Callisto

## General Information

The second-largest moon of Jupiter is Callisto, sometimes known as Jupiter IV, after Ganymede. It is as big as the smallest planet Mercury, but only roughly a third as massive. It is the third biggest moon in the Solar System, behind Ganymede and Titan, the largest moon of Saturn. Callisto orbits Jupiter on average at a distance of 1883000 km, which is nearly six times further away than the Moon around Earth. With a diameter of 4821 km, Callisto is about a third larger than the Moon. Known as the outermost of Jupiter's four giant Galilean moons, it was originally observed in 1610 using one of the earliest telescopes and could be seen with regular binoculars from Earth.

The Solar System's oldest and most severely cratered object is Callisto's surface. Impact craters cover the whole surface of it. It is believed to have evolved mostly under the effect of impacts and does not exhibit any indicators of subsurface processes like as plate tectonics or volcanism, nor does it offer any indications that geological activity in general has ever happened. In Multi-ring formations, differently shaped impact craters, and chains of craters with related deposits, ridges, and scarps are notable surface features.In Small, glittering frost deposits at the apex of high points make up the surface's variable small-scale structure, which is encircled by a smooth, dark material blanket that lies low to the ground.

The likelihood that Callisto contains an ocean raises the prospect that life may exist there. Conditions are believed to be less favorable than on neighboring Europa, though. Callisto has been explored by several space probes, including Pioneers 10 and 11, Galileo, and Cassini. For a long time, Callisto was thought to be the best spacecraft to build any future crewed expeditions to research the Jovian system upon due to its low radiation levels.

#### History

Together with the other three major Jovian moons, Ganymede, Io, and Europa, Callisto was independently discovered in 1610 by Simon Marius and Galileo Galilei.

### Properties

Callisto's typical density of 1.83 g per cubic cm indicates that it is mostly composed of water ice and rocky material in almost equal amounts, with a small amount of other volatile ices like ammonia.

The beaten surface of Callisto resides atop an 80–150 km deep layer of frigid, ice, and rigid lithosphere. Research on the magnetic fields surrounding Jupiter and its moons suggests that there may be a salty ocean 150–200 km deep beneath the surface. The discovery revealed that Callisto reacts to the fluctuating background magnetic field of Jupiter like a perfectly conducting sphere, that is, the field is unable to enter Callisto itself, indicating the presence of a layer of highly conductive fluid within that is at least 10 km thick. An ocean is more likely to exist if the water has up to 5% of ammonia or another type of antifreeze in it. The water and ice layer in this instance might be as thick as 250–300 km.

One of the Solar System's most severely cratered surfaces is that of Callisto's ancient past. Since any new crater tends to obliterate existing ones, the crater density is actually very nearly saturation. Large mountains, volcanoes, or other endogenic tectonic features are absent from Callisto, making the large-scale geology there very basic. Large features on the surface consist only of impact craters and multi-ring structures along with related fractures, scarps, and deposits.

Multi-ring basins are the biggest impact landforms found on Callisto's surface. Two are in fact enormous.  The largest is Valhalla, which has rings that reach up to 1,800 kilometers from the center and a luminous central zone that is 600 kilometers in circumference. At almost 1,600 kilometers in diameter, Asgard is the second largest. Probably originating from a post-impact concentric lithosphere fracture that was resting on a layer of liquid or soft material, perhaps an ocean, are multi-ring structures. The surface's extensive chains of impact craters arranged in straight lines are known as the catenae. Very oblique impacts or objects that were tidally disrupted as they traveled close to Jupiter before the impact on Callisto are most likely the sources of them.

#### Water

On the surface of Callisto, there are tiny areas of pure water ice that have an albedo of up to 80%, encircled by material that is far darker. Thin water frost deposits are most likely what they are. Usually found in lowlands, dark material has a smooth appearance and surrounds brilliant features. It frequently appears in areas of the crater floors and inter-crater depressions that are up to 5 km broad.

#### Atmosphere

Carbon dioxide makes up the atmosphere of Callisto, which is highly cloudy.  The Galileo Near Infrared Mapping Spectrometer picked it up thanks to an absorption characteristic that was close to 4.2 micrometers in wavelength. Such a thin atmosphere would be lost in roughly 4 years, thus it would need to be continuously replaced. One possible way to do this would be by slow sublimation of carbon dioxide ice from Callisto's frozen crust, which would be consistent with the surface knob creation theory based on sublimation–degradation.

Recent study of the Hubble Space Telescope's data from 2001 has also revealed the presence of atomic hydrogen in the atmosphere of Callisto. Reexamination of spectral pictures obtained on December 15, 24, and 2001 revealed a weak signal of scattered light indicative of a hydrogen corona. When the leading hemisphere is examined, the observed brightness from the dispersed sunlight in the hydrogen corona of Callisto is around two times larger. Different hydrogen abundances in the leading and trailing hemispheres could be the cause of this asymmetry. But this hemispheric variation in the brightness of Callisto's hydrogen corona is probably due to the signal in Earth's geocorona extinction, which increases when the trailing hemisphere is seen.

## Appearance

Castillo appears to be tan with lighter bands. There also appears to be craters on the moon.