Rhea is one of Saturn’s many moons and is known for being the second largest of all of them. While the biggest moon might be Titan, Rhea has a mean radius of 475 mi/764 km, which is fairly big compared to all of the other moons.
Rhea is cold, small, and airless and appears very similar to two of its sister moons, Tethys and Dione. Like them, Rhea is tidally locked with Saturn so only one side faces Saturn as it goes through its 4.5 Earth-day orbit around its parent planet. Also like Dione and Tethys, Rhea’s surface temperatures run from the warmest at -281 degrees F/-174 degrees C in the areas that are lit by the sun, to the coldest at around -364 degrees F/-220 degrees C in the areas that are shady. All three of these moons are highly reflective and this means that their surfaces are probably made up of mostly water ice.
Scientists have researched to find that the density of Rhea is about 1.233 that of liquid water which indicates that Rhea may be made up of ¼ rock and ¾ ice. Measurements from the Cassini spacecraft show that the area that measures how difficult it would be for Rhea to alter its rotation is higher than if Rhea had a core that was rocky. Due to this information, scientists assume that Rhea is made up of a mixture of rock and ice, something that they sometimes refer to as a “dirty snowball.”
- Orbits: Saturn
- Discovered By: G.D. Cassini
- Discovery Date: December 23, 1672
- Diameter: 1,528.6 km
- Mass: 2.31 × 10^21 kg (3.1% Moon)
- Orbital Period: 4.5 days
- Orbit Distance: 527,068 km
- Surface Temperature: -179 to -220 degrees C
On December 23, 1672, Giovanni Cassini, an Italian astronomer spotted Rhea as the second moon that he observed and the third moon that was orbiting around Saturn. He discovered a total of four moons, with Rhea at around 949 mi/1,528 km in diameter. Rhea was about one third the size of Saturn’s largest moon, Titan.
Cassini assigned the names of the four moons as Sidera Lodoicea, which translates to the “Stars of Louis,” named after King Louis IV. He assign Roman numerals to the moons and Rhea was Saturn V. The names stuck until 1847 when the son of famed astronomer William Herschel, John Herschel, made the suggestion that the moons should be named for the Titans. This is a mythological group of sisters and brothers of the Greek god Cronus, known in Roman mythology as Saturn.
Rhea was the daughter of Gaea and Uranus and became the wife of Cronus. She was called the mother of gods and was the mother of all of the gods and goddesses of Olympus, including Zeus, Hera, Hestia, Hades, Poseidon, and Demeter.
Formation, Structure and Surface:
Rhea is 327,500 miles/527,000 km from its parent planet Saturn and farther away that its sister moons, Tethys and Dione. Due to the distance, Rhea doesn’t receive the kind of tidal variations that are required to create internal heating. This is important to note because both of the sister moons have a lot smoother plains than Rhea, suggesting that they do get sufficient tidal variations to create enough heat to cause liquid water to reach the surface and replenish it. Both Dione and Tethys appear to have a surface with ponded craters where liquids arrived and then froze. Rhea seems to lack the warmth and therefore the surface never had the opportunity to replenish itself. This explains why Rhea’s surface is more heavily cratered than its sister moons.
Until the Voyager 1 and 2 spacecraft missions, Rhea only appeared as a small dot to the astronomers and very little was known about it. The images sent back from the Voyager spacecraft showed that Rhea had two main regions. The first area was bright as well as heavily cratered, with craters as big as 25 mi/40 km across. The second area in parts of the equatorial and Polar regions had fewer craters and those that did exist were less than 25 mi/40 km across. The difference between the two regions indicate that at one time in Rhea’s history there was resurfacing that covered the craters in the second region. It would have had to be a very long time ago since the average age of Rhea’s plains is believed to be around 4 billion years old.
The images from Voyager also included wispy lines that have lengths that extend out as far as tens to hundreds of miles and cut through the craters and the plains. It wasn’t until the Cassini spacecraft images in 2006 that scientists began to look at the wispy areas as possible fractures that made the canyons. The canyons themselves are several hundred meters high and have walls that are bright due to darker material falling off and exposing bright fresh ice that is underneath. The fractures indicate that at one time in Rhea’s history, the moon might have been tectonically active. This same kind of feature is also on the surface of Tethys and Dione.
The NASA Cassini-Huygens spacecraft sent back data that showed that the moon was an icy body that has a density of 1.233 times of liquid water; indicating it was three quarters ice and one quarter rock. This led to the assumption that there wasn’t any icy core but instead a mixture of ice and rock together. The temperatures on Rhea are so cold that the ice acts like rock.
Rhea is considered to be one of the most heavily cratered satellites that we have in our solar system and is more cratered than its sister moons.
