Dione (moon)
| Dione | |
|---|---|
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| Dione, mosaic image of the Cassini space probe , taken on October 11, 2005 during the probe's closest flyby to date; Distance in the range from 27,180 to 55,280 km | |
| Central body | Saturn |
| Properties of the orbit | |
| Major semi-axis | 377,420 km |
| Periapsis | 376,580 km |
| Apoapsis | 378,260 km |
| eccentricity | 0.00223 |
| Orbit inclination | 0.02 ° |
| Orbital time | 2.737 d |
| Mean orbital velocity | 10.03 km / s |
| Physical Properties | |
| Albedo | 0.55 |
| Apparent brightness | 10.4 mag |
| Medium diameter | (1123.4 ± 0.9) 1127.6 × 1122 × 1120.6 km |
| Dimensions | 1.096 x 10 21 kg |
| surface | 3.93 million km 2 |
| Medium density | (1.476 ± 0.004) g / cm 3 |
| Sidereal rotation | 2.737 d |
| Axis inclination | 0.006 ° |
| Acceleration of gravity on the surface | 0.22 m / s 2 |
| Escape speed | 512 m / s |
| Surface temperature | 87 K |
| discovery | |
| Explorer | |
| Date of discovery | March 21, 1684 |
| Remarks | Extremely thin atmosphere of molecular, ionized oxygen; simply bound rotation |
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| Size comparison between Dione (bottom left), Earth's moon (top left) and Earth (true-to-scale photo montage) | |
Dione (also Saturn IV ) is one of the larger moons of the planet Saturn . It is an ice moon and has an average diameter of 1118 km.
discovery
Dione was discovered by Giovanni Domenico Cassini on March 21, 1684 .
The moon was named after the titan Dione , mother of Aphrodite , from Greek mythology . The name "Dione" and seven other Saturn moons were suggested by Wilhelm Herschel's son, the astronomer John Herschel , in a publication published in 1847, "Results of Astronomical Observations made at the Cape of Good Hope " .
Track properties
Dione orbits Saturn at a mean distance of 377,420 km in 2.737 days. The orbit has an eccentricity of 0.0022 and is inclined 0.02 ° to the equatorial plane of Saturn.
The moon Helene orbits Saturn at the same distance and leads Dione at an angular distance of 60 ° in the leading Lagrangian point , L 4 . In the following Lagrangian point L 5 , the moon Polydeuces Dione follows at an angle of 60 °. Dione is in a 2: 1 orbit resonance with Saturn's moon Enceladus .
Dione rotates around its own axis in 65 hours and 41 minutes and thus, like the earth's moon , exhibits a bound rotation . The axis of rotation is inclined 0.006 ° from the vertical.
Structure and physical properties
It is mainly composed of water ice . With 1.47 g / cm 3 it has the fourth highest density of all Saturn's moons (surpassed only by Enceladus, Phoebe and Titan , whose density is increased by gravitational compression). There must therefore be a larger proportion of denser material inside, such as silicate rock. Their albedo is 0.55, which means that 55% of the incident sunlight is reflected. Compared to the moons Tethys and Enceladus, their surface is relatively dark. The surface temperature is −187 ° C.
Research by the Cassini spacecraft suggests that Dione, similar to Enceladus, may have a layer of liquid material beneath the surface. Bends in the crust beneath the Janiculum Dorsa mountain range are evidence of internal warming in what was, for astronomical standards, more recent.
surface
Dione has an area of 3.93 million km², which is roughly twice the size of Mexico or eleven times the size of Germany. Dione resembles the Saturn moon Rhea in its composition, albedo and the structure of its surface . Both moons have different hemispheres . On the following hemisphere of Dione, a network of bright stripes on a dark surface and impact craters are visible. The stripes cover the craters, indicating that they are of more recent origin. The leading hemisphere is heavily cratered and does not show any major differences in brightness. The origin of the light-colored strip material is not entirely clear. It could have originated from eruptions along crevices and cracks and deposited as thin deposits of ice or dust.
Craters and mountains
Some (also larger) impact craters have central mountains , as they are typical for the Earth's moon or the planet Mercury . Obviously, the thin ice crust of Dione has not yielded over geological time to the same 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 its formation. Through processes such as cryovolcanism (cold volcanism), part of its surface was renewed, whereby the observed stripe patterns remained. After the decline in activity, the leading hemisphere was altered by impacts from asteroids , the craters of which largely obliterated the stripe patterns.
