APOD: Mimas, Crater, and Mountain (2017 Jan 11)

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APOD: Mimas, Crater, and Mountain (2017 Jan 11)

Postby APOD Robot » Wed Jan 11, 2017 5:27 am

[img]http://apod.nasa.gov/apod/calendar/S_170111.jpg[/img] Mimas, Crater, and Mountain

Explanation: Mimas is an icy, crater-pocked moon of Saturn a mere 400 kilometers (250 miles) in diameter. Its largest crater Herschel is nearly 140 kilometers wide. About a third the diameter of Mimas itself, Herschel crater gives the small moon an ominous appearance, especially for scifi fans of the Death Star battlestation of Star Wars fame. In fact, only a slightly bigger impact than the one that created such a large crater on a small moon could have destroyed Mimas entirely. In this Cassini image from October 2016, the anti-Saturn hemisphere of the synchronously rotating moon is bathed in sunlight, its large crater near the right limb. Casting a long shadow across the crater floor, Herschel's central mountain peak is nearly as tall as Mount Everest on planet Earth.

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Re: APOD: Mimas, Crater, and Mountain (2017 Jan 11)

Postby gadieid » Wed Jan 11, 2017 6:14 am

I am glad it is a moon of Saturn and not Alde(ba)ran ...
May the force be with us
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Re: APOD: Mimas, Crater, and Mountain (2017 Jan 11)

Postby Guest » Wed Jan 11, 2017 7:54 am

Herschel's central mountain peak is nearly as tall as Mount Everest on planet Earth.


Well, at the gravity of that little sphere, climbing the mountain would be easier, but you still have to pack your oxygen (and lunch).

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Re: APOD: Mimas, Crater, and Mountain (2017 Jan 11)

Postby starsurfer » Wed Jan 11, 2017 2:01 pm

Click to play embedded YouTube video.


Fun fact: Mimas was discovered in 1978! :D

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Re: APOD: Mimas, Crater, and Mountain (2017 Jan 11)

Postby neufer » Wed Jan 11, 2017 3:02 pm

https://www.nasa.gov/mission_pages/cass ... 16198.html wrote:
Click to play embedded YouTube video.
    A Pair of 'Pac-Men'
<<Scientists with NASA's Cassini mission have spotted two features shaped like the 1980s video game icon "Pac-Man" on moons of Saturn. One was observed on the moon Mimas in 2010 and the latest was observed on the moon Tethys. The pattern appears in thermal data obtained by Cassini's composite infrared spectrometer, with warmer areas making up the Pac-Man shape. At Tethys, unlike Mimas, the Pac-Man pattern can also be seen subtly in visible-light images of the surface, as a dark lens-shaped region.

Scientists theorize that the Pac-Man thermal shape on the Saturnian moons occurs because high-energy electrons bombard low latitudes on the side of the moon that faces forward as it orbits around Saturn, turning a fluffy surface into hard-packed ice. As a result, the altered surface does not heat as rapidly in the sunshine or cool down as quickly at night as the rest of the surface, similar to how a boardwalk at the beach feels cooler during the day but warmer at night than the nearby sand. Finding another PacMan on Tethys confirms that high-energy electrons can dramatically alter the surface of an icy moon. Also, because the altered region on Tethys, unlike on Mimas, is also bombarded by icy particles from Enceladus' plumes, it implies the surface alteration is occurring more quickly than its recoating by plume particles.>>
Art Neuendorffer

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The seventh satellite of a seventh 'satellite'

Postby neufer » Wed Jan 11, 2017 3:10 pm

https://en.wikipedia.org/wiki/Mimas_(moon) wrote:
<<With a diameter of 396 kilometres it is the smallest astronomical body that is known to be rounded in shape because of self-gravitation. [However,] due to the tidal forces acting on it, Mimas is noticeably prolate; its longest axis is about 10% longer than the shortest.

Mimas is responsible for clearing the material from the Cassini Division, the gap between Saturn's two widest rings, the A Ring and B Ring. Particles in the Huygens Gap at the inner edge of the Cassini division are in a 2:1 resonance with Mimas. They orbit twice for each orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap. The boundary between the C and B ring is in a 3:1 resonance with Mimas. Recently, the G Ring was found to be in a 7:6 co-rotation eccentricity resonance with Mimas; the ring's inner edge is about 15,000 km inside Mimas's orbit. Mimas is also in a 2:1 mean-motion resonance with the larger moon Tethys, and in a 2:3 resonance with the outer F Ring shepherd moonlet, Pandora.

