APOD: Herschel Crater on Mimas (2022 Aug 13)

[APOD Robot]

Herschel Crater on Mimas

Explanation: Mimas, small 400 kilometer-diameter moon of Saturn, is host to 130 kilometer-diameter Herschel crater, one of the larger impact craters in the entire Solar System. The robotic Cassini spacecraft orbiting Saturn in 2010 recorded this startling view of small moon and big crater while making a 10,000-kilometer record close pass by the diminutive icy world. Shown in contrast-enhanced false color, the image data reveal more clearly that Herschel's landscape is colored slightly differently from heavily cratered terrain nearby. The color difference could yield surface composition clues to the violent history of Mimas. Of course, an impact on Mimas any larger than the one that created the 130-kilometer Herschel might have destroyed the small moon of Saturn.

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[Holger Nielsen]

This is an intriguing image. There seems to be no exterior crater wall. Perhaps this is due to the low gravity on Mimas, resulting in permanent ejection of all material which was blown out at the impact?
Also, the crater has a steep, smooth wall. Is the reason, that the inside of the crater at some time sank back into the interior, and if so why? Or was the crater left by the impact gradually filled with water from the interior, which froze solid before reaching the surface? But in that case, why is there a central mountain?

[orin stepanek]

You can almost make out something under
the ice; something darker anyway! IMO

[Bird_Man]

It looks like there are two "volcanos" within the crater. Both close to the rim. One at about 5:30 and the other at about 7:30. Were these the result of the impactor melting ice with in Mimas or other forces?

[rstevenson]

This is an intriguing image. There seems to be no exterior crater wall. Perhaps this is due to the low gravity on Mimas, resulting in permanent ejection of all material which was blown out at the impact?
Also, the crater has a steep, smooth wall. Is the reason, that the inside of the crater at some time sank back into the interior, and if so why? Or was the crater left by the impact gradually filled with water from the interior, which froze solid before reaching the surface? But in that case, why is there a central mountain?

I wonder if you’re seeing it in a sort of reverse 3D. I sometimes have a very hard time seeing the correct 3D effect in, for example, some Mars images from orbit, but if I import the image to an editor and rotate it 180* it clicks into place. There is certainly an exterior crater wall, most easily seen at the top of the crater in this image, the silvery white areas. They slope down (physically and in this image) to the crater floor.

[ later edit ] … Ah, I see what you mean. The crater walls don’t seem to have a lip at the top, relative to the surrounding terrain. I think I can see such a lip in a few areas, but not generally or obviously. So, yes, in such a small object I guess the low escape velocity allowed a great deal of excavated material to be ejected away from the impact site, some of it likely landing later all over the poor moon.

Rob