Callisto, Moon

Callisto

Union World - Jupiter Moon - Sol System

has been colonized since the the early 22.nd century and has been a major world of the Sol System. It has been the Fleet Headquarter of the United Earth Fleet and became Fleet Head Quarters for the 2nd Fleet after the founding of the Union. It is still a Fleet base for the 2nd Fleet and also HQ of  UMBRELLA ( Sol Defense Force). As all Sol System Citizens must serve 24 Month in the SDF, Callisto is always a very busy place with millions of SDF Draftees performing basic training, While only about 4 million call Callisto home, there ar close to 100 Million there every day. Callisto is represented by an appointed Representative and governed by a Military Govaneur (appointed by the Sol System Council) Laws: Union Laws + Military Laws and Ordinances. No Exports of significance (Other than seewage, Trade and waste, Trade) Imports: Groceries

Callisto  /k ə ˈl ɪst oʊ/[8 ] (Jupiter IV) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1,880,000 km.[2 ] It does not form part of the orbital resonance that affects three inner Galilean satellites—Io, Europa and Ganymede—and thus does not experience appreciable tidal heating.[9 ] Callisto's rotation is tidally locked to its revolution around Jupiter, so that the same hemisphere always faces inward; Jupiter appears to stand still in Callisto's sky. Callisto is less affected by Jupiter's magnetosphere than the other inner satellites because it orbits farther away.[10 ]

Callisto is composed of approximately equal amounts of rock and ices, with a mean density of about 1.83 g/cm3. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates, and organic compounds. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than 100 km.[11 ] [12 ]

The surface of Callisto is heavily cratered and extremely old. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, and is thought to have evolved predominantly under the influence of impacts.[13 ] Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges and deposits.[13 ] At a small scale, the surface is varied and consists of small, bright frost deposits at the tops of elevations, surrounded by a low-lying, smooth blanket of dark material.[4 ] This is thought to result from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants.[14 ] The absolute ages of the landforms are not known.

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide[6 ] and probably molecular oxygen,[7 ] as well as by a rather intense ionosphere.<sup class="reference" id="cite_ref-Kliore_2002_15-0">[15 ] Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation.<sup class="reference" id="cite_ref-Canup2002_16-0">[16 ] Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation. The slow convection in the interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core.<sup class="reference" id="cite_ref-Spohn_2003_17-0">[17 ]

The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa.<sup class="reference" id="cite_ref-Lipps2004_18-0">[18 ] Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied the moon. Because of its low radiation levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system.<sup class="reference" id="cite_ref-HOPE_19-0">[19

SURFACE

Surface features
See also: List of geological features on CallistoGalileo image of cratered plains, illustrating the pervasive local smoothing of Callisto's surfaceThe ancient surface of Callisto is one of the most heavily cratered in the Solar System.<sup class="reference" id="cite_ref-Zahnle_1998_35-0">[35] In fact, the crater density is close to saturation: any new crater will tend to erase an older one. The large-scale geology is relatively simple; there are no large Callistoan mountains, volcanoes or other endogenic tectonic features.<sup class="reference" id="cite_ref-Bender_1997_36-0">[36] The impact craters and multi-ring structures—together with associated fractures, scarps and deposits—are the only large features to be found on the surface.<sup class="reference" id="cite_ref-Greeley_2000_13-4">[13] <sup class="reference" id="cite_ref-Bender_1997_36-1">[36]

