Rhea, second largest of Saturn’s natural satellites or moons, was discovered in the year 1672 by Giovanni Domenico Cassini, an Italian-French astronomer.
Giovanni Domenico Cassini. © National Portrait Gallery, London
It is also the ninth biggest moon in the existing solar system in terms of size. The moon got its name from the Greek mythological character Rhea who is the revered mother of all the gods. Also given the name Saturn V, it is the fifth moon that moves outward from the planet. The four moons of Saturn that Cassini discovered were named “Stars of Louis” (Tethys, Lapetus, Rhea, and Dione) to honor the French king. Most astronomers, however, referred to these moons (and Titan) as Saturn I through V. With the discovery of two more moons in 1789 (Mimas and Enceladus), the names extended to Saturn VII. The names of the seven moons of Saturn were named after Greek mythological Titans (brothers and sisters of Cronos) by John Herschel who was the son of William Herschel, the discoverer of Uranus.
Physical structure and characteristics of Rhea
Rhea’s density of 1.236 g/cm3 indicates that it is primarily composed of water ice constituting about 75 percent and rocky material makes up for the balance 25 percent. Though it is ninth largest in size, it holds the tenth position in terms of mass.
You can see Rhea on the edge of Saturn. © NASA
Rhea has an icy body and was initially assumed to have a core made of rocks. However, research by various astronomers has only proved controversial. Whereas some of them argued that the moment of inertia reading suggested a rocky core, others have pointed out to a homogenous interior. The scientific community points out to the fact that additional measurements may be required to resolve the problem and come to a conclusion. The shape of Rhea suggests that it is in hydrostatic equilibrium and has a homogenous body. Rhea is about 763 km in radius and has a surface area of 7.3 million km2. Rhea’s distance from Saturn is about 527,040 km and it takes the same time to rotate on its axis as well as revolve around Saturn (4.516 Earth days). Its orbit is almost a circle and it has a surface gravity of about 0.265 m/s2. Scientific models that have been created to point out that Rhea may be capable of sustaining a liquid ocean of water inside through the heating produced by radioactive decay. One side of Rhea is always facing Saturn.
Surface and geological characteristics of Rhea
Thanks to Cassini space probe we have picture of Rhea’s surface. © NASA
Rhea’s surface features very closely resemble that of another moon Dione and therefore suggest similar histories of formation. Its leading and trailing hemispheres (the hemisphere that faces the direction opposite to the orbital movement) have different features that bear close semblance to those of Dione. Under direct light of the sun Rhea’s temperature is about 174 degrees centigrade and under shade it is between -200 and -220 degrees centigrade.
Rhea’s leading hemisphere is very reflective. The surface of Rhea has several craters like that of our moon and its trailing hemispheres are covered with a wispy terrain very similar to that found in Dione. These appear as brighter streaks in the darker trailing hemisphere. Deposits at its equatorial region due to material that has deorbited from its rings region show up as a line on Rhea. The craters of Rhea are not walled high nor do they have a peak towering their centers.
Two large impact craters dot the trailing hemisphere of Rhea and are approximately 400 and 500 km across. The impact crater Tirawa lies more to the north and is comparable in structure to Odysseus basin on the moon Tethys. The other impact crater is called Mamaldi. Inktomi also nicknamed as ‘Splat’ is prominent and one of the youngest craters of Rhea with a diameter of 48 km.Internal structure of Rhea. © NASA images
Craters on Rhea’s surface. © NASA
The crater densities are also starkly different on the surface of Rhea. This has given rise to two different geologically characterized areas: one that contains craters larger than 40 km in diameter and the second containing craters smaller in size. The second is found in polar and equatorial regions of Rhea. The wispy terrain on the trailing hemisphere (that is bright and prominent) is thought to have been caused by ejections from ice volcanoes when Rhea was just forming and its interiors were filled with liquid.
The bright streaks extend for hundreds of kilometers across the terrain. Some of the canyons are several hundred meters high. The walls of the canyon appear bright when dark material falls off and exposes fresh water ice. The brighter streaks in Dione’s darker trailing hemisphere has shown the existence of ice cliffs formed due to extensive fracturing of the surface of that moon.
In the year 2010, the space probe called Cassini discovered that Rhea has a very thin atmosphere made up of carbon dioxide and oxygen existing in an approximate proportion of 2 to 5. This is the first time that a spacecraft has captured an atmosphere of oxygen other than in Earth.
The source of oxygen is purported to be the dissociation of water ice molecules at the surface through ions created by the magnetic cavity of Saturn. In simple words, the oxygen arises when Saturn’s magnetic field rotates over the moon Rhea. The source of carbon dioxide is not very clear. It is thought to come from the organics present in the water ice or it may have occurred due to the release of the gas which was dissolved, frozen, or absorbed in other material.
Rhea’s ring system
Cassini space probe. © NASA
There were initial suggestions about the existence of rings around Rhea, as announced by NASA in 2008. Rings were thought to exist because of a changed pattern of the flow of electrons that was observed. It was thought that dust and debris were denser near the moon forming three rings of high density. However, later observations by the space probe Cassini in 2010 did not reveal any material and made way for further conclusive evidence to be found to establish the presence of rings.
Rhea’s first images were got through pictures taken by Voyager I and II in the years 1980 and 1981. The Cassini orbiter has been able to procure images of Rhea after its four fly-bys in the years 2005, 2007, 2010 and 2011. The last targeted fly-by by the orbiter was as close as 69 km from the moon. Many additional images from long to moderate distances shot by the orbiter also exist.