Mimas was discovered by William Herschel on 17 September 1789. He recorded his discovery as follows: "The great light of my forty-foot telescope was so useful that on the 17th of September, 1789, I remarked the seventh satellite, then situated at its greatest western elongation." The names of all seven then-known satellites of Saturn, including Mimas, were suggested by William Herschel's son John in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope. He named them after Titans specifically because Saturn (the Roman equivalent of Cronus in Greek mythology), was the leader of the Titans and ruler of the world for some time. Mimas's most distinctive feature is a giant impact crater 130 km across, named Herschel. Herschel's diameter is almost a third of Mimas's own diameter; its walls are approximately 5 km high, parts of its floor measure 10 km deep, and its central peak rises 6 km above the crater floor. When seen from certain angles, Mimas resembles the Death Star, a fictional space station known from the 1977 film Star Wars Episode IV: A New Hope, which is said to be roughly 140 km in diameter. Herschel resembles the concave disc of the Death Star's "superlaser". This is coincidental, as the film was made nearly three years before Mimas was resolved well enough to see the crater.>>
https://en.wikipedia.org/wiki/Mimas_(Giant) wrote:
<<In Greek mythology, Mimas was one of the Gigantes (Giants) killed, during the Gigantomachy, the cosmic battle of the Giants with the Olympian gods, by Hephaestus with "missiles of red-hot metal" from his forge. For the Greeks, the Gigantomachy represented a victory for order over chaos—the victory of the divine order and rationalism of the Olympian gods over the discord and excessive violence of the earth-born chthonic Giants. More specifically, for sixth and fifth century BC Greeks, it represented a victory for civilization over barbarism. The attempt of the Giants to overthrow the Olympians also represented the ultimate example of hubris, with the gods themselves punishing the Giants for their arrogant challenge to the gods' divine authority. Plato compares the Gigantomachy to a philosophical dispute about existence, wherein the materialist philosophers, who believe that only physical things exist, like the Giants, wish to "drag down everything from heaven and the invisible to earth".>>
Art Neuendorffer

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Re: APOD: Mimas, Crater, and Mountain (2017 Jan 11)

Postby julianm3 » Wed Jan 11, 2017 5:42 pm

That's no moon . . .

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Re: APOD: Mimas, Crater, and Mountain (2017 Jan 11)

Postby MarkBour » Wed Jan 11, 2017 9:53 pm

I see lots of bright parts on the surface of Mimas in this image. Are they comparable to the spots on Ceres?
(I can never tell if the processing has rendered a faint thing bright, or vice versa.)
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Re: APOD: Mimas, Crater, and Mountain (2017 Jan 11)

Postby Ann » Wed Jan 11, 2017 11:42 pm

An interesting aspect of Mimas is that it is right at the borderline between bodies that are massive enough to become spherical due to their own self-gravity, and those that are not massive enough for that. Or so I think, anyway. If true, it would mean that there are very few bodies in the solar system that are more lightweight than Mimas and still spherical.

An interesting case is the minor planet Haumea in the Kuiper Belt, which is, or so I think, the most elongated body in the solar system. How did it get that way, and is it more massive than Mimas? I guess the answer to the second question is yes.

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Re: APOD: Mimas, Crater, and Mountain (2017 Jan 11)

Postby neufer » Thu Jan 12, 2017 4:09 am

Ann wrote:
An interesting case is the minor planet Haumea in the Kuiper Belt, which is, or so I think, the most elongated body in the solar system. How did it get that way, and is it more massive than Mimas? I guess the answer to the second question is yes.

Mimas orbits Saturn in about 23 hours at a little over 3 Saturn radii. Tidal forces plus centrifugal forces combine to squeeze Mimas into a prolate spheroid in the radial direction). And Mimas would probably be pulled apart if it got anywhere near 1.5 Saturn radii (= orbital period ~ 8 hours).

Haumea, OTOH, is a freely rotating prolate dwarf planet with a very short rotational period of ~4 hours. Haumea is rotating too fast (period <5 hours ?) to be an oblate spheroid (thanks to symmetry breaking from oblate to very prolate). If its rotation period got to be less than three hours Haumea too would be pulled apart ... but by centrifugal forces alone.

The situations are similar but different. (E.g., a freely rotating dwarf planet probably cannot be just slightly prolate like the moon Mimas. Freely rotating dwarf planets are either oblate spheroids or extremely prolate spheroids.)
Art Neuendorffer


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