Callisto (moon) Totally Explained

Everything about Callisto Moon totally explained

Callisto (kə-LIS-toe, or as Greek Καλλιστώ) is a moon of the planet Jupiter, 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 kilometers. Marius attributed the suggestion to Johannes Kepler. However, the names of the Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral designation, a system introduced by Galileo, as or as "the fourth satellite of Jupiter". In scientific writing, the adjectival form of the name is Callistoan, or Callistan. This is significantly larger than the orbital radius—1,070,000 km—of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto doesn't participate in the mean-motion resonance—in which the three inner Galilean satellites are locked—and probably never has.
Like most other regular planetary moons, Callisto's rotation is locked to be synchronous with its orbit.
The dynamic isolation of Callisto means that it has never been appreciably tidally heated, which has had important consequences for its internal structure and evolution.

Physical characteristics

Composition

The average density of Callisto, 1.83 g/cm³, The mass fraction of ices is between 49–55%. The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed various non-ice materials: magnesium- and iron-bearing hydrated silicates, and possibly ammonia and various organic compounds. Many fresh impact craters like Lofn also show enrichment in carbon dioxide. It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly conducting sphere; that is, the field can't penetrate inside the moon, suggesting a layer of highly conductive fluid within it with a thickness of at least 10 km. In this case the ocean can be as thick as 250–300 km. In fact, the crater density is close to : 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. 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. 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 craters called palimpsests, the central parts of multi-ring structures, and isolated patches in the cratered plains. 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. The second largest is Asgard, measuring about 1,600 kilometers in diameter. 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. Absolute dating hasn't 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. 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 bar and particle density 4 cm^-3. Because such a thin atmosphere would be lost in only about 4 days (see atmospheric escape), it must be constantly being replenished, possibly by slow sublimation of carbon dioxide ice from the satellite's icy crust, its high electron density of 7–17 cm^-3 can't be explained by the photoionization of the atmospheric carbon dioxide alone. Hence, it's suspected that the atmosphere of Callisto is actually dominated by molecular oxygen (10–100 times more of this than ). However, oxygen hasn't 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. At the same time HST was able to detect condensed oxygen trapped on the surface of Callisto.

Origin and evolution

The partial differentiation of Callisto (inferred for example from moment of inertia measurements) means that it has never been heated enough to melt its ice component.The allowable timescale of formation of Callisto lies then in the range 0.1–10 million years. Details of the subsolidus convection in the ice is the main source of uncertainty in the models of all icy moons. It is known to develop when the temperature is sufficiently close to the melting point, due to the temperature dependence of ice viscosity. It is thought to proceed in the so-called "stagnant lid" regime, where a stiff, cold outer layer of the moon conducts heat without convection, while the ice beneath it convects in the subsolidus regime.
The current understanding of the evolution of Callisto allows for the existence of a layer, an "ocean", of liquid water in its interior. This is connected with the anomalous behavior of the ice I phase's melting temperature, which decreases with pressure, achieving temperatures as low as 251 K at 2,070 bar. However, the conditions for life appear to be less favourable on Callisto than on Europa. The principal reasons are: the lack of contact with rocky material and the lower heat flux from the interior of Callisto.
Based on the considerations mentioned above and on other scientific observations, it's thought that of all of Jupiter's Galilean moons, Europa has the greatest chance of supporting microbial life.

Exploration

[[Image:Callisto base.PNG|thumb|right|300px|Artist's impression of a human base on Callisto in the future The real breakthrough happened later with the Voyager 1 and 2 flybys in 1979–1980. They imaged more than half of the Callistoan surface with a resolution of 1–2 km, and precisely measured its temperature, mass and shape. In February–March 2007, the New Horizons probe on its way to Pluto obtained new images and spectra of Callisto.

Potential colonization

In 2003 NASA conducted a conceptual study called "Human Outer Planets Exploration" (HOPE) regarding the future human exploration of the outer solar system. The target chosen to consider in detail was Callisto. It was proposed that it could be possible to build a surface base on Callisto that would produce fuel for further exploration of the solar system. Advantages of this localization include the low radiation at Callisto's distance from Jupiter, and Callisto's geological stability. A base there could facilitate remote exploration of Europa, or be an ideal location for a Jovian system waystation servicing spacecraft heading farther into the outer Solar System, using a gravity assist from a close flyby of Jupiter after departing Callisto.Further Information

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