This page describes an image Ganymede seen by Juno spacecraft
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Ganymede is the largest and most massive natural moon of Jupiter, and at the same time, the largest moon in the solar system. It also stands out as it is the only moon in the solar system which has an internally generated magnetic field. This is likely created by a liquid iron or iron sulfide core.
With a diameter of over 5,000 kilometers, it exceeds Mercury in size. Although it has more than twice the mass of Earth's Moon, it hosts weaker surface gravity than Mercury, Io, and the Moon due to its lower mean density.
Ganymede's appearance is primarily composed of darker, heavily cratered regions, and brighter, less bombarded ones with deep grooves and ridges indicating geological activity. Whether this might be caused by tidal heating is still subject to further analysis. The brighter regions make up around two-thirds of Ganymede's surface.
The remaining area consists of dark regions, saturated with impact craters, which have been dated back to four billion years. It is assumed that the moon hosts a large saltwater ocean under its icy surface with extreme depths of up to 800 kilometers.
Ganymede likely has a very tenuous oxygen atmosphere, comparable to that found on the Jupiter moon Europa. It cannot be interpreted as a sign of life. Instead, it is thought that hydrogen and oxygen are split up by radiation on the moon's surface.
The first space probe to visit Ganymede was Pioneer 10, performing a high-speed flyby manoeuvre through the Jupiter system. Since then, it has been investigated by several other probes such as Galileo, New Horizons, and Juno. The Jupiter Icy Moons Explorer (JUICE) will be the first to actually enter orbit around Ganymede itself (in around 2034) and impact it in later stages to further study the moon's surface composition.
Image credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill Credit Link
Related glossary terms:
Galilean Satellites
, Moons
Image license: Creative Commons Attribution 2.0 Generic Creative Commons Attribution 2.0 Generic icons
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