单选题：TEXT EIn its first month orbiting Saturn, the Cassini spacec

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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 tings. 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 does the passage mainly discuss? A．The discovery of a new radiation belt has become a breakthrough in astronomical researches
B．After a recent discovery of a radiation belt, a relatively small radiation belt has been perceive
C．A belt around Saturn unlike any ever seen before has been detecte
D．Some mystifying patterns have been observed at Satur