NASAのインスタグラム(nasa) - 3月13日 03時11分

This view of Saturn’s dramatic, icy moon Dione, aptly named for a Titaness in Greek mythology, was eyed by our Cassini spacecraft on July 23, 2012 at a distance of approximately 260,000 miles.

Dione, measuring about 698 miles across, has a density that suggests that almost a third of the moon is made up of a solid core (probably silicate rock) with the remainder of its material being water ice. At Dione's average temperature of -304 degrees Fahrenheit, ice is so hard it behaves like rock.

Astronomer Giovanni Domenico Cassini discovered Dione (1684) and three of Saturn’s other moons — lapetus (1671), Rhea (1672) and Tethys — as well as the large gap in Saturn's rings, now called the Cassini division. His namesake, our Cassini spacecraft, ended its more than 13 years exploring his findings and making many more with a dive into Saturn’s atmosphere on Sept. 15, 2017.

Credit: NASA/JPL-Caltech/Space Science Institute

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