Astronomy Practice Exam

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Which moon is the largest in the solar system?

Io
Europa
Ganymede
Callisto

The correct answer is: Ganymede

Ganymede is the largest moon in the solar system, and its significance is highlighted by its impressive size, which surpasses that of the planet Mercury. Ganymede has a diameter of about 5,268 kilometers (3,273 miles), making it not only the largest moon of Jupiter but also the largest in the entire solar system. In addition to its size, Ganymede is unique because it is the only moon known to have a substantial magnetic field, likely generated by a partially liquid iron or iron-sulfide core. This has implications for understanding both the moon's geological history and its potential for harboring subsurface oceans, which may be of interest in astrobiology. The other moons listed, while significant in their own right—Io, Europa, and Callisto—are smaller in comparison. Io is known for its volcanic activity, Europa for its icy surface and possibility of an ocean beneath, and Callisto for its heavily cratered surface. However, none of these can match Ganymede’s size.