If you looked at Rhea, the first thing that you notice is the many craters on its surface.
|Discovered By:||G.D. Cassini December 23, 1672|
|Mass:||2.31 × 10^21 kg (3.1% Moon)|
|Orbit Distance:||527,068 km|
|Orbit Length:||4.5 days|
|Surface Temperature:||-179 to -220 °C|
There are so many craters that it is rated as one of the most heavily cratered satellites in our solar system.
Discovered in 1672 by Giovanni Cassini, the Italian astronomer, he found all four moons orbiting around Saturn. Rhea was the second moon that he discovered and at the time, he named all four moons the Sidera Lodoicea, “the Stars of Louis,” after King Louis XIV.
Although numerically, Rhea was called Saturn V, the moons were renamed in 1847 by John Herschel, the son of famed astronomer William Herschel, so that they were the relatives of the mythological Greek god Cronus (whom the Romans called Saturn.) Rhea was Cronus’ wife and was the mother to many of the Olympian gods and goddesses.
Rhea is Saturn’s second largest moon and its surface is icy with a density that is 1.233 times the density of liquid water.
Scientists assume that the surface, which is highly reflective, is ¾ ice and ¼ rock.
The spacecraft mission Cassini-Huygens did an examination of Rhea and scientists have said that it kind of looks like a dirty frozen snowball.
Rhea’s temperatures range from -281 F/-174 C in the sunlit and warmer areas to -364 F/-220 C in the shade.
In this drastically cold temperature, the ice that is on the moon can behave as rock. Rhea is most known for the incredible number of craters on its surface.
Scientists use cratering as a way to classify specific areas. On Rhea, the brighter terrain that is heavily craters has impacts that are bigger than 25 mi/40 km. The smaller impacts and craters are near both the equatorial area and the poles.
Saturn’s gravitational pull is less on Rhea because it orbits farther out than some of the other icy moons.
Scientists think this may be a reason that Saturn’s other moons are smoother, as they have internal heat created by Saturn’s gravity that has melted and re-iced those regions that are cratered.
Not so on Rhea, however, the moon does have a series of wispy lines that cover hundreds of kilometers over the surface.
It was the Cassini spacecraft that showed that these lines are surface fractures that have created canyons as large as several hundred meters in height.
Cassini found that when dark material fell from the canyon walls, there was bright ice material exposed.
Rhea has a very thin oxygen atmosphere and is the first evidence of any celestial body with an oxygen atmosphere in our solar system, other than Earth.
The oxygen may be released as the surface is irradiation by Saturn’s magnetosphere ions. The oxygen is 5 trillion times less dense than the oxygen on Earth.
There is also the presence of carbon dioxide, but scientists don’t know what the source is. Some have guessed that it could be delivered from dry ice that are found on comets.