Why is Saturn so low-density?
Quick Answer
Saturn is made mostly of hydrogen and helium, so its average density is less than water. If there were a bathtub large enough, Saturn would float in it.
The Full Story
Saturn formed from the same gas-rich disk as Jupiter, so its composition is dominated by light hydrogen and helium. Although it is enormous, its mass is spread over a very large volume, giving it an average density of about 0.69 grams per cubic centimetre. That is lower than water's density of 1 gram per cubic centimetre. The well-known claim that Saturn would float is a fun simplification, but it captures the basic idea. Saturn's low density is one reason it looks so different from the dense rocky planets.
Key Facts
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Suggested video title for this topic:
"Saturn is so light for its size it would float in water!"
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