Starship Asterisk*

Mimas: Small Moon with a Big Crater

Explanation: Whatever hit Mimas nearly destroyed it. What remains is one of the largest impact craters on one of Saturn's smallest moons. The crater, named Herschel after the 1789 discoverer of Mimas, Sir William Herschel, spans about 130 kilometers and is pictured above. Mimas' low mass produces a surface gravity just strong enough to create a spherical body but weak enough to allow such relatively large surface features. Mimas is made of mostly water ice with a smattering of rock - so it is accurately described as a big dirty snowball. The above image was taken during the 2010 February flyby of the robot spacecraft Cassini now in orbit around Saturn. A recent analysis of Mimas's unusual wobble indicates that it might house a liquid water interior ocean.

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http://www.jpl.nasa.gov/news/news.php?feature=4342
http://asterisk.apod.com/viewtopic.php?t=33978

Ahh, the golf ball returns. I like Mimas. It looks so... clean and spiffy.

Seeing Titan behind Mimas is interesting. Are the colors and the albedos "true"? That dirty-orange atmosphere inside a thin coating of blue gas is probably true for Titan, and I don't doubt that Mimas is virtually colorless. Is it so reflective, though?

Ann

Ann wrote:Seeing Titan behind Mimas is interesting. Are the colors and the albedos "true"? That dirty-orange atmosphere inside a thin coating of blue gas is probably true for Titan, and I don't doubt that Mimas is virtually colorless. Is it so reflective, though?

Ann

I believe that's Saturn, not Titan behind Mimas. It was recorded in visible green light, with Saturn coloured afterwards based on other true-colour images, according to the links.

At first I sincerely thought it was a well made model photographed against a wall The way it's lit and no curvature in the background objects - unusual for your average space photo.

Nitpicker wrote:

Ann wrote:Seeing Titan behind Mimas is interesting. Are the colors and the albedos "true"? That dirty-orange atmosphere inside a thin coating of blue gas is probably true for Titan, and I don't doubt that Mimas is virtually colorless. Is it so reflective, though?

Ann

I believe that's Saturn, not Titan behind Mimas. It was recorded in visible green light, with Saturn coloured afterwards based on other true-colour images, according to the links.

You're correct, Nit. Here are two consecutive images from the archive, at least one of which was obviously used for this composite. As you can see, Mimas is passing rapidly in front of Saturn's limb, causing the motion blur. You can see it at the left edge in the first image and then the middle in the second. Planetary image processing is often tricky.

Nitpicker wrote:

Ann wrote:Seeing Titan behind Mimas is interesting. Are the colors and the albedos "true"? That dirty-orange atmosphere inside a thin coating of blue gas is probably true for Titan, and I don't doubt that Mimas is virtually colorless. Is it so reflective, though?

Ann

I believe that's Saturn, not Titan behind Mimas. It was recorded in visible green light, with Saturn coloured afterwards based on other true-colour images, according to the links.

Interesting. I wouldn't expect Saturn to have that sort of "blue border". You mean it is a product of processing?

Ann

Ann wrote:I wouldn't expect Saturn to have that sort of "blue border". You mean it is a product of processing?
Ann

I had to look hard to find a another colour image of Saturn's limb up close, which showed a blue hue. I found one (along with a lot of other spectacular probe images) here:

http://saturn.jpl.nasa.gov/photos/image ... ageId=3444
http://www.boston.com/bigpicture/2009/0 ... ssion.html

For some reason, only one of Saturn's hemispheres has blue skies. It's also very easy to end up with weird fringes when objects are moving around between exposures. That picture of the thin rings in front of Saturn's limb would have used the rings as the registration point rather than the background Saturn. Maybe it's really like that but there's a good chance that the misalignment between channels is emphasizing it, too.

I found the original image release for that picture.
http://saturn.jpl.nasa.gov/photos/image ... ageId=3444

Beyond wrote:Ahh, the golf ball returns. I like Mimas. It looks so... clean and spiffy.

Usually golf balls often find the water hazard but "Mimas" has one on the inside. From the looks of it this one was pounded although it looks more like it was the driving range. Judging from the size of the Herschel crater some "Samim" really got a hold of one and left quite the ball mark.

I love symmetrys - well close enough, one simian to another. Hope that's reciprocated.

On a serious note, today's APOD does have a 3D character to it. Is it the crater shadows on the left or conversation above re: the background? (Guess that would be 1/3D to me but I digress)...

What I don't understand is why the so called huge impact crater on this thiny snowball moon has the form of a Hexagon. On some pictures of Jupiter and other planets on the pole area also Hexagons are present , WHY?"

When it was FREE....it might have made a great COMET.....but it got captured.

:---[===] *

KarelVreeburg wrote:What I don't understand is why the so called huge impact crater on this thiny snowball moon has the form of a Hexagon. On some pictures of Jupiter and other planets on the pole area also Hexagons are present , WHY?"

As far as Mimas is concerned...I think it is the "Chunky-ness" of the material. Mostly Ice evidently...so maybe it "cracks" wrong. In the photo there are many such smaller craters.
As for Hexagon shaped Polar clouds on Jupiter and Saturn, there is some information on Wikipedia...about Saturn's North Pole Hexagon Pattern.
"The pattern's origin is a matter of much speculation. Most astronomers believe it was caused by some standing-wave pattern in the atmosphere. Polygonal shapes have been replicated in the laboratory through differential rotation of fluids"

:---[===] *

geckzilla wrote:For some reason, only one of Saturn's hemispheres has blue skies. It's also very easy to end up with weird fringes when objects are moving around between exposures. That picture of the thin rings in front of Saturn's limb would have used the rings as the registration point rather than the background Saturn. Maybe it's really like that but there's a good chance that the misalignment between channels is emphasizing it, too.

