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An impact crater is a depression in the surface of a solid astronomical body formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters are typically circular, though they can be elliptical in shape or even irregular due to events such as landslides. Impact craters range in size from microscopic craters seen on lunar rocks returned by the Apollo Program to simple bowl-shaped depressions and vast, complex, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth.
Tethys, or Saturn III, is the fifth-largest moon of Saturn, measuring about 1,060 km (660 mi) across. It was discovered by Giovanni Domenico Cassini in 1684, and is named after the titan Tethys of Greek mythology.
Iapetus is the outermost of Saturn's large moons. With an estimated diameter of 1,469 km (913 mi), it is the third-largest moon of Saturn and the eleventh-largest in the Solar System. Named after the Titan Iapetus, the moon was discovered in 1671 by Giovanni Domenico Cassini.
Hyperion, also known as Saturn VII, is the eighth-largest moon of Saturn. It is distinguished by its highly irregular shape, chaotic rotation, low density, and its unusual sponge-like appearance. It was the first non-rounded moon to be discovered.
Phoebe is the most massive irregular satellite of Saturn with a mean diameter of 213 km (132 mi). It was discovered by William Henry Pickering on 18 March 1899 from photographic plates that had been taken by DeLisle Stewart starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru. It was the first natural satellite to be discovered photographically.
The moons of Saturn are numerous and diverse, ranging from tiny moonlets only tens of meters across to the enormous Titan, which is larger than the planet Mercury. There are 146 moons with confirmed orbits, the most of any planet in the solar system. This number does not include the many thousands of moonlets embedded within Saturn's dense rings, nor hundreds of possible kilometer-sized distant moons that have been observed on single occasions. Seven Saturnian moons are large enough to have collapsed into a relaxed, ellipsoidal shape, though only one or two of those, Titan and possibly Rhea, are currently in hydrostatic equilibrium. Three moons are particularly notable. Titan is the second-largest moon in the Solar System, with a nitrogen-rich Earth-like atmosphere and a landscape featuring river networks and hydrocarbon lakes. Enceladus emits jets of ice from its south-polar region and is covered in a deep layer of snow. Iapetus has contrasting black and white hemispheres as well as an extensive ridge of equatorial mountains among the tallest in the solar system.
Cassini Regio is the enigmatic dark area that covers the leading half of Saturn's moon Iapetus. It is named after Giovanni Cassini, the discoverer of Iapetus; 'Regio' is a term used in planetary geology for a large area that is strongly differentiated in colour or albedo from its surroundings. The brighter half of Iapetus is composed of the Roncevaux Terra and the Saragossa Terra.
Roncevaux Terra is the name given to the northern part of the highly reflective side of Saturn's moon Iapetus. The southern half of this side is named Saragossa Terra. The other half of Iapetus, named Cassini Regio, is extremely dark. It is believed that Roncevaux Terra's colour is the underlying colour of Iapetus, while Cassini Regio was formed either by a substance that has covered up the brighter ice of the rest of the moon, or by a residue left from the sublimation of Roncevaux-type water ice.
Turgis is the largest known crater on Saturn's moon Iapetus. It is 580 km in diameter, 40% of the moon's diameter and one of the larger craters in the Solar System. It is named after a Saracen baron, Turgis of Turtelose (Tortosa).
Columbus is a crater in the Terra Sirenum of Mars. It is 119 km in diameter and was named after Christopher Columbus, Italian explorer (1451–1506). The discovery of sulfates and clay minerals in sediments within Columbus crater are strong evidence that a lake once existed in the crater. Research with an orbiting near-infrared spectrometer, which reveals the types of minerals present based on the wavelengths of light they absorb, found evidence of layers of both clay and sulfates in Columbus crater. This is exactly what would appear if a large lake had slowly evaporated. Moreover, because some layers contained gypsum, a sulfate which forms in relatively fresh water, life could have formed in the crater.
The equatorial ridge is the tallest mountain feature on Saturn's moon Iapetus. It is 20 km (12 mi) high, and is the third tallest mountain structure in the Solar System. It runs along most of Iapetus' equator. It was discovered by the Cassini probe in 2004. The ridge's origin is unknown. There are bright areas on the sides of the equatorial ridge near Iapetus' bright trailing hemisphere, which were already visible in Voyager 2 images appearing like mountains and were nicknamed the "Voyager Mountains".
Engelier is a 310-mile (500-kilometers) large crater on Saturn's moon Iapetus in Saragossa Terra. It partially obscures the slightly smaller crater Gerin.
Gerin is 445 km wide impact crater on Saturn's moon Iapetus, located on Saragossa Terra. It is partially obscured by the larger crater, Engelier, whose formation has destroyed about half of Gerin. Gerin is located at 45.6°S 233°W
Abisme /æˈbiːm/ is a large impact basin located in Cassini Regio at 37.5°N, 267.1°E. Craters Climborin, Clarin, Dapamort, Johun and Valdebron can be found inside Abisme. It was imaged for the first time by the Cassini spacecraft in 2004.
Roland is a 144 km (89-mile)-wide crater near the north pole of Iapetus, a moon of the planet Saturn. It lies inside one of the bright areas on Iapetus called Roncevaux Terra. Images of Roland shows that it has a central peak.
References
- ↑ "Planetary Names: terra, terrae: Saragossa Terra on Iapetus". United States Geological Survey. August 5, 2008. Retrieved August 7, 2015.
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