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Callisto
Details
Callisto is Jupiter's second-largest moon and the outermost of the four Galilean moons. Its ancient, heavily cratered surface records a quieter history than Io, Europa, and Ganymede, while Galileo spacecraft data suggest a possible salty ocean deep below the ice.
Callisto matters because it changes the mood of Jupiter's moon system. Io, Europa, and Ganymede all feel dynamically restless in different ways, but Callisto looks like a world that has preserved an older surface memory. Its dark, cratered face is not empty or boring; it is a record of impacts that have remained visible because the moon has not been globally resurfaced in the same way as some of its neighboring worlds.
That old surface makes Callisto useful as more than a size marker. It is Jupiter's second-largest moon and nearly the size of Mercury, yet its distance from Jupiter gives it a different history from the more active inner Galilean moons. The result is a large icy world where scale, location, and visible activity do not line up in the simple way people might expect.
The deeper twist is that Callisto may not be entirely inactive. NASA notes that Galileo spacecraft data suggest a possible salty ocean beneath the surface, deep under the ice. That possibility gives Callisto a quiet kind of tension: it is one of the most visibly ancient bodies in the Solar System, but it may still hide a liquid layer below a battered exterior.
2,107km
Key facts
- Scale fact
- 4,821 kmdiameter
- Estimated age
- ~4.5 billion years
- Composition
- Rock and water ice
Scale context
Where Callisto sits on the full axis
Together, these objects make the size change around Callisto easy to compare at a glance.
Sources
References for Callisto
Measurements and descriptive context are compiled by the Scale of Space team from the references below. If you find an error, please let us know.
- Callisto — Wikipedia
- Visual reference: Wikimedia Commons (NASA / JPL-Caltech / Kevin M. Gill, CC BY 2.0)
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Scale of Space is a scroll-based journey through the universe, placing objects on a single logarithmic scale so you can compare size across an unbroken range.
Guides turn parts of that scale into curated essays, while focused views let you explore the same range through specific groups of objects.