While scientists have speculated(推测) since 1970s about the presence of an ocean on Ganymede—the largest moon in our solar system—until now the only observational evidence came from a brief flyby by the Galileo spacecraft, which didn't observe the moon long enough to confirm a liquid ocean.

　　This discovery marks an important milestone, highlighting what only Hubble can accomplish. In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy surface of Ganymede opens up further exciting possibilities for life beyond Earth.

　　Scientists_have_already_confirmed_the_existence_of_an_ocean_on_Europe,_another moon orbiting Jupiter, and NASA has announced plans to send an unmanned mission there searching for the life that might come with liquid water.

　　Hubble is a telescope that orbits Earth, but because of its impressive gravitational analyses it can be used to study the inside of far distant planets. Using these same principles, scientists could theoretically detect oceans on distant planets as well.

　　文章大意：这是一篇关于天文知识的资讯报道。科学家们通过Hubble望远镜判断在木星上发现海洋的存在，人们期待更多的地球以外的生命存在。

　　4．What can we learn about the Ganymede from the passage?

　　A．It is a planet with the largest moon in our whole solar system.

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