Erich Karkoschka/Univ. of Arizona, Voyager 2, NASA
If you could put Uranus’ moon Cressida in a gigantic tub of water, it would float.
Cressida is one of at least 27 moons that circle Uranus. Robert Chancia of the University of Idaho in Moscow and colleagues calculated Cressida’s density and mass using visible variations in an inner ring of Uranus as the planet passed in front of a distant star. The moon’s density is 0.86 grams per cubic centimeter and its mass is 2.5 x 1017 kilograms. These results, reported online August 28 at arXiv.org, are the first to reveal any details about the moon. Knowing its density and mass helps researchers determine if and when Cressida might collide with another of Uranus’ moons.
Voyager 2 discovered Cressida and several other moons when the spacecraft flew by Uranus in 1986. Those moons, plus two others found later, are the