7th Largest Moon of Saturn - Mimas (9th Moon outwards from Saturn)

Mimas, is the seventh largest moon of Saturn, and 20th largest moon in the Solar System.

Mimas is the smallest known astronomical body that is rounded in shape due to self-gravitation.



Discovery

Mimas was discovered by the astronomer Fredrick William Herschel on 17 September 1789.


Naming

Mimas the moon is named after one of the Titans in Greek mythology, Mimas.

Mimas was one of the Gigantes of Greek mythology. Like the other giant sons of Gaia, Mimas had serpents for legs and was born fully armoured. Mimas was slain by Hephaestus during the war against the Olympians by a volley of molten iron.

Stats

Diameter: 396 km

Semi-major axis: 185,404 km

Orbital Period: 0.942 days

Orbit

A number of features in Saturn's rings are related to resonances with Mimas. 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 kilometres inside Mimas' 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 rotates synchronously with its orbital period, keeping one face pointed toward Saturn.

Physical characteristics

The low density of Mimas, 1.15 g/cm3, indicates that it is composed mostly of water ice with only a small amount of rock.

Mimas' most distinctive feature is a colossal impact crater 130 kilometres across, named Herschel after the moon's discoverer. Herschel's diameter is almost a third of the moon's own diameter; its walls are approximately 5 kilometres high, parts of its floor measure 10 kilometres deep, and its central peak rises 6 kilometres above the crater floor.

The impact that made this crater must have nearly shattered Mimas: fractures can be seen on the opposite side of Mimas that may have been created by shock waves from the impact travelling through the moon's body.

The Mimantean surface is saturated with smaller impact craters, but no others are anywhere near the size of Herschel. Although Mimas is heavily cratered, the cratering is not uniform. Most of the surface is covered with craters greater than 40 kilometres in diameter, but in the south polar region, craters greater than 20 kilometres are generally lacking.

Curious Resemblances

When seen from certain angles, Mimas closely resembles the Death Star, a fictional space station known from the film Star Wars Episode IV: A New Hope, which is said to be roughly 140 kilometres in diameter. This resemblance stems from the fact that Herschel can appear in Mimas' northern hemisphere, much like the concave disc of the Death Star's "superlaser".

This is purely coincidental, as the first film was made three years before the first close-up photographs of Mimas were taken.