Callisto (moon)

Callisto is the second-largest moon of Jupiter, after Ganymede. It is also the third-largest moon in the Solar System, after Ganymede and Saturn's moon Titan. It is almost as big as the planet Mercury.

Callisto is much farther from Jupiter than Earth's Moon. It orbits Jupiter at a distance of 1.883 million km, about five times farther than the Moon is from Earth. It is the outermost of the four large Galilean moons of Jupiter. These moons were discovered in 1610 using one of the first telescopes. Today, Callisto can be seen from Earth with common binoculars.

The surface of Callisto is very old and full of craters. It does not have processes like plate tectonics or volcanism. Scientists think it formed mostly from impacts. Callisto may have an ocean inside, which raises the possibility that it could harbor life, though it might be harder for life to exist there than on Europa. Several space probes, including Pioneers 10 and Galileo, have visited Callisto. Because it has low radiation levels, Callisto is a good place for future human missions to study Jupiter and its moons.

History

Discovery

Callisto was found in 1610 by two astronomers, Simon Marius and Galileo Galilei. They saw it with three other large moons of Jupiter: Ganymede, Io, and Europa.

Name

Callisto is named after a character from Greek mythology. Callisto was linked to the goddess Artemis, who was known for her connection to the hunt. The name was suggested by Simon Marius after he found Callisto. For a long time, people used numbers instead of names, calling Callisto Jupiter IV, meaning the fourth moon of Jupiter. Today, we usually use the name Callisto.

Orbit and rotation

Callisto is the farthest out of the four large moons that orbit Jupiter. It travels around Jupiter at a distance of about 1.88 million km, much farther than the next moon, Ganymede. Because it is so far away, Callisto does not follow the same patterns as the inner moons.

Callisto spins in a way that matches its orbit around Jupiter, meaning it shows the same face to Jupiter at all times. It takes about 16.7 Earth days to complete one orbit. Its path is not a perfect circle and tilts slightly, but these changes happen very slowly over many years. This distant orbit protects Callisto from strong effects of Jupiter's magnetic field, keeping its surface safer from harmful radiation.

Physical characteristics

The density of Callisto is 1.83 g/cm³. This shows it is made of about equal parts rocky material and water ice, with some other icy materials such as ammonia. We can see water ice on its surface using a tool called near-infrared spectroscopy. Other materials on the surface include magnesium- and iron-bearing hydrated silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and various organic compounds.

Callisto's surface sits on a cold, stiff icy layer that is between 80 and 150 km thick. There may be a salty ocean 150–200 km deep below the surface. We know this from studying the magnetic fields around Jupiter and its moons. Inside, Callisto is made of compressed rocks and ices, with more rock as you go deeper. The surface is covered with many craters from impacts. The biggest feature is Valhalla, which has a bright center 600 km wide and rings that stretch out to 1,800 km from the center. The surface has different types of areas: cratered plains, light plains, bright and dark smooth plains, and areas linked to big impact structures. Callisto also has a very thin atmosphere made mostly of carbon dioxide and possibly oxygen.

Origin and evolution

Callisto is Jupiter's second-largest moon. It has never been warm enough to melt its icy surface. Scientists think it formed slowly from a cloud of gas and dust around Jupiter. This slow growth let it stay cool as it got bigger.

After it formed, Callisto changed because of heat from radioactive materials. The heat, cooling near the surface, and slow movement of ice deep inside all played a part. This movement is very slow—about one centimeter each year—and helped cool the moon over time. Callisto might also have a layer of liquid water deep inside, where the pressure makes ice melt at lower temperatures. Unlike its neighbor Ganymede, Callisto shows little sign of activity inside. This is because it formed under different conditions and had fewer strong impacts. This makes Callisto a simpler example for scientists studying more active moons.

Habitability

Like some other moons in our solar system, Callisto might have a hidden ocean of salt water deep inside it. Scientists think that this ocean could possibly have conditions where tiny, microscopic life might survive.

However, Callisto may not be as good a place for life as another moon called Europa. This is because Callisto does not get as much heat from Jupiter, and its ocean is only warmed by natural radioactive processes. Europa, being closer to Jupiter, gets extra heat from gravitational forces, making it more likely to support tiny life forms.

Exploration

The Pioneer 10 and Pioneer 11 missions in the 1970s gave us more information about Callisto. The big discoveries came with the Voyager 1 and Voyager 2 missions in 1979. These spacecraft took pictures of most of Callisto and measured its temperature, mass, and shape.

Later, the Galileo spacecraft flew close to Callisto between 1994 and 2003, getting as close as 138 kilometres to its surface. Other spacecraft like Cassini and New Horizons also studied Callisto. In the future, more spacecraft will visit Callisto, including the European Space Agency's Jupiter Icy Moons Explorer, NASA's Europa Clipper, and China's CNSA Tianwen-4.