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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
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 ]
The seven largest natural satellites in the Solar System (those bigger than 2,500 km across) are Jupiter's Galilean moons (Ganymede, Callisto, Io, and Europa), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton. Triton, the smallest of these, has more mass than all smaller natural satellites together.
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.
The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and the Moon have synchronized the Moon's orbital period (lunar month) with its rotation period at 29.5 Earth days, causing the same side of the Moon to always face Earth.
The sizes are listed in units of Jupiter radii (R J, 71 492 km).This list is designed to include all planets that are larger than 1.6 times the size of Jupiter.Some well-known planets that are smaller than 1.6 R J (17.93 R 🜨 or 114 387.2 km) have been included for the sake of comparison.