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Moons of our Solar System by Mr. McKinsey, November, 2007 A Moon for Every Planet? • Most planets have moons. Even dwarf planets have moons. Only Mercury and Venus do not have moons. • Moons vary in size, in composition, and in many other ways. Gravity • The sun’s gravity keeps planets, moons, comets and asteroids in our solar system. • Each planet’s gravity keeps its moon or moons in orbit around the planet. • The pull of gravity depends on the size of the objects and their distance from each other. • Close objects pull harder than distant ones. Moon Characteristics • • • • • • Atmosphere Moon diameter Surface appearance Distance from host planet Size compared to host planet Amount of reflected light (albedo) Comparing Moons Moons of our Solar System are large and small, smooth and rough, near and far. Moon Atmospheres Moon Atmosphere? Earth’s Moon No Phobos No Io Yes! Ganymede No Triton Yes! Jupiter’s Galilean moons Io has an atmosphere. Moon Diameters Moon Diameter in Kilometers Earth’s Moon 3,476.2 Phobos 22.4 (average) Io 3,643.2 Ganymede 5,262.4 Triton 2,706.8 Earth’s Moon and Neptune’s Triton to the same scale. Ganymede -8th largest world • Largest moon in the solar system Io 12th largest world Surface Appearance Moon Surface Appearance Earth’s Moon Rocky, cratered Mars’ Phobos Rocky, smooth Jupiter’s Io Rocky, volcanic Jupiter’s Ganymede Cratered, icy, grooved Triton Icy, volcanic Phobos photographed from Mars Phobos, moon of Mars Io has active volcanoes. Io’s Volcanoes • Volcanic eruptions change Io’s surface Ganymede Ganymede’s complex surface has grooves, craters, lava flows, mountains, and water ice. Neptune's Triton from a distance of 530,000 kilometers (330,000 miles). Distance from Moon to Planet Moon Moon Phobos Io Ganymede Triton Distance from Moon to Planet in Kilometers 384,467 6,000 421,600 1,070,400 354,760 Neptune (top) and its moon, Triton (bottom) Ratio of Moon Diameter to Planet Diameter, as % of Planet Diameter Moon Ratio Expressed as a % Earth’s Moon 27.37% Mars’ Phobos 0.33% Jupiter’s Io 2.60% Jupiter’s Ganymede 3.76% Triton 5.52% Jupiter seen below its moon, Io Reflected Sunlight (Albedo) Moon Earth’s Moon Percent of Sunlight Reflected by Moon Surface (Albedo) 7% (0.07) Phobos 6% (0.06) Io 61% (0.61) Ganymede 42% (0.42) Triton 7% (0.07)