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Io (/ ˈ aɪ. oʊ /), or Jupiter I, is the innermost and second-smallest of the four Galilean moons of the planet Jupiter.Slightly larger than Earth's moon, Io is the fourth-largest moon in the Solar System, has the highest density of any moon, the strongest surface gravity of any moon, and the lowest amount of water by atomic ratio of any known astronomical object in the Solar System.
The Earth seen from Apollo 17.jpg FullMoon2010.jpg Io_highest_resolution_true_color.jpg: Author: Apollo 17 Picture of the Whole Earth: NASA. Telescopic Image of the Full Moon: Gregory H. Revera True color image of Io: NASA / JPL / University of Arizona
The largest, Ganymede, is the largest moon in the Solar System and surpasses the planet Mercury in size (though not mass). Callisto is only slightly smaller than Mercury in size; the smaller ones, Io and Europa, are about the size of the Moon. The three inner moons — Io, Europa, and Ganymede — are in a 4:2:1 orbital resonance with
Size comparison of Europa (lower left) with the Moon (top left) and Earth (right) Europa is slightly smaller than the Earth's moon . At just over 3,100 kilometres (1,900 mi) in diameter , it is the sixth-largest moon and fifteenth-largest object in the Solar System .
The size of solid bodies does not include an object's atmosphere. For example, Titan looks bigger than Ganymede, but its solid body is smaller. For the giant planets , the "radius" is defined as the distance from the center at which the atmosphere reaches 1 bar of atmospheric pressure.
Like Saturn's largest moon Titan, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io, or the Moon due to its lower density compared to the three. [18] Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.
Io (1.9 nbar) and Callisto (26 pbar) have very thin atmospheres, but still enough to have collisions between atmospheric molecules. Other planetary-mass moons only have exospheres at most. [ 26 ] Exospheres have been detected around Earth's Moon, Europa, Ganymede, [ 26 ] Enceladus, [ 27 ] Dione, [ 28 ] and Rhea. [ 29 ]
Size comparison between Enceladus ... [102] [103] For comparison, Earth's ocean has an average depth of 3.7 ... compared to Jupiter's highly active moon Io) ...