I found the original image release for that picture.
http://saturn.jpl.nasa.gov/photos/image ... ageId=3444

I was just looking for other examples upon which Supportstorm (the final digital processor of the APOD) might have based the colourisation. I was expecting to find more examples of a blue limb on an otherwise golden Saturn and was surprised to find only one. (And apologies for originally providing questionable links in need of replacement, geck -- my Greek extends no further than knowledge of the alphabet.)

I have been alerted that the flyby in this image occurred in 2010 February, and so the APOD text has been updated to indicate this. I apologize for the oversight. - RJN

Nitpicker wrote:

geckzilla wrote:For some reason, only one of Saturn's hemispheres has blue skies. It's also very easy to end up with weird fringes when objects are moving around between exposures. That picture of the thin rings in front of Saturn's limb would have used the rings as the registration point rather than the background Saturn. Maybe it's really like that but there's a good chance that the misalignment between channels is emphasizing it, too.

I found the original image release for that picture.
http://saturn.jpl.nasa.gov/photos/image ... ageId=3444

I was just looking for other examples upon which Supportstorm (the final digital processor of the APOD) might have based the colourisation. I was expecting to find more examples of a blue limb on an otherwise golden Saturn and was surprised to find only one. (And apologies for originally providing questionable links in need of replacement, geck -- my Greek extends no further than knowledge of the alphabet.)

I have never seen a blue limb on Saturn. (And as some people know, I look for blue details quite carefully.)

Titan, on the other hand, really has a blue limb. This blue limb isn't visible in all pictures of Titan, but it is really there. Here is one good picture of it.

On the other hand, as I searched for information about the nature about this blue limb, I came across this site. It says that the blue limb on Titan is a layer of haze, and that its true color is ultraviolet, not actually blue.

Ann

Sure looks like one of the beat up golf balls we put in the chicken nest boxes (fake eggs.)

Ann wrote:

Nitpicker wrote:

geckzilla wrote:For some reason, only one of Saturn's hemispheres has blue skies. It's also very easy to end up with weird fringes when objects are moving around between exposures. That picture of the thin rings in front of Saturn's limb would have used the rings as the registration point rather than the background Saturn. Maybe it's really like that but there's a good chance that the misalignment between channels is emphasizing it, too.

I found the original image release for that picture.
http://saturn.jpl.nasa.gov/photos/image ... ageId=3444

I was just looking for other examples upon which Supportstorm (the final digital processor of the APOD) might have based the colourisation. I was expecting to find more examples of a blue limb on an otherwise golden Saturn and was surprised to find only one. (And apologies for originally providing questionable links in need of replacement, geck -- my Greek extends no further than knowledge of the alphabet.)

I have never seen a blue limb on Saturn. (And as some people know, I look for blue details quite carefully.)

Titan, on the other hand, really has a blue limb. This blue limb isn't visible in all pictures of Titan, but it is really there. Here is one good picture of it.

On the other hand, as I searched for information about the nature about this blue limb, I came across this site. It says that the blue limb on Titan is a layer of haze, and that its true color is ultraviolet, not actually blue.

Ann

Yes, but Titan could never look so big behind Mimas, unless they were imaged from a much greater distance (which then couldn't give us the spectacular resolution on Mimas). I notice geometry more than colour, but it is hard to miss Titan's colourful atmosphere in Cassini images. The blue in the APOD is much more tenuous, but still mysterious.

The wobble, could it not just be the rocky center is off center? Liquid water seems unlikely since Mimas has been there for a very long time, at very low temperatures.

Historian wrote:The wobble, could it not just be the rocky center is off center? Liquid water seems unlikely since Mimas has been there for a very long time, at very low temperatures.

Yes, a somewhat oval core is suggested as another possibility in the recent study. Neither possibility has conclusive evidence at this stage.

Hmmm, I think I know where Lucas got his idea for what a death star should look like. At least that's what I thought when I first saw this picture

Nitpicker wrote:

Historian wrote:The wobble, could it not just be the rocky center is off center? Liquid water seems unlikely since Mimas has been there for a very long time, at very low temperatures.

Yes, a somewhat oval core is suggested as another possibility in the recent study. Neither possibility has conclusive evidence at this stage.

I wonder if tidal flexing could be strong enough to keep Mimas' interior partially above the freezing temperature. Or, could the great impact that created the crator Hershel have knocked Mimas' core off center? Or perhaps the remnants of the impacter itself could be unbalancing this moon?

BDanielMayfield wrote:
I wonder if tidal flexing could be strong enough to keep Mimas' interior partially above the freezing temperature. Or, could the great impact that created the crator Hershel have knocked Mimas' core off center? Or perhaps the remnants of the impacter itself could be unbalancing this moon?

Very interesting possibilities, Bruce. We have seen that other moons in the solar system are strongly affected by tidal flexing. Whether that is possible for Mimas depends, I guess, on how much this little moon gets tugged "this way and that" by other bodies in the "Saturninan system".

And certainly the "bulls-eye" hit that caused the huge crater could have unbalanced Mimas. Just look at Uranus for comparison.

Ann

BDanielMayfield wrote:I wonder if tidal flexing could be strong enough to keep Mimas' interior partially above the freezing temperature.

Well, it's quite close to Saturn, and has a pretty eccentric orbit, which is what's required for high tidal forces. It's twice as close to Saturn as Io is to Jupiter, and has a considerably more eccentric orbit. But it's awfully small; I'm not sure how effective such a small body would be at holding internal heat.