Saturn: Moons: Hyperion

This stunning false-color view of Saturn's moon Hyperion reveals crisp details across the strange, tumbling moon's surface. Differences in color could represent differences in the composition of surface materials. The view was obtained during Cassini's very close flyby on Sept. 26, 2005.

Hyperion [pronounced hi-PEER-ee-en; adjective form: Hyperionian] is the largest known irregular (nonspherical) body in the Solar System. Hyperion's average diameter is 270 kilometers (168 miles), but since Hyperion is rather potato-shaped, its shape can be described in terms of its diameter along its three axes: 410 x 260 x220 kilometers (255 x163 x137 miles, respectively). Considering its odd shape, Hyperion is probably a remnant of a larger moon that was destroyed by a major impact.

Hyperion appeared as a tiny dot to astronomers until the Voyager II spacecraft passed through the Saturnian system in 1981 but this view was from a distance. The Voyager images showed the irregular shape, the heavy cratoring, and the chaotic spin. However, Cassini has filled in details including the curious sponge-like appearance and the presence of complexed compounds.

The most noticeable close-up feature of Hyperion is its deeply cratered surface. Hyperion and its sister outer moons, Phoebe and Iapetus, all show extensive cratering because they are Saturn's most distant moons and have experienced very little tidal warming that might blur or erase earlier features. However, the Hyperion craters are particularly deep and do not have significant rays of ejecta (although there appears to have been slumping or landslides inside many of the bigger craters). The result is a curiously punched-in look, somewhat like the surface of a sponge or a wasp nest. Planetary geologists have theorized that Hyperion's high-porosity and low density would crater more by compression than excavation.

Many of the crater walls on Hyperion are bright, which suggests an abundance of water ice. The crater floors are mostly the areas of the lowest albedo and greatest red coloration. This may be because the average temperature of roughly -180 Celsius (-300 Fahrenheit) might be close enough to a temperature that would cause volatiles to sublimate, leaving the darker materials accumulated on the crater floors. This scenario fits with some of the newer crater floors being bright water ice.

Some have theorized that these dark materials at the crater floors would absorb more sunlight and that the resulting "warmth" would deepen the craters as the more volatile materials sublimed away. Those arguing against this theory say that sunlight at Saturn's distance is too weak to have that effect, particularly in crater floors that are shaded from all but the most direct sunlight.

Celestial Mechanics of Hyperion and Its Near Neighbors

Hyperion orbits at a mean distance of 1,481,100 kilometers (920,300 miles) from Saturn in an eccentric orbit. This contributes to variation in the spin or rotation of Hyperion. A stronger effect on Hyperion's rotation is that it is in resonance with Saturn's largest moon, Titan, which orbits at 1,221, 850 kilometers (759,200 miles). Thus, the two objects speed up and slow down as they pass each other in a complex set of variations. Because Hyperion is much smaller than Titan, its rotation and orbit are affected vastly more than the larger moon, and Titan apparently keeps the Hyperion orbit eccentric rather than growing more circular over time.

The great distance from Saturn and resonance with Titan has also kept Hyperion from becoming tidally locked facing Saturn. Hyperion rotates roughly every 13 days during its 21-day orbit.

Hyperion shows a dull reddish color with a low reflectivity (or albedo) averaging 0.3. This is similar to neighboring Phoebe (0.05) and Iapetus (0.03-0.05 in the dark leading side), but it is much lower than the inner moons such as Rhea (0.7), Dione (1.0), and Enceladus (1.4). This low albedo may be due to frozen carbon dioxide complexed with frozen water and other molecules such as hydrocarbons. Another contributor might be methane from Titan's atmosphere being stripped of its hydrogen by solar radiation and the resulting carbon dust making its way to Hyperion. A third possibility is that dark material from Phoebe may be coloring both Hyperion and Iapetus.

This image of Hyperion was acquired by the Voyager 2 spacecraft on August 25, 1981.

Hyperion's density is slightly more than half that of water. This could be due to water ice with gaps (porosity) of more than 40 percent. Also, lighter materials, such as frozen methane or carbon dioxide, could make up part of Hyperion. This is consistent with the concept of Hyperion accreting from a number of smaller ice and rock bodies but not having enough gravity to compact them. Thus, Hyperion might be similar to a large rubble pile.

Discovery and Naming

William Lassel discovered Hyperion in 1848. The same year William Cranch Bond, with his son George Phillips Bond, independently discovered the moon, and all three men are jointly credited with the discovery.

John Herschel suggested that the moons of Saturn be associated with mythical brothers and sisters of Kronus, known to the Romans as Saturn. The name Hyperion comes from the Greek god (or titan) Hyperion (he who watches over). Hyperion was the son of Uranus and Gaia and the husband of Thea. The children of Hyperion and Thea included Helios (the Sun), Eos (the dawn), and Selene (the Moon).

Astronomers also refer to Hyperion as Saturn VII. The International Astronomical Union now controls naming of astronomical bodies.