Callisto's surface can be divided into several geologically different parts: cratered plains, light plains, bright and dark smooth plains, and various units associated with particular multi-ring structures and impact craters.<sup class="reference" id="cite_ref-Greeley_2000_13-5">[13] <sup class="reference" id="cite_ref-Bender_1997_36-2">[36] The cratered plains constitute most of the surface area and represent the ancient lithosphere, a mixture of ice and rocky material. The light plains include bright impact craters like Burr and Lofn, as well as the effaced remnants of old large craters called palimpsests,<sup class="reference" id="ref_Inone">[i] the central parts of multi-ring structures, and isolated patches in the cratered plains.<sup class="reference" id="cite_ref-Greeley_2000_13-6">[13] These light plains are thought to be icy impact deposits. The bright, smooth plains constitute a small fraction of the Callistoan surface and are found in the ridge and trough zones of the Valhalla and Asgard formations and as isolated spots in the cratered plains. They were believed to be connected with endogenic activity, but the high-resolution Galileo images showed that the bright, smooth plains correlate with heavily fractured and knobby terrain and do not show any signs of resurfacing.<sup class="reference" id="cite_ref-Greeley_2000_13-7">[13] The Galileo images also revealed small, dark, smooth areas with overall coverage less than 10,000 km2, which appear to embay<sup class="reference" id="ref_Jnone">[j] the surrounding terrain. They are possible cryovolcanic deposits.<sup class="reference" id="cite_ref-Greeley_2000_13-8">[13] Both the light and the various smooth plains are somewhat younger and less cratered than the background cratered plains.<sup class="reference" id="cite_ref-Greeley_2000_13-9">[13] <sup class="reference" id="cite_ref-Wagner_2001_37-0">[37] Impact crater Hár with a central dome. Chains of secondary craters from formation of the more recent crater Tindr at upper right crosscut the terrain.Impact crater diameters seen range from 0.1 km—a limit defined by the imaging resolution—to over 100 km, not counting the multi-ring structures.<sup class="reference" id="cite_ref-Greeley_2000_13-10">[13] Small craters, with diameters less than 5 km, have simple bowl or flat-floored shapes. Those 5–40 km across usually have a central peak. Larger impact features, with diameters in the range 25–100 km, have central pits instead of peaks, such as Tindr crater.<sup class="reference" id="cite_ref-Greeley_2000_13-11">[13] The largest craters with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact;<sup class="reference" id="cite_ref-Greeley_2000_13-12">[13] examples include Doh and Hár craters. A small number of very large—more 100 km in diameter—and bright impact craters show anomalous dome geometry. These are unusually shallow and may be a transitional landform to the multi-ring structures, as with the Lofn impact feature.<sup class="reference" id="cite_ref-Greeley_2000_13-13">[13] Callistoan craters are generally shallower than those on the Moon. Voyager 1 image of Valhalla, a multi-ring impact structure 3800 km in diameterThe largest impact features on the Callistoan surface are multi-ring basins.<sup class="reference" id="cite_ref-Greeley_2000_13-14">[13] <sup class="reference" id="cite_ref-Bender_1997_36-3">[36] Two are enormous. Valhalla is the largest, with a bright central region 600 kilometers in diameter, and rings extending as far as 1,800 kilometers from the center (see figure).<sup class="reference" id="cite_ref-Map_2002_38-0">[38] The second largest is Asgard, measuring about 1,600 kilometers in diameter.<sup class="reference" id="cite_ref-Map_2002_38-1">[38] Multi-ring structures probably originated as a result of a post-impact concentric fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an ocean.<sup class="reference" id="cite_ref-Klemaszewski2001_23-1">[23] The catenae—for example Gomul Catena—are long chains of impact craters lined up in straight lines across the surface. They were probably created by objects that were tidally disrupted as they passed close to Jupiter prior to the impact on Callisto, or by very oblique impacts.<sup class="reference" id="cite_ref-Greeley_2000_13-15">[13] A historical example of a disruption was Comet Shoemaker-Levy 9.

