Rhea, Saturn's second-largest moon

Updated October 19, 2026 · 4 min read

Rhea is Saturn's second-largest moon, 1,527 km across, and the ninth-largest moon in the solar system. It is a cold, heavily cratered world of ice and rock that appears to have changed little in billions of years. In 2010 the Cassini spacecraft discovered that Rhea is surrounded by an extremely thin exosphere of oxygen and carbon dioxide. NASA described it as the first time a spacecraft had directly captured molecules of an oxygen atmosphere at a world other than Earth.

Discovery and name

Giovanni Domenico Cassini found Rhea on 23 December 1672, his second discovery after Iapetus the previous year. It was the third moon of Saturn to be found, after Titan. John Herschel named it in 1847 after Rhea, the Titaness who was the sister and wife of Cronus and mother of Zeus; in Roman tradition she corresponds to Ops, consort of Saturn (Saturn's mythological family).

Rhea is only slightly smaller than Uranus' Titania and slightly larger than Oberon, with which it is often compared. With a density of 1.24 g/cm³, it is roughly three quarters ice and one quarter rock by mass.

An ancient surface

Rhea's surface is saturated with craters, many of them old and degraded, which makes it one of the best-preserved records of bombardment in the outer solar system. Its two largest basins, Tirawa and Mamaldi, span several hundred kilometers each. Against this old background, one crater stands out: Inktomi, about 48 km wide, surrounded by a bright splash of rays that makes it one of the youngest large features on the moon.

As on Dione, the trailing hemisphere carries bright, wispy streaks. Cassini showed that they too are fracture systems and ice cliffs, evidence that even this apparently quiet moon was stretched and cracked at some point in its history. Unlike Dione, Rhea shows few other signs of internal activity, and it is not known to have an ocean.

The interior remains debated. Gravity data from Cassini's 2005 flyby were first interpreted as indicating a nearly uniform mixture of ice and rock; later analyses argued for partial separation into a rocky core and an icy mantle. The measurements are sensitive to assumptions about Rhea's shape and rotation, so the question is still open.

The oxygen and carbon dioxide exosphere

During a close flyby in March 2010, Cassini's instruments sniffed a faint envelope of molecular oxygen and carbon dioxide around Rhea. The oxygen is produced when energetic particles from Saturn's magnetosphere strike the icy surface and break water molecules apart, a process called radiolysis. The carbon dioxide may come from the same process acting on carbon-bearing material on the surface, or from organics delivered by dust.

This exosphere is extraordinarily tenuous, more than a trillion times less dense than Earth's air at sea level, and could never support life or weather. Its interest lies elsewhere: it shows how radiation chemistry can produce oxidants on icy worlds. On moons with subsurface oceans, such as Europa or Enceladus, similar oxidants carried down into the water could provide chemical energy, a key ingredient in assessing habitability (life on Titan and Enceladus).

Does Rhea have rings?

In 2008, a team analyzing Cassini's charged-particle data reported sharp, symmetric drops in electron counts on either side of Rhea. They proposed that a disk of debris, perhaps with narrow rings, orbited the moon, which would have made Rhea the first moon known to have rings. Dedicated imaging searches by Cassini in 2009 and 2010, looking for sunlight scattered by such material, found nothing. Most scientists now consider the ring hypothesis unconfirmed and unlikely; the electron dropouts may have another explanation.

Exploration

The Voyagers photographed Rhea in 1980 and 1981, Voyager 1 from about 74,000 km. Cassini made several targeted flybys between 2005 and 2013, some passing less than 100 km above the surface, and imaged the moon repeatedly during its 13 years at Saturn. Rhea is a common target for amateur astronomers too: at around magnitude 10 it is the second-brightest moon of Saturn after Titan, visible in a modest telescope (telescope observing tips).

Sources

• NASA Science: Rhea
• Teolis, B. D., et al., "Cassini finds an oxygen–carbon dioxide atmosphere at Saturn's icy moon Rhea", Science 330 (2010)
• Jones, G. H., et al., "The dust halo of Saturn's largest icy moon, Rhea", Science 319 (2008)
• Tiscareno, M. S., et al., Cassini imaging search for rings around Rhea, Geophysical Research Letters 37 (2010)