Dione (moon)

Giovanni Cassini

Dione ( Saturn IV) is one of the larger moons of the planet Saturn. Diones having a mean diameter of 1118 km.

Discovery

Dione was discovered on March 21, 1684 by Giovanni Cassini.

It was named after the moon Titan Dione, mother of Aphrodite, from Greek mythology. The name " Dione " and seven other moons of Saturn was suggested by William Herschel's son, the astronomer John Herschel, in a 1847 published paper " Results of Astronomical Observations made ​​at the Cape of Good Hope ."

Web properties

Dione orbits Saturn at an average distance of 377,420 kilometers in 2,737 days. The track has an eccentricity of 0.0022 and 0.02 ° inclined relative to the equatorial plane of the Saturn.

The moon Helene orbits at the same distance to Saturn and Dione runs at an angular distance of 60 ° in the leading Lagrange point, L4 advance. The following L5 Lagrange point of the moon Polydeuces Dione at an angular distance of 60 ° runs behind. Dione is in a 2:1 orbital resonance with Saturn's moon Enceladus.

Dione rotates in 65 hours and 41 minutes around its own axis, thus showing how the Earth's moon, a tidal locking on. The rotational axis is inclined by 0.006 degrees from the vertical.

Structure and physical properties

It is composed mainly of water ice. With 1.47 g/cm3, it has the third highest density of all Saturn's moons on ( surpassed only by Enceladus and Titan, whose density is increased by gravitational compression). Therefore, larger proportions of denser material must be present in its interior, such as silicate rock. Your albedo is 0.55, ie 55 % of the incident sunlight is reflected. Compared to the moons Tethys and Enceladus its surface is relatively dark. The surface temperature is -187 ° C.

Investigations by the Cassini space probe indicate that Dione a layer of liquid material could have below the surface similar to Enceladus. Bending of the crust beneath the mountain range Janiculum Dorsa evidence of an internal heating in astronomical standards recently.

Surface

Dione is similar to Saturn 's moon Rhea in composition, albedo, and the structures of their surface. Both moons have different hemispheres. On the trailing hemisphere of Dione a network brighter stripes on a dark surface and impact craters are visible. The strips overlay the craters, indicating that they are of recent origin. The leading hemisphere is heavily cratered and shows no major differences in brightness. The origin of the bright stripes material is not fully understood. It could come from eruptions along fissures and cracks and have been deposited as a thin deposit of ice or dust.

Craters and mountains

Some ( larger ) impact craters have central mountains, as they are typical of the Earth's moon or the planet Mercury. Obviously, the thin crust of ice Dione has yielded over geological periods not to the extent as on Jupiter's moon Callisto, where such structures were almost completely leveled.

It is believed that Dione was geologically active in the early days of their creation. Processes like the Kryovulkanismus ( Kältevulkanismus ) a portion of its surface was renewed, with the observed fringe pattern remained. Following the decline of the activity, the leading hemisphere by impacts of asteroids has been changed, whose crater largely wiped out the stripe pattern.

On Dione's surface are heavily cratered regions and plains with few craters visible. The first-mentioned regions have a number of craters with diameters of more than 100 km, the layers are covered with craters of less than 30 km in diameter. Some of the levels are very heavily cratered, others barely. Here are heavily cratered regions on the following, slightly cratered plains on the leading hemisphere visible. That's actually exactly the opposite of what scientists had expected. The astronomer and planetary geologist Shoemaker had developed a model that describes the cratering of moons with bound rotation. Thus, most craters are on the leading, to find the least craters on the trailing hemisphere of a moon. In the case of Dione the Moon during the most violent phase of the asteroid bombardment seems to have been oriented exactly the opposite. Because Dione is relatively small, may already impact event, that leaves a 35 km wide crater, disturb the rotation of the Moon. Since Dione has a plurality of such large craters, the rotation could have been repeatedly disturbed during the period of intense bombardment. The distribution of the craters and the high albedo of the leading hemisphere indicate that the moon has retained the current focus over the last several billion years.

Atmosphere

Dione has an atmosphere of ionized oxygen atoms, which is so thin with 90,000 ions per cubic meter as the Earth's atmosphere in 480 km altitude.

Observation

Dione has an apparent magnitude of 10.4 m and is therefore, seen from Earth, one of the brightest moons of Saturn. To watch them, however, you need a telescope with a 10 cm lens opening.

On 14 December 2004, the space probe Cassini -Huygens Dione has happened for the first time at a distance of 83,000 km. On 11 October 2005 an even greater rapprochement took place with only 500 kilometers distance.

On December 12, 2011, Cassini flew by in only 99 km Distance to Dione. Cassini's next encounter with Dione 28 March 2012 was carried out at a distance of 44,000 km. For the following studies of the moon is hoped that further insights into the composition and geological processes.

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