Mimas: Overview

In this view captured by NASA's Cassini spacecraft on 13 February 2010, Herschel Crater dominates Mimas, making the moon look like the Death Star in the movie "Star Wars."

In this view captured by NASA's Cassini spacecraft on 13 February 2010, Herschel Crater dominates Mimas, making the moon look like the Death Star in the movie "Star Wars."


Herschel Crater is 130 km, or 80 miles, wide and covers most of the right of this image. Scientists continue to study this impact basin and its surrounding terrain (see PIA12569 and PIA12571).


Cassini came within about 9,500 km (5,900 miles) of Mimas on 13 February 2010.This mosaic was created from six images taken that day in visible light with Cassini's narrow-angle camera. The images were re-projected into an orthographic map projection. This view looks toward the area between the region that leads on Mimas' orbit around Saturn and the region of the moon facing away from Saturn. Mimas is 396 km (246 miles) across. This view is centered on terrain at 11 degrees south latitude, 158 degrees west longitude. North is up. This view was obtained at a distance of approximately 50,000 km (31,000 miles) from Mimas and at a sun-Mimas-spacecraft, or phase, angle of 17 degrees. Image scale is 240 m (790 feet) per pixel.


The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

Less than 123 miles (198 km) in mean radius, crater-covered Mimas is the smallest and innermost of Saturn's major moons. It is not quite big enough to hold a round shape, so it is somewhat ovoid with dimensions of 129 x 122 x 119 (miles 207 x 197 x 191 km, respectively). Its low density suggests that it consists almost entirely of water ice, which is the only substance ever detected on Mimas.

At a mean distance just over 115,000 miles (186,000 km) from the massive planet, Mimas takes only 22 hours and 36 minutes to complete an orbit. Mimas is tidally locked: it keeps the same face toward Saturn as it flies around the planet, just as our Moon does with Earth.

Surface Features

Most of the Mimas surface is saturated with impact craters ranging in size up to greater than 25 miles (40 km) in diameter. However, the craters in the South Pole region of Mimas are generally 12.4 miles (20 km) in diameter or less. This suggests that some melting or other resurfacing processes occurred there later than on the rest of the moon. (Interestingly, the South Pole area of Enceladus appears to be the source of that moon's geysers.)

Its most distinguishing feature is a giant impact crater -- named Herschel after the moon's discoverer -- which stretches a third of the way across the face of the moon, making it look like the Death Star from "Star Wars." The Herschel Crater is 80 miles (130 km) across -- one third of the diameter of the moon itself -- with outer walls about 3 miles (5 km) high and a central peak 3.5 miles (6 km) high. The impact that blasted this crater out of Mimas probably came close to breaking the moon apart. Shock waves from the Herschel impact may have caused the fractures, also called chasmata, on the opposite side of Mimas.

That Mimas appears to be frozen solid is puzzling because Mimas is closer to Saturn and has a much more eccentric (elongated) orbit than Enceladus, which should mean that Mimas has more tidal heating than Enceladus. Yet Enceladus displays geysers of water, which implies internal heat, while Mimas has one of the most heavily cratered surfaces in the solar system, which suggests a frozen surface that has persisted for enough time to preserve all those craters. This paradox has prompted the "Mimas Test" by which any theory that claims to explain the partially thawed water of Enceladus must also explain the entirely frozen water of Mimas.

Orbit

Mimas orbits Saturn exactly twice as often as the more distant moon, Tethys, a phenomenon known as "orbital resonance." Similar orbital resonances between Mimas and parts of Saturn's rings are thought to be responsible for the Huygens gap, which marks the boundary between the B Ring and the Cassini Division, and for several density waves within the A Ring. In addition, Mimas' slight inclination (1.574 degrees with respect to the ring plane) gives rise to several vertical bending waves within the A Ring.

Mimas is in resonance with two nearby moons, Dione and Enceladus. That is, these moons speed up slightly as they approach each other and slow down as they draw away, causing their orbits to vary slightly in a long series of complex changes, which help keep them locked in their positions.

Mimas strongly perturbs the tiny 2-mile (3-km) diameter moon Methone, the 3-mile (4-km) diameter moon Pallene, and the 1-mile (2-km) diameter moon Anthe, all of which orbit between Mimas and the next major moon going out from Saturn, Enceladus. The vastly more massive Mimas causes the Methone orbit to vary by as much as 12.4 miles (20 km). The perturbations are larger for tiny Anthe, and slightly smaller for Pallene.

Discovery

Mimas was discovered on 17 September 1789 by English astronomer William Herschel, using his 40-foot reflector telescope.

Ground-based astronomers could only see Mimas as little more than a dot until Voyagers I and II imaged it in 1980. The Cassini spacecraft has made several close approaches and provided detailed images of Mimas since Cassini achieved orbit around Saturn in 2004.

How Mimas Got its Name

The mythological Mimas was a giant who was killed by Mars in the war between the Titans and the gods of Olympus. Even after his death, Mimas' legs -- which were serpents -- hissed vengeance and sought to attack his killer.

Mimas was named by John Herschel, the son of discoverer William Herschel, who explained his choice of names for the first seven of Saturn's moons to be discovered by writing, "As Saturn devoured his children, his family could not be assembled round him, so that the choice lay among his brothers and sisters, the Titans and Titanesses."

Astronomers also refer to Mimas as "Saturn I" based on its distance being the closest to Saturn. The International Astronomical Union now controls the official naming of astronomical bodies.