What Is Callisto?
QUICK ANSWER
Callisto is the outermost of Jupiter's four Galilean moons and the most heavily cratered body in the solar system. About 3,000 miles across (the third-largest moon in the solar system), Callisto has an ancient surface that has remained nearly unchanged for about 4 billion years. It may have a deep subsurface ocean.
Callisto is the quiet, ancient member of the Galilean moons. While Io is volcanically violent, Europa harbors a young icy ocean, and Ganymede has a magnetic field, Callisto preserves what the early solar system looked like billions of years ago. Its heavily battered surface tells the story of the asteroid bombardment that shaped the early solar system. Despite its quiet appearance, Callisto likely has a deep subsurface ocean.
How big is Callisto?
Third-largest moon in the solar system. According to NASA, Callisto is Jupiter's second largest moon and the third largest moon in our solar system, with a diameter of about 3,000 miles. Only Ganymede (also at Jupiter) and Saturn's Titan are larger. Callisto orbits about 1.2 million miles from Jupiter, the farthest of the Galilean moons. This distance places it outside Jupiter's strongest radiation belts, making it potentially more accessible for future human exploration than the inner Galilean moons.
Why is it so cratered?
An ancient, undisturbed surface. Callisto's surface is the most heavily cratered of any body in the solar system. Almost every part is covered with overlapping impact craters of various ages. This intense cratering reveals that Callisto's surface has remained essentially unchanged for about 4 billion years, since the heavy bombardment phase of the early solar system. Unlike Io (volcanic resurfacing), Europa (ice flow), or even Ganymede (some tectonic activity), Callisto has had no significant geological activity to erase craters.
What's the Valhalla crater?
One of the largest impact features in the solar system. Callisto contains the Valhalla impact basin, a multi-ring structure about 2,400 miles across (4,000 km). It's one of the largest impact features anywhere in the solar system. The basin's distinctive rings spread outward from a bright central point like ripples in a pond, frozen in place after a massive impact billions of years ago. Several other large impact basins on Callisto, including Asgard, are similar multi-ring structures preserved from the early solar system.
Could it support exploration?
Yes, in some ways better than the inner Galilean moons. Callisto's distance from Jupiter means it's outside the main radiation belts that bombard Io, Europa, and (less intensely) Ganymede. Surface radiation on Callisto is much lower, making it relatively safer for human or sensitive robotic missions. NASA has studied Callisto as a possible base for future human exploration of the Jupiter system. The moon may also have a subsurface ocean, though if so it's deeper and harder to access than Europa's, making it less of an astrobiology priority.
Callisto is the outermost of Jupiter's four Galilean moons and the most heavily cratered body in the solar system. About 3,000 miles across (the third-largest moon), it preserves an ancient surface unchanged for about 4 billion years. The Valhalla impact basin is one of the largest impact features in the solar system. Callisto may have a subsurface ocean, though deeper than Europa's. Its distance from Jupiter's radiation belts makes it potentially attractive for future human exploration.
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