Jupiter’s moon Callisto is the most heavily cratered object in the solar system. It also has the oldest landscape. It is thought to be a long dead world, with hardly any geologic activity on its surface.
Facts about Callisto
Age: Callisto is about 4.5 billion years old, about the same age as Jupiter.
Distance from Jupiter: It is the outermost of the Galilean moons. Because of its orbiting distance from Jupiter (about 1,168,000 miles) it takes about seven Earth days to make one complete orbit of the planet. It also experiences less tidal influences than the other Galilean moons because it orbits beyond Jupiter’s main radiation belt.
Size: At 3,000 miles (4,800 km) in diameter, Castillo is roughly the same size as Mercury. It is the third largest moon in the entire solar system. It has the lowest density of the four Galilean moons.
Temperature: The mean surface temperature of Callisto is -218.47 Fahrenheit.
The Discovery of Callisto
Callisto is the fourth of Jupiter’s moons to be discovered by Galileo Galilei on Jan. 7, 1610. The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth. Galileo’s discovery eventually led to the understanding that planets orbit the sun, instead of our solar system revolving around Earth.
Galileo called this moon Jupiter IV. When the numerical naming system was abandoned in the mid-1800s, the moon was named after the daughter of the King of Arcadia, Lycaon, who was a companion of Artemis, the chaste hunting goddess. Like all of the Greek mythological figures used to name the Jupiter moons, Callisto was seduced by Zeus and became pregnant. To protect Callisto and their son from his jealous wife, Hera, Zeus transformed them into bears and positioned them in the sky where he could watch over them.
Exploration of Callisto
Several spacecraft have flown by Jupiter and its moons. Pioneer 10 arrived first, in 1973, followed by Pioneer 11 in 1974. Voyager 1 and Voyager 2 returned striking photos during their flybys. The Galileo spacecraft passed as low as 162 miles (261 km) over the surfaces of the Galilean moons and produced detailed images.
Characteristics of Callisto
The most prominent feature of Callisto is its craters, as it has the most craters of any object in the solar system. Scientists also estimate that it has the oldest surface of any object in the solar system. There are no signs that its geological landscape has changed in 4 billion years, which has sparked keen scientific interest in the planet.
Without the impact of plate tectonics or volcanoes to change the surface, it is believed that any changes in the surface have come as the results of being hit by objects. Distinguishing features of Callisto’s surface include multi-ring structures, impact craters of varying shapes, and lines of craters known as catenae. Icy peaks also dot the moon’s surface.
While craters are Castillo’s signature feature, its surface coloring is also the darkest of all of the Galilean moons. Castillo’s composition is about half water ice and half rocky material, which consists of magnesium- and iron-bearing hydrated silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and other organic compounds.
The likely presence of an ocean within Callisto leaves open the possibility that it could sustain life. Because of its low radiation levels, Callisto has long been considered the suitable place among the Galilean moons for future exploration.
Callisto’s thin atmosphere is composed of carbon dioxide and likely some molecular oxygen. Callisto is thought to have formed as a result of slow accretion from the disk of gas and dust that surrounded Jupiter after its formation.
- Kim Ann Zimmermann
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