Overview

Hyperion is Saturn's only small, outer-regular satellite. It's known for its unique, irregular, spongy-like appearance, and as far as discoveries go, it was the first non-round moon. Hyperion has a moderately eccentric orbit, and its orbit is near that of Titan's. Hyperion is in a 3:4 orbital resonance with Titan, and it isn't tidally locked with Saturn but instead rotates chaotically, the orbital resonance may contribute to Hyperion's unpredictable rotation.^[1] The low density is believed to be caused by its composition, primarily water ice with a small amount of rock; it may be a loosely accreted pile of rubble. It is the only moon other than the Moon that has an electrically charged surface.^[2]

Discovery and Exploration

Separate discoveries of Hyperion were made on September 16, 1848, by British astronomer, William Lassell and American astronomer, William Cranch Bond and his son George Phillips Bond.^[3]

Voyager 2 explored across the Saturnian system but only photographed Hyperion from afar. It could see individual craters and a large ridge, but not the detailed characteristics of Hyperion's surface. Early Cassini orbiter photographs hinted an odd appearance, but it wasn't until Cassini's first planned flyby of Hyperion on September 25, 2005, that the peculiarity was fully revealed. Roughly 40% of Hyperion is empty space, according to recent analysis of data Cassini acquired throughout its flybys in 2005 and 2006.^[4] It was proposed in July 2007 that Hyperion's porosity of 0.46^[5] is what keeps craters almost intact over eons. The latest investigations also verified that the majority of Hyperion's composition is water ice, with very little rock.^[4]

During the Cassini-Huygens's mission of exploring the Saturnian system, it made various flybys of Hyperion. On September 26, 2005, the first close targeted flyby was conducted by Cassini at a distance of 500 km (310 mi).^[6] On August 25, 2011, Cassini made another near approach to Hyperion, passing by at about 25,000 km (16,000 km) from the moon. On September 16, 2011, Cassini conducted a third close encounter, coming the closest at 58,000 km (36,000 mi).^[7] On May 31, 2015, Cassini made its last flyby, traveling around 34,000 kilometers (21,000 miles).^[6]

Composition and Surface features

Hyperion, like the majority of Saturn's moons, consists mainly of water ice with hardly any rocks, as seen by its low density. Hyperion is thought to have a physical composition akin to a loosely accreted mound of rubble. But unlike the majority of Saturn's moons, Hyperion has a low albedo (0.2–0.3), suggesting that a thin coating of dark material covers it. This might be dark material from Saturn's other further moon, Phoebe, albeit Hyperion is redder and more closely resembles the hue of the black material on Iapetus.

Hyperion resembles a massive sponge due to its surface's plethora of deep craters with sharp edges. Every crater has a bottom filled with dark stuff. Long chains of hydrogen and carbon make up the reddish substance, which resembles material from other Saturnian natural satellites, most notably Iapetus. Scientists believe that Hyperion's extraordinarily low density for such a huge object is the reason behind its peculiar, sponge-like appearance. Hyperion has a weak surface gravity and is very porous due to its low density. Due to these features, impactors often compress the surface as opposed to digging it, and the majority of material that is thrown off the surface never comes back.^[6]

Motion

Among the huge moons, Hyperion is special because of its very lopsided shape, a somewhat eccentric orbit, and proximity to Titan, a much larger moon. The combination of these elements limits the range of circumstances under which a steady rotation may occur. Additionally, a chaotic rotation could be more plausible due to Titan and Hyperion's 3:4 orbital resonance. Unlike several of Saturn's other moons, which have distinct leading and trailing hemispheres, Hyperion's surface is presumably rather uniform due to its unconstrained rotation.^[1]

Hyperion's axis of rotation wobbles so much that its orientation in space is unpredictable, as evidenced by the Voyager 2 photos and subsequent ground-based photometry. About 30 days is its Lyapunov time.^[8][9][10] Though it is anticipated to be widespread among binary asteroids,^[11] Hyperion is one of the few moons in the Solar System known to rotate chaotically, along with Nix and Hydra, moons of Pluto.^[12][13] In the Solar System, it is also the only regular moon that does not appear to be tidally locked.

Origin

One of the biggest bodies in the Solar system with an highly irregular shape is Hyperion. The biggest crater on Hyperion is 10.2 km (6.3 mi) deep and has a diameter of around 121.57 km (75.54 mi). Hyperion's asymmetric shape could be explained by being a fragment of a bigger body that was shattered by a significant impact a long time ago.^[14] 350–1,000 km (220–620 mi) is a potential diameter range for a proto-Hyperion.^[15] Ejecta from a potential proto-Hyperion breakup would have struck Titan gradually over a period of 1,000 years, accumulating volatiles in Titan's atmosphere. The unusual appearance of Hyperion is due to its low density, which causes it to be porous and has a low surface gravity. The surface is compressed by impactors, which prevents excavation. As a result, Hyperion resembles a sponge.^[15]

Gallery

Alternate view of Hyperion.

Hyperion with Saturn and Titan being clearly visible