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Explore Jupiter S Galilean, Galilean Moons, and more!

Internal structure of Io, Europa, Ganymede and Callisto. © based on NASA images. More about the Galilean moons in Mona Evans, "Jupiter's Galilean Moons"

from National Geographic

Jupiter Moon Io, Jupiter Moon Picture, Io Picture

Jupiter and Its Moons Photograph courtesy NASA This family portrait, a composite of the Jovian system, includes the edge of Jupiter (with the Great Red Spot visible) and Jupiter's four largest moons, known as the Galilean satellites. From top to bottom are Io, Europa, Ganymede, and Callisto. The smallest of these four moons, Europa is about the size of Earth's moon.

"Jupiter with Io and Ganymede" by Damien Peach. Winner of the Astrophotographer of the Year 2011 competition. An impressive image that compares well with those taken with large telescopes. At the lower left is Io and the larger moon is Ganymede. ©Mona Evans, "Jupiter's Galilean Moons"

Io. The most volcanically active body in the Solar System, it's about the same size as the Moon and orbits at about the same distance from Jupiter as the Moon does from Earth. Yet it has over 400 volcanoes and a “month” on Io lasts only 42 hours. This is all down to gravity. Caught between Jupiter's strong gravity and that of its companions Callisto & Ganymede, Io is mercilessly squeezed, releasing heat. ©Mona Evans,"Jupiter's Galilean Moons"

Jupiter’s moon Io and its tiny shadow sweep across the giant planet’s face back in 1999, as snapped by the Hubble Space Telescope. [Credit: John Spencer (Lowell Observatory) and NASA] Mona Evans, "Jupiter's Galilean Moons"

As befits the colossus of the Solar System, Jupiter has four of the system's largest moons: Ganymede, Callisto, Io and Europa. See them compared to other Solar System objects. ©Mona Evans,"Jupiter's Galilean Moons"

Io is the first Galilean moon of Jupiter, it is slightly larger than Earth’s moon.  Io experiences intense tidal heating due to its elliptical orbit and orbital resonance with Europa and Ganymede.  This makes Io the most geologically active moon in our solar system.

The sizes of Jupiter's four major moons in contrast to Jupiter's Great Red Spot. From top to bottom, the moons are Io, Europa, Ganymede and Callisto. Image credit: NASA/JPL/DLR

Hubble Space Telescope observations show water vapor on Jupiter's moon Europa. If this is coming through surface cracks from Europa's subsurface ocean, it could have a lot of information on what's there. This is an artist's impression showing Europa with Jupiter in the background. (Image credit: NASA, ESA, and M. Kornmesser) Mona Evans, "Jupiter's Galilean Moons"

The 1:2:4 orbital resonance of the three inner Galilean moons. They are tidally locked, which means they keep the same face towards Jupiter as they orbit. In addition, they orbit in what's called a resonance created by a combination of Jupiter's gravity and the gravity of the moons themselves. Each time Ganymede orbits Jupiter, Europa orbits twice and Io four times. ©Mona Evans,"Jupiter's Galilean Moons"