What is an Alkyonide Moon?
Saturn herds a big batch of moons, of which the Alkyonide moons such as the Methone moon, are just one category. Having 82 of them in total (so far), it surpassed the previous holder, Jupiter, of having the most known natural satellites in the solar