Europa and Titan are moons, as the definition of a moon is a natural satellite (object orbiting another object). Earth's moon is called Earth's moon.
Jupiter, Saturn, Neptune, and Uranus don't have surfaces to begin with.
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Says who? They're pretty big not to have something in there
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Says NASA radar scans
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Actually it is still a theory. Because the atmosphere's are too thick to penetrate and probes are basically torn apart on entry. We really have not been low enough to Verify they have no surface.
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There are also certain gravitational and magnetic distinctions of a solid surface.
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NASA hasn't ever been to those planets except in like 1980 and they sent those things in like 1970 with just a camera on it. no radar
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Voyager 1, Voyager 2, and many other probes had cameras, radar, and more. They also sent a probe that was designed to determine what jupiter was made of by dropping into it's atmosphere and determining the composition.
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Wtf does GAS GIANT mean to you