Hyperion (astronomy)

Hyperion (astronomy), large moon of the planet Saturn. Hyperion’s strange spongy appearance, odd shape, and low density make it one of the most unusual objects known in the solar system. Hyperion orbits Saturn at a distance of about 1.48 million km (about 920,312 mi). It completes an orbit about once every 21 Earth days. Hyperion’s orbit parallels Saturn’s equator and is slightly elliptical, or oval-shaped.

Hyperion is irregular in shape, measuring about 364 km (about 226 mi) at its widest diameter and about 190 km (118 mi) at its narrowest diameter. Data from the Cassini space probe indicate that Hyperion is made of “dirty ice”—water ice contaminated with complex organic materials that contain hydrocarbons (combinations of hydrogen and carbon). The hydrocarbons are similar to compounds found in meteorites and comets, and are related to the basic chemical building blocks needed for life. Carbon dioxide ice that is chemically bound to other molecules is also mixed with the water ice. The moon’s low density—slightly more than half the density of water—indicates that it is porous all through its structure rather than compact and solid. About 42 percent of Hyperion may be empty space. Hyperion has no atmosphere.

Among known irregular moons, Hyperion is second in size only to Neptune’s moon Proteus. Gravity tends to pull worlds above a certain size into spheres, a sphere being the most compact shape possible. Hyperion, however, is one of the few exceptions to that rule. Planetary scientists think that the moon likely formed by the gradual accumulation of small particles. It retained a lumpy shape because its low mass could not provide enough gravity to pull material together into a more regular, rounded form.

The moon’s porous structure means that impacting objects penetrate deeper into its surface and send up much less material than would occur when striking a denser, more compact body. At the same time Hyperion’s low gravity lets most of the debris from the impacts escape into space rather than pulling ejected material back onto the moon’s surface. Dark material in the bottom of some craters also warms from sunlight and melts deeper into the surface in a process called thermal erosion. The deep, sharp-edged impact craters that result give the moon an odd spongy appearance. Some parts of Hyperion’s surface and the bottoms of some craters are covered with dark, reddish black material similar to that seen on Saturn’s moons Phoebe and Iapetus. The source of the dark material is not known.

Hyperion’s rotation is chaotic and the moon tumbles erratically as gravity from Saturn and nearby moons tugs on its irregular shape. Unlike Hyperion, most moons rotate once on their axes in the same amount of time they take to orbit the planet once; they also keep the same hemisphere pointed toward their planet at all times.

Hyperion was discovered in 1848 by British astronomers William Cranch Bond, George Bond, and William Lassell. Hyperion is named for the Titan Hyperion from Greek mythology. Individual features on Hyperion are named for moon and sun gods from cultures around the world.