APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

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APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by APOD Robot » Mon May 31, 2021 4:05 am

Image 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 round moons. Analysis indicates that a slightly larger impact would have destroyed Mimas entirely. The huge crater, named Herschel after the 1789 discoverer of Mimas, Sir William Herschel, spans about 130 kilometers and is featured here. 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 featured image was taken during the closest-ever flyby of the robot spacecraft Cassini past Mimas in 2010 while in orbit around Saturn.

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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by wolfie138 » Mon May 31, 2021 6:31 am

for such a big/wide crater, it doesn't seem very deep, ratio-wise.

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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by WWW » Mon May 31, 2021 9:11 am

So that's not a space station?

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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by Davebard » Mon May 31, 2021 12:36 pm

Why are there so many angular, geometric craters there? The big one is almost square, as are a number of the smaller ones. There's even a Pentagon up-left of the biggest one. It's a whisper of Saturn's hexagonal polar cap.

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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by orin stepanek » Mon May 31, 2021 12:59 pm

Mimas_Cassini_960.jpg

Wow; this moon really got clobbered! I wonder if all those other
craters were also created by incoming meteors or fallout from the
large impact meteor?
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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by Chris Peterson » Mon May 31, 2021 1:10 pm

orin stepanek wrote:
Mon May 31, 2021 12:59 pm
Mimas_Cassini_960.jpg


Wow; this moon really got clobbered! I wonder if all those other
craters were also created by incoming meteors or fallout from the
large impact meteor?
Mimas's escape velocity is only 159 m/s, which is the fastest that any debris could fall back down. That's not fast enough to create craters.
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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by neufer » Mon May 31, 2021 1:58 pm

Chris Peterson wrote:
Mon May 31, 2021 1:10 pm
orin stepanek wrote:
Mon May 31, 2021 12:59 pm

Wow; this moon really got clobbered! I wonder if all those other
craters were also created by incoming meteors or fallout from the
large impact meteor?
Mimas's escape velocity is only 159 m/s, which is the fastest that any debris could fall back down. That's not fast enough to create craters.
Mimas has an orbital velocity of 14.28 km/s.

A rock can be ejected from the back end of Mimas at a relative velocity of -2.4(14.28 km/s) and go into a near parabolic retrograde orbit around Saturn that eventually crashes head on into Mimas at a relative velocity of +2.4(14.28 km/s) = +34.27 km/s.

Much of the ejecta from Herschel's formation went into orbit around Saturn that eventually crashed back onto Mimas (although mostly on the backside from Herschel crater itself).
Last edited by neufer on Mon May 31, 2021 2:09 pm, edited 2 times in total.
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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by XgeoX » Mon May 31, 2021 2:03 pm

wolfie138 wrote:
Mon May 31, 2021 6:31 am
for such a big/wide crater, it doesn't seem very deep, ratio-wise.
Actually, compared to the Lunar craters I browsed that were the same size it was actually a few kilometers deeper. It plunges 10-12 km deep at it’s deepest spots. Of course I only looked at a few lunar craters so I might have got a misleading impression...

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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by Chris Peterson » Mon May 31, 2021 2:03 pm

neufer wrote:
Mon May 31, 2021 1:58 pm
Chris Peterson wrote:
Mon May 31, 2021 1:10 pm
orin stepanek wrote:
Mon May 31, 2021 12:59 pm

Wow; this moon really got clobbered! I wonder if all those other
craters were also created by incoming meteors or fallout from the
large impact meteor?
Mimas's escape velocity is only 159 m/s, which is the fastest that any debris could fall back down. That's not fast enough to create craters.
Mimas has an orbital velocity of 14.28 km/s.

A rock can be ejected from the back end of Mimas at a relative velocity of -2.4(14.28 km/s) and go into a near parabolic retrograde orbit around Saturn that eventually crashes head on into Mimas at a relative velocity of +2.4(14.28 km/s) = +34.27 km/s.
It's possible. But the cratering pattern on Mimas doesn't seem to suggest it intersected a debris cloud on its leading edge. The cratering pattern is remarkably uniform for a tidally locked body, with the variation appearing to involve resurfacing, not an uneven pattern of impacts.
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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by XgeoX » Mon May 31, 2021 2:06 pm

neufer wrote:
Mon May 31, 2021 1:58 pm
Chris Peterson wrote:
Mon May 31, 2021 1:10 pm
orin stepanek wrote:
Mon May 31, 2021 12:59 pm

Wow; this moon really got clobbered! I wonder if all those other
craters were also created by incoming meteors or fallout from the
large impact meteor?
Mimas's escape velocity is only 159 m/s, which is the fastest that any debris could fall back down. That's not fast enough to create craters.
Mimas has an orbital velocity of 14.28 km/s.