As mentioned above, small patches of pure water ice with an albedo as high as 80% are found on the surface of Callisto, surrounded by much darker material.<sup class="reference" id="cite_ref-Moore2004_4-9">[4] High-resolution Galileo images showed the bright patches to be predominately located on elevated surface features: crater rims, scarps, ridges and knobs.<sup class="reference" id="cite_ref-Moore2004_4-10">[4] They are likely to be thin water frost deposits. Dark material usually lies in the lowlands surrounding and mantling bright features and appears to be smooth. It often forms patches up to 5 km across within the crater floors and in the intercrater depressions.<sup class="reference" id="cite_ref-Moore2004_4-11">[4] Two landslides 3–3.5 km long are visible on the right sides of the floors of the two large craters on the right.On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy Galilean moons.<sup class="reference" id="cite_ref-Moore2004_4-12">[4] Typically there is a deficit of small impact craters with diameters less than 1 km as compared with, for instance, the dark plains on Ganymede.<sup class="reference" id="cite_ref-Greeley_2000_13-16">[13] Instead of small craters, the almost ubiquitous surface features are small knobs and pits.<sup class="reference" id="cite_ref-Moore2004_4-13">[4] The knobs are thought to represent remnants of crater rims degraded by an as-yet uncertain process.<sup class="reference" id="cite_ref-Moore1999_14-2">[14] The most likely candidate process is the slow sublimation of ice, which is enabled by a temperature of up to 165 K, reached at a subsolar point.<sup class="reference" id="cite_ref-Moore2004_4-14">[4] Such sublimation of water or other volatiles from the dirty ice that is the bedrock causes its decomposition. The non-ice remnants form debris avalanches descending from the slopes of the crater walls.<sup class="reference" id="cite_ref-Moore1999_14-3">[14] Such avalanches are often observed near and inside impact craters and termed "debris aprons".<sup class="reference" id="cite_ref-Moore2004_4-15">[4] <sup class="reference" id="cite_ref-Greeley_2000_13-17">[13] <sup class="reference" id="cite_ref-Moore1999_14-4">[14] Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain Martian surface features.<sup class="reference" id="cite_ref-Moore2004_4-16">[4] In the ice sublimation hypothesis, the low-lying dark material is interpreted as a blanket of primarily non-ice debris, which originated from the degraded rims of craters and has covered a predominantly icy bedrock.

The relative ages of the different surface units on Callisto can be determined from the density of impact craters on them. The older the surface, the denser the crater population.<sup class="reference" id="cite_ref-Chapman1997_39-0">[39] Absolute dating has not been carried out, but based on theoretical considerations, the cratered plains are thought to be ~4.5 billion years old, dating back almost to the formation of the Solar System. The ages of multi-ring structures and impact craters depend on chosen background cratering rates and are estimated by different authors to vary between 1 and 4 billion years.<sup class="reference" id="cite_ref-Greeley_2000_13-18">[13] <sup class="reference" id="cite_ref-Zahnle_1998_35-1">[35]

[edit] Atmosphere and ionosphere
Induced magnetic field around CallistoCallisto has a very tenuous atmosphere composed of carbon dioxide.<sup class="reference" id="cite_ref-Carlson_1999_6-3">[6] It was detected by the Galileo Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength 4.2 micrometers. The surface pressure is estimated to be 7.5  × 10−12 bar (0.75 µPa) and particle density 4 × 108 cm−3. Because such a thin atmosphere would be lost in only about 4 days (see atmospheric escape), it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from the satellite's icy crust,<sup class="reference" id="cite_ref-Carlson_1999_6-4">[6] which would be compatible with the sublimation–degradation hypothesis for the formation of the surface knobs.

Callisto's ionosphere was first detected during Galileo flybys;<sup class="reference" id="cite_ref-Kliore_2002_15-1">[15] its high electron density of 7–17 × 104 cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated by molecular oxygen (in amounts 10–100 times greater than CO2 ).<sup class="reference" id="cite_ref-Liang_2005_7-2">[7] However, oxygen has not yet been directly detected in the atmosphere of Callisto. Observations with the Hubble Space Telescope (HST) placed an upper limit on its possible concentration in the atmosphere, based on lack of detection, which is still compatible with the ionospheric measurements.<sup class="reference" id="cite_ref-Strobel2002_40-0">[40] At the same time HST was able to detect condensed oxygen trapped on the surface of Callisto.

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