Heavily cratered regions and plains with only a few craters are visible on Dione's surface. The former regions have numerous craters with diameters of more than 100 km, the plains are covered with craters of less than 30 km in diameter. Some of the levels are very badly cratered, others hardly. Strongly cratered regions are visible on the following, weakly cratered levels on the leading hemisphere. This is actually exactly the opposite of what the scientists expected. The astronomer and planetary geologist Shoemaker had developed a model that describes the cratering of moons with bound rotation. According to this, most of the craters are to be found on the leading hemisphere, the fewest on the following hemisphere. In the case of Dione, the moon appears to have been oriented exactly in the opposite direction during the phase of the most violent asteroid bombardment. Since Dione is relatively small, an impact event that leaves a 35 km crater can disrupt the rotation of the moon. Since Dione has a large number of such large craters, the rotation could have been disrupted repeatedly during the phase of the intense bombardment. The distribution of the craters and the high albedo in the leading hemisphere indicate that the moon has maintained its current orientation for billions of years.
By far the largest named crater called Evander has a diameter of 350 km and is located in the south of the part of the leading hemisphere facing away from Saturn.
the atmosphere
Dione has an atmosphere of ionized oxygen atoms that, at 90,000 ions per cubic meter, is as thin as the earth's atmosphere at an altitude of 480 km .
ocean
Similar to Enceladus , Dione could have a liquid ocean beneath its thick layer of ice, as indicated by magnetic measurements from the Cassini space probe . Another indication of liquid water is the equatorial mountain range Janiculum Dorsa , the weight of which lowers the crust of the moon by up to 500 meters. The ice crust must therefore have been warmer earlier, which can best be explained with the assumption of a subglacial ocean (at the time the mountain range was formed).
observation
Dione has an apparent brightness of 10.4 m and is therefore one of the brightest moons of Saturn when viewed from Earth. To observe them, however, you need a telescope with a lens opening of at least 10 cm.
The Cassini-Huygens space probe first passed Dione on December 14, 2004 at a distance of 83,000 km. An even closer convergence took place on October 11, 2005 with a distance of only 500 kilometers. On December 12, 2011, Cassini passed Dione just 99 km away. Cassini's next encounter with the moon took place on March 28, 2012 at a distance of approximately 44,000 km. Cassini's fifth and final meeting with Dione was on August 17, 2015, with an approximation to within 474 kilometers. More precise data on the North Pole and the internal composition and structure of the moon should be obtained.
Web links
- Current Cassini recordings from Dione a picture gallery of the field of planetology and remote sensing at the Free University of Berlin under the direction of Cassini team member Gerhard Neukum
- Voyager 1 and 2 - Atlas of Six Saturnian Satellites NASA maps of six Saturn moons based on images from the Voyager probes
- Ralph-Mirco Richter: Does the moon Dione harbor an underground ocean? in Raumfahrer.net
- Dione @ nasa.gov
- astronews.com: Picture of the day February 3, 2017 , February 22, 2017 , March 14, 2018
Individual evidence
- ^ PC Thomas, Veverka, J .; Helfenstein, P .; Porco, C .; Burns, J .; Thinks.; Turtle, E .; Jacobson, RA : Shapes of the Saturnian Icy Satellites . (PDF) In: 37th Annual Lunar and Planetary Science Conference . .
- ^ Cassini: An Extract of the Journal Des Scavans. of April 22 st. N. 1686. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory at Paris. Phil. Trans. 1686 16: 79-85; doi : 10.1098 / rstl.1686.0013 ( full text )
- ↑ Dione. nineplanets, March 15, 2007, accessed on October 24, 2016 (German).
- ↑ a b Hidden ocean also on Saturn's moon Dione? on astronews.com
- ↑ Dione. (No longer available online.) Saturn moons, archived from the original on January 30, 2010 ; Retrieved February 8, 2010 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Calvin J. Hamilton .: Dione. Views of the Solar System, accessed February 8, 2010 .
- ↑ Shoemaker, EM; and Wolfe, RF; Cratering time scales for the Galilean satellites , in Morrison, D., editor; Satellites of Jupiter , University of Arizona Press , Tucson (AZ) (1982), pp. 277-339, bibcode : 1982stjp.conf..277S
- ^ Dione in the Gazetteer of Planetary Nomenclature of the IAU (WGPSN) / USGS ; last accessed on September 25, 2016.
- Jump up ↑ Jia-Rui C. Cook: Cassini Detects Hint of Fresh Air at Dione , Jet Propulsion Laboratory, Date: March 2, 2012, Accessed: March 4, 2012
- ^ RL Tokar, RE Johnson, MF Thomsen, et alt .: Detection of exospheric O2 + at Saturn's moon Dione. In: Geophysical Research Lettwers. (Geophys. Res. Lett.) Vol. 39, 2012, L03105, p. 7, doi : 10.1029 / 2011GL050452 , full text online .
- ↑ Ralph-Mirko Richter: Cassini discovers thin atmosphere around the moon Dione. In: Raumfahrer.net, March 4, 2012, accessed March 7, 2012
- ↑ jme / dpa: "Cassini" space probe. Last rendezvous with Saturn's moon Dione. Spiegel.de, August 18, 2015
| further inside | Saturn moons | further outside |
| Calypso |
Semi- major axis (km) Dione 377.400 |
Helene |