Jupiter - Io, Europa, Ganymede, Callisto
Jupiter has over 60 known moons, but four of them stand out due to their exceptional size and the fact that they have been studied extensively: Io, Europa, Ganymede, and Callisto. These moons were first discovered in 1610 by Galileo Galilei, thanks to their size, and are therefore known as the Galilean moons.
The smallest of the four is Europa, which is slightly smaller than Earth's Moon. The largest is Ganymede, with a diameter slightly greater than that of Mercury. As their distance from Jupiter increases (from Io to Europa, Ganymede, and Callisto), their density decreases from 3.56 grams per cubic centimeter to 1.851 grams per cubic centimeter. Callisto, the farthest, is composed almost entirely of ice rather than rock.
All four moons were photographed by the Voyager 1 probe, which revealed that Io has active volcanoes—an extraordinary feature unique to a moon in our solar system.
On December 30, 2023, the Juno spacecraft flew past Io at a distance of 1,500 km, capturing new images. No spacecraft has ever been this close to Jupiter’s moon.
Juno has been on its mission since 2011, aiming to learn more about Jupiter’s formation and composition. Since Juno’s camera is somewhat old and the image quality is not as sharp anymore, NASA has released the raw images for public processing. The edited version shown above is one of them.