Uranian year, though in that case an orbiter doesn't need to bother with night illumination at all since the full globe gets sunlight through one orbit and you can just target a flyby for the appropriate portion of the local day. An orbiter around 2050, the next equinox, would be ideal!

[HSchirmer]

Unless you catch the system during one of the two equinoxes per Uranian year, though in that case an orbiter doesn't need to bother with night illumination at all since the full globe gets sunlight through one orbit and you can just target a flyby for the appropriate portion of the local day. An orbiter around 2050, the next equinox, would be ideal!

Interesting timing, but with Uranus, as with Pluto, don't we run into the problem of ephemeral ices?
What I mean is, when Uranus and its moons are in 'rotisserie mode" and essentially 100% of the surface receives sunlight, then you're going to have a sublimation driven 'fluffed up" atmosphere.

When you've got polar only illumination, you're got an interesting possibility that a fair-fraction of the atmosphere is going to freeze out into a lump on the dark side of the moon.

So, yes at the equinoxes we'd be seeing "the entire surface" but each solstice, you'd get a new surface as the atmosphere freezes out and snows down over half the moon.

[JRehling]

Interesting timing, but with Uranus, as with Pluto, don't we run into the problem of ephemeral ices?
[...]
So, yes at the equinoxes we'd be seeing "the entire surface" but each solstice, you'd get a new surface as the atmosphere freezes out and snows down over half the moon.

There is no known atmosphere on four of Uranus's five biggest moons. CO2 has been detected at Titania. There's no indication that it would freeze out or snow down.

Even without the ability to resolve these bodies, we can observe their light curves even with modest equipment. Unless there's a published finding that Titania shows this, I wouldn't presume that it occurs.

Even if so, we'd be seeing the sunlit portion in all cases. If something happens in the long winter night, no, we will never see that, because we're observing from the same direction as the Sun. We also didn't see that Saturn's winter polar regions are relatively blue for the same reason.

[HSchirmer]

Even if so, we'd be seeing the sunlit portion in all cases. If something happens in the long winter night, no, we will never see that,

Agreed, and accurate as always! On rethinking, I should have said two things differently.
- First, what about those 'short winter nights" when the subsolar point is almost tracing a great-circle along the Uranian moon's equator, in contrast to when the sub solar point is locked on a pole?
- Second, I should have been more precise, it's also about sublimation and deposition under exospheric conditions, not just atmospheric conditions: similar to Mercurian and Lunar craters trapping ices, even though there's no atmosphere.
Perhaps there is a sublimation / deposition driven latitude dependent 'hoarfrost line' during solstice, where ices accumulate and freeze out.
During equinox, that's a more interesting question, would there be a longitude dependent 'hoarfrost line" from sublimating ices condensing back behind the dawn?

[JRehling]

I doubt that there will be anything like this observable from Earth. We don't see it at Callisto or Iapetus. We see Mars's winter cap emerge from springtime and thaw. Titania has a trace atmosphere at best.