Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Saturn: Rhea Link for sharing this page on Facebook
Voyager 1 images of Rhea taken in 1980: On the left, the heavily cratered leading side; on the right the trailing side, showing wisps similar to those on Dione.

      Rhea is the second largest of Saturn's moons, approximately 950 miles in diameter. Although somewhat larger than Dione it is otherwise very similar. Its density is slightly lower, but still greater than that of most Saturnian moons, indicating that it is made mostly of ice, but has a substantial minority of heavier materials. In addition it has the same two-sided appearance as Dione, with a heavily cratered leading edge and a trailing edge with wispy features originally thought to be related to some sort of ancient ice vulcanism, but now believed to be similar to the cliff scarps on Dione.
      In the close-up of the leading edge below, we can see not only huge numbers of larger craters, but also the suggestion of a streaky alignment of the smaller craters. This is probably the result of multiple impacts of moderate-sized objects ejected from larger impact craters in a way similar to the formation of "rays" on our own Moon. Similar features probably exist on many other moons in the Solar System, but in most cases we don't have close-up images of the moons, and such small details are not visible.

     Closeup of leading side of Rhea, showing intense cratering, taken with the Voyager spacecraft in 1980. Note the streaky alignment of smaller craters at the "top" of the picture, suggesting multiple collisions by fragments of individual objects such as ejecta from larger collisions or fragile comets. (Voyager, NASA, Calvin J. Hamilton, apod030608)

     An October 2009 Cassini image of the intensely cratered surface of Rhea. (NASA/JPL/Space Science Institute, Planetary Photojournal)

     A natural-color image of the trailing side of Rhea and its "wispy" features obtained by the Cassini spacecraft in January 2005. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod050215)

     A January 17, 2006 Cassini image of Rhea shows icy scarps similar to those on Dione, confirming the suspected nature of the wispy features on its trailing surface. (NASA/JPL/Space Science Institute, Wikipedia posting of cleaned-up version of this raw Cassini image)

     A portion of the side of Rhea that always faces Saturn, taken by the Cassini spacecraft during a close (60,000 miles distance) approach in March of 2006. An intensely cratered surface which can have hardly changed in more than a billion years, save by additional cratering so frequent that the older craters no longer appear round, having been shattered by more recent cratering. About 1000 miles in diameter, Rhea is Saturn's second largest moon, although much smaller than Titan, which is four times its diameter and sixty times its volume. Primarily made of water ice, Rhea is presumed to have a small rocky core, as mixtures of ice and rock this large should have generated some heat during their formation, allowing heavier materials to sink to the center. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod060530)

     The southern portion of the side of Rhea which always faces Saturn as imaged by the Cassini spacecraft in April 2008. Ancient craters cover most of the surface, but a portion of the bright streaks found on the trailing side of the moon can be seen near the top. Although not revealed as such in this image, the streaks are probably, as in the case of Dione, cliffs caused by faulting which are reflecting sunlight more brightly than the more horizontal surfaces on either side of them. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod080513)

     Whitish deposits mark what is probably a recent impact crater and material ejected by the impact in this April 2005 Cassini image of Rhea. Even for nearly white objects like the icy moons of Saturn, surfaces tend to darken with exposure to the Sun's ultraviolet radiation. As a result, fresh material exposed by impacts within the last few hundred million years tends to be brighter and whiter than the surrounding landscape. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod050530)

     Another view of the "rays" emanating from the impact crater in the previous image. The first image has reduced brightness and increased contrast to show sharper detail. This image shows the brightness difference more clearly, but at the expense of reduced contrast. (NASA/JPL/Space Science Institute, Planetary Photojournal)

     An earlier image of Rhea happened to photograph the eastern rim of the bright rayed crater from a distance of only 300 miles. The entire region shown is covered with brighter than usual material, so the difference in brightness -- particularly noticeable on the inside edge of the crater -- is not due to differences in the brightness of the material, but the angle of lighting by the Sun. The crater walls are brightly lit by sunlight striking them more directly, while the crater floor and surrounding terrain are less brightly lit by sunlight striking them at a low angle. The same phenomenon is responsible for the appearance of the wispy structures seen on Rhea's trailing edge, which like those on Dione have proven to be ice cliffs presumably caused by faulting. (NASA/JPL/Space Science Institute, Planetary Photojournal)

     Rhea's scarred surface is shown in exceptional detail in this image taken by the Cassini spacecraft from a distance of less than 400 miles in November 2005. The worn-down rim of an ancient crater (worn down by subsequent impacts, not weathering or erosion such as found on the Earth) runs across the bottom of the image, while smaller craters fill the crater and the region outside it. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod051213)

Data for Rhea

Discovered by Giovanni Cassini in 1672
Named after one of the daughters of Uranus and Gaea
Orbital size 527,000 km (approximately 328,000 miles)
Orbital eccentricity 0%
Orbital inclination 0.3 degrees
Orbital period 4.5175 days
Rotation period 4.5175 days (synchronous rotation, one side permanently facing Saturn)
Diameter 1530 km (about 950 miles)
Mass 1/2400 of Earth, 1/30 of Earth's Moon
Surface gravity 3% of Earth, 1/6 of Earth's Moon
Density 1.3 times density of water (Composition probably mostly ice, some rock)
Albedo (reflectivity) 70%
Surface temperature 280 to 360 degrees below zero Fahrenheit (day to night variation)