A rock can be ejected from the back end of Mimas at a relative velocity of -2.4(14.28 km/s) and go into a near parabolic retrograde orbit around Saturn that eventually crashes head on into Mimas at a relative velocity of +2.4(14.28 km/s) = +34.27 km/s.
Ouch, that will definitely leave a mark!

https://youtu.be/sVSfcWUxu_Q

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Ice moom and sublimation

Post by Filboid » Mon May 31, 2021 2:18 pm

After reading the caption of today's picture of Mimas, I wondered: If it is mostly frozen water, why doesn't the ice sublimate? How can any ice moon exist in the vacuum of space? After all, the ice cubes in my refrigerator eventually sublimate completely.

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Re: Ice moom and sublimation

Post by Chris Peterson » Mon May 31, 2021 2:26 pm

Filboid wrote:
Mon May 31, 2021 2:18 pm
After reading the caption of today's picture of Mimas, I wondered: If it is mostly frozen water, why doesn't the ice sublimate? How can any ice moon exist in the vacuum of space? After all, the ice cubes in my refrigerator eventually sublimate completely.
If you move the temperature knob on your freezer down to -300°F you might find that the ice cubes last a lot longer!
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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by neufer » Mon May 31, 2021 5:40 pm

Chris Peterson wrote:
Mon May 31, 2021 2:03 pm
neufer wrote:
Mon May 31, 2021 1:58 pm
Chris Peterson wrote:
Mon May 31, 2021 1:10 pm

Mimas's escape velocity is only 159 m/s, which is the fastest that any debris could fall back down. That's not fast enough to create craters.
Mimas has an orbital velocity of 14.28 km/s.

A rock can be ejected from the back end of Mimas at a relative velocity of -2.4(14.28 km/s) and go into a near parabolic retrograde orbit around Saturn that eventually crashes head on into Mimas at a relative velocity of +2.4(14.28 km/s) = +34.27 km/s.
It's possible. But the cratering pattern on Mimas doesn't seem to suggest it intersected a debris cloud on its leading edge. The cratering pattern is remarkably uniform for a tidally locked body, with the variation appearing to involve resurfacing, not an uneven pattern of impacts.
The leading edge has the potential for the fastest return collisions; however, even the trailing edge could receive returning debris at +0.4(14.28 km/s) = +5.71 km/s. And Mimas may well not have been tidally locked after the Herschel collision (and for a long time after).

(The point was that Mimas's escape velocity has little to do with the danger of returning debris.)
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Re: APOD: Mimas: Small Moon with a Big Crater (2021 May 31)

Post by Chris Peterson » Mon May 31, 2021 5:44 pm

neufer wrote:
Mon May 31, 2021 5:40 pm
Chris Peterson wrote:
Mon May 31, 2021 2:03 pm
neufer wrote:
Mon May 31, 2021 1:58 pm

Mimas has an orbital velocity of 14.28 km/s.

A rock can be ejected from the back end of Mimas at a relative velocity of -2.4(14.28 km/s) and go into a near parabolic retrograde orbit around Saturn that eventually crashes head on into Mimas at a relative velocity of +2.4(14.28 km/s) = +34.27 km/s.
It's possible. But the cratering pattern on Mimas doesn't seem to suggest it intersected a debris cloud on its leading edge. The cratering pattern is remarkably uniform for a tidally locked body, with the variation appearing to involve resurfacing, not an uneven pattern of impacts.
The leading edge has the potential[ for the fastest return collisions/b] but even the trailing edge could receive returning debris at +0.4(14.28 km/s) = +5.71 km/s. And Mimas may well not have been tidally locked after the Herschel collision (and for a long time after).

I think all of the cratering is relatively young compared with the time of tidal locking, which would have occurred early in the development of the Solar System, especially given the evidence for resurfacing events.

(The point was that Mimas's escape velocity has little to do with the danger of returning debris.)

Agreed, I was simply referring to the original suggestion of scattered debris, which is different than a debris cloud in orbit around Saturn. That said, it's a complex question as to whether such a debris cloud would form, or just how long it would survive in that highly perturbed environment.
Chris

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