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单词 Callisto
释义
Callisto

Astronomy
  • The second-largest satellite of Jupiter, and the outermost of the four Galilean satellites; also known as Jupiter IV. It orbits 1 882 700 km from Jupiter’s centre in 16.69 days, keeping the same face turned towards the planet. Callisto is 4821 km in diameter but, at magnitude 5.7, is the faintest of the Galilean satellites, because of its low albedo of 0.17. Callisto’s density is 1.83 g/cm3. It has an icy surface uniformly covered with impact craters (the largest, Heimdall and Lofn, are 210 and 200 km wide respectively), with little sign of the tectonic resurfacing activity seen on the other Galilean satellites. It has several multi-ringed basins, the two largest being Valhalla (3000 km across, the largest in the Solar System) and Asgard (1400 km across), which are surrounded by numerous concentric rings.


Space Exploration
  • The second-largest moon of the planet Jupiter, 4 800 km in diameter, orbiting every 16.7 days at a distance of 1.9 million km. Its surface is covered with large craters.

    The US space probe Galileo detected oxygen on Callisto in 1997, suggesting that its surface is made of water ice. In 1998 Galileo's magnetometer measured electrical currents near Callisto's surface that are consistent with the existence of a buried salty ocean beneath the icy crust.


Geology and Earth Sciences
  • The third largest of the Galilean satellites, with the lowest density, the body is believed not to have differentiated into a core and mantle, consisting throughout of a mixture of rock and ice. Its surface is the darkest of the Galileans (albedo 0.20; though twice as bright as that of the Moon) and Callisto is the most heavily cratered body in the solar system. There is believed to be an almost complete absence of geologic activity at the surface, which has an age of about 4 billion years and shows no sign of any extensive resurfacing. The surface is of dirty ice and there are no large mountains. Surface craters and rings are shallow; the largest structures are Valhalla, a bright patch about 600 × 3000 km, and Asgard, a ring about 1600 km across. The surface temperature is about 134 K. Callisto was discovered on 7 January 1610 by Galileo. Its diameter is 4820 km; mass 1.076 × 1023 kg; density 1.8344 kg/m3; surface gravity 0.126 g (Earth = 1); mean distance from Jupiter 1.883 × 106 km; mean distance from Sun 5.203 AU; orbital period 16.68902 days; rotational period 16.68902 days.


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