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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 Galilean moons are named after Galileo Galilei, who observed them in either December 1609 or January 1610, and recognized them as satellites of Jupiter in March 1610; [2] they remained the only known moons of Jupiter until the discovery of the fifth largest moon of Jupiter Amalthea in 1892. [3]
The sizes and masses of many of the moons of Jupiter and Saturn are fairly well known due to numerous observations and interactions of the Galileo and Cassini orbiters; however, many of the moons with a radius less than ~100 km, such as Jupiter's Himalia, have far less certain masses. [5]
A montage of Jupiter and its four largest moons (distance and sizes not to scale) There are 95 moons of Jupiter with confirmed orbits as of 5 February 2024. [1] [note 1] This number does not include a number of meter-sized moonlets thought to be shed from the inner moons, nor hundreds of possible kilometer-sized outer irregular moons that were only briefly captured by telescopes. [4]
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.
They were named after lovers of Zeus, the Greek equivalent of Jupiter. Among them is Ganymede, the largest and most massive moon in the Solar System. Its 87 known irregular moons are organized into two categories: prograde and retrograde. The prograde satellites consist of the Himalia group and three others in groups of
Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn and the sixth known planetary satellite (after Earth's moon and the four Galilean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii or 1,200,000 km above Saturn's apparent surface.
Phoebe, at 213 ± 1.4 km in diameter, is by far the largest of Saturn's irregular satellites. [30] It has a retrograde orbit and rotates on its axis every 9.3 hours. [ 87 ] Phoebe was the first moon of Saturn to be studied in detail by Cassini , in June 2004 ; during this encounter Cassini was able to map nearly 90% of the moon's surface.