Using the data from the Cassini spacecraft, scientists have been attempting to map Rhea and to figure out more information about the age of the surface. A 2014 study showed that Rhea’s Inktomi crater and the debris that was around it had more resistance to being at the same temperature than area that were around it. The suggestion that was given was that Rhea as well some of the other moons may have been hit by debris from older moons. A 2015 study showed that there were good amounts of crystalline and amorphous water ice. A team studying both the Inktomi and Oblata craters did find evidence of crystalline ice at their centers. Over time this ice can cause ice transformation.
Atmosphere and Magnetosphere:
The 2010 flyby by the Cassini spacecraft did detect a thin atmosphere on Rhea that scientists call an exosphere. The exosphere is infused with both carbon dioxide and oxygen and the molecules that Cassini captured in space became the first ones ever captured from any atmosphere, other than our own planet Earth.
Examining the molecules let scientists understand that the oxygen seems to occur when the magnetic field of Saturn rotates over Rhea. This occurrence allows the energetic particles that are trapped in Saturn’s magnetic field scatter over Rhea’s water-ice surface, causing a chemical reaction of decomposition, releasing the oxygen. Scientists are still unsure where the carbon dioxide originates from.
Rhea’s surface oxygen is around 5 trillion times less dense that the oxygen that we have on Earth. However, the capture of the atmosphere molecules did show that surface decomposition can contribute a lot of oxygen as well as possible surface densities that are greater than either Earth’s moon or Mercury. Researchers are looking into whether the formation of carbon dioxide and oxygen could be a reason for the surface chemistry of many of the surfaces of the icy objects.
The Cassini spacecraft did seem to find evidence of material orbiting Rhea in 2008 that scientists believed could be rings. If they were, this would make Rhea the first moon to have a ring. Cassini detected a broad disk of debris and what appeared to be at least a single ring with many of its suite of instruments. These were designed for the study of particles and atmospheres around the moons and the parent planet of Saturn. However, later observations showed no evidence of any rings and the source of the original observation remains a mystery.
The samples taken by Cassini indicated that the atmosphere of Rhea is about 100 times thinner than the air around Ganymede and Europa. It’s so thin that it couldn’t be remotely detected. The oxygen is around 70% of the thin atmosphere with carbon dioxide as the remaining 30%. To compare, our Earth has oxygen concentrations that are around 5 trillion times higher than those on Rhea. It means that Rhea’s atmosphere is nearing one hundred times thicker than the atmospheres on Mercury or our moon.
Could Life Exist?
While oxygen atmospheres have been discovered on some of the other icy bodies, Rhea’s atmosphere is very thin. Rhea doesn’t have any other confirmed pieces that are required to sustain life as we know it.
Both the Voyager 1 and Voyager 2 spacecraft discovered mysterious wispy lines on Rhea as well as a surface filled with craters. It wasn’t until the Cassini spacecraft that we learned the wispy lines were fractures and cliffs that are all over Rhea.
The Cassini spacecraft images showed a hemisphere that is dark in color and another one that is lighter in color. The dark hemisphere is believed to be caused by millions of years of radiation exposure of the water-ice. The lighter colored hemisphere is thought to be covered in young ice dust that has landed from Saturn’s E-ring and Enceladus tiny particles.
In the 1990s, NASA’s Hubble Space Telescope found a thin oxygen atmosphere on both Ganymede and Europa. It led scientists to question if other moons had atmospheres and to figure out the mechanisms needed to create them.
The Hubble Space Telescope team began to research whether atmospheres existed on moons in the Saturn system, which were thought to be mostly frozen moons.
In two flybys in 2005 and 2007, the Cassini explored to look for an oxygen atmosphere on Rhea. A later flyby got much closer over the north pole of Rhea and managed to use the mass spectrometer to confirm that the thin atmosphere had both oxygen and carbon dioxide.
Facts about Rhea Moon for Kids:
- Giovanni Cassini discovered two moons around Saturn and then discovered Rhea as the third moon and then a fourth moon.
- The original name of Rhea was Saturn V as the fifth moon from Saturn.
- After the asteroid and dwarf planet Ceres, Rhea is the second smallest body within our solar system that is confirmed to be in a state of constant fluid balance over a long period of time.
- Many of the moons in our solar system are tidally locked so that only one side of the moon faces its parent planet. Rhea is tidally locked to Saturn.
- Rhea has a lot of water ice on the surface and it makes the moon highly reflective.
- Rhea is thought to have a surface that is older than its two sister moons of Tethys and Dione.
- Rhea’s two biggest craters are on the side that faces away from the parent planet, are incredibly old and are from 400-500 km across.
- Rhea has a smaller impact crater that is around 48 km across and has extension rays that spread out from it. It has been nicknamed “The Splat” and is thought to be one of the youngest craters that exist on any of Saturn’s inner moons.