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.〕 | rotation = synchronous | axial_tilt = 0–0.33°〔 | albedo = | magnitude = 4.61 (opposition)〔 4.38 (in 1951)〔 (4.38 on 1951-Oct-03)〕 | temperatures=yes | temp_name1 = K | max_temp_1 = 152 | mean_temp_1 = 110 | min_temp_1 = 70〔 | atmosphere = yes | surface_pressure = Trace | atmosphere_composition = Oxygen〔 }} Ganymede 〔, or as (ギリシア語:Γανυμήδης )〕 (Jupiter III) is the largest moon of Jupiter and in the Solar System, and the only moon known to have a magnetosphere. It is the seventh satellite outward from Jupiter〔 and third of the Galilean moons, the first group of objects discovered orbiting another planet. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. With a diameter of , it is 8% larger than the planet Mercury, but has only 45% as much mass.〔(【引用サイトリンク】title=Ganymede Fact Sheet )〕 Its diameter is 2% larger than that of Saturn's Titan, the Solar System's second-largest moon. At 2.02 times the mass of the Moon, it is the most massive planetary satellite. It is the 9th largest object in the Solar System, and the largest without a substantial atmosphere. Ganymede is composed of approximately equal amounts of silicate rock and water ice. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans together. Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity brought about by tidal heating.〔 Ganymede's magnetosphere was probably created through convection within its liquid iron core.〔 The meager magnetosphere is buried within Jupiter's much larger magnetic field and would show only as a local perturbation of the field lines. The satellite has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone).〔 Atomic hydrogen is a minor atmospheric constituent. Whether the satellite has an ionosphere associated with its atmosphere is unresolved.〔 Ganymede's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610. The satellite's name was soon suggested by astronomer Simon Marius, for the mythological Ganymede, cupbearer of the Greek gods and Zeus's lover.〔 Beginning with ''Pioneer 10'', spacecraft have been able to examine Ganymede closely.〔 The ''Voyager'' probes refined measurements of its size, whereas the ''Galileo'' craft discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. After flybys of all three icy Galilean moons, the probe is planned to enter orbit around Ganymede. ==History== Chinese astronomical records report that in 365 BC, Gan De detected what appears to have been a moon of Jupiter, probably Ganymede, with the naked eye. On January 7, 1610, Galileo Galilei observed what he thought were three stars near Jupiter, including what turned out to be Ganymede, Callisto, and one body that turned out to be the combined light from Io and Europa; the next night he noticed that they had moved. On January 13, he saw all four at once for the first time, but had seen each of the moons before this date at least once. By January 15, Galileo came to the conclusion that the stars were actually bodies orbiting Jupiter.〔〔〔 He claimed the right to name the moons; he considered "Cosmian Stars" and settled on "Medicean Stars".〔(【引用サイトリンク】title=Satellites of Jupiter )〕 The French astronomer Nicolas-Claude Fabri de Peiresc suggested individual names from the Medici family for the moons, but his proposal was not taken up.〔 Simon Marius, who had originally claimed to have found the Galilean satellites, tried to name the moons the "Saturn of Jupiter", the "Jupiter of Jupiter" (this was Ganymede), the "Venus of Jupiter", and the "Mercury of Jupiter", another nomenclature that never caught on. From a suggestion by Johannes Kepler, Marius once again tried to name the moons:〔 This name and those of the other Galilean satellites fell into disfavor for a considerable time, and were not in common use until the mid-20th century. In much of the earlier astronomical literature, Ganymede is referred to instead by its Roman numeral designation (a system introduced by Galileo) as or as the "third satellite of Jupiter". Following the discovery of moons of Saturn, a naming system based on that of Kepler and Marius was used for Jupiter's moons.〔 Ganymede is the only Galilean moon of Jupiter named after a male figure — like Io, Europa, and Callisto, he was a lover of Zeus. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ganymede (moon)」の詳細全文を読む スポンサード リンク
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