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  3. In its first month orbiting Saturn, the Cassini spacecraft has detected a new radiation belt in an unexpected place, its invisib

In its first month orbiting Saturn, the Cassini spacecraft has detected a new radiation belt in an unexpected place, its invisib

admin2016-12-06  29
问题     In its first month orbiting Saturn, the Cassini spacecraft has detected a new radiation belt in an unexpected place, its invisible swarm of trapped high-energy particles circling the planet inside the inner edge of Saturn’s signature disk of luminous rings.
    Scientists who reported the discovery said yesterday that they were surprised to find a relatively small radiation belt completely isolated from a planet’s main radiation belts, which lie at much greater distances. They had not expected that such a concentration of energetic particles could be sustained inside the famous Saturnian rings.
    The phenomenon of a single isolated belt, the scientists said, has never been observed at any other planet in the solar system. By contrast, the Van Allen belts of Earth consist of two related regions of intense radiation trapped by the planet’s magnetic field.
    In a NASA conference call with reporters, Dr. Donald G. Mitchell, a Cassini mission scientist from Johns Hopkins University, said the newly discovered radiation belt was detected as the spacecraft made its closest approach to Saturn immediately after rocketing into its orbit on July 1. No previous spacecraft visiting Saturn had been in the proper position to make such a discovery.
    Dr. Mitchell said it was the spacecraft’s imaging magnetometer that had observed the radiation belt, which extended around Saturn from about 15,000 miles above the planet’s cloud tops to the inner edge of the innermost of its spectacular rings. The belt is much smaller, and the energies of its particles are less intense, than Saturn’s main radiation belts.
    With its discovery, Dr. Mitchell said, "we have seen something that we did not expect: that radiation belt particles can hop over obstructions like Saturn’s rings."
    Other Cassini mission scientists reported puzzling patterns of lightning and thunderstorms on Saturn and observations of a striking glow emanating day and night from the planet’s largest moon, Titan.
What is special about the belt discovered latest?
选项 A、It is an insular belt which unexpectedly pierces through Saturnian rings.
B、It is much smaller and less intense than the formerly-discovered belts.
C、It is different from the Van Allen belts of Earth.
D、It is the outermost belt of Saturn.
答案B
解析 原文第5段说：“发光带要小一些，粒子能量也不如土星主要发光带强。”故正确答案为B。A称该辐射带是孤立的，它出人意料地穿透了土星环；原文说到，辐射带的颗粒可以穿透土星环，二者并不一致，故不选A。C本身没错，但说它与地球上的范·艾伦辐带不同，并未揭示该辐射带的特点，与题意不符。D错误，该辐射带在土星环内侧，远比土星的主要辐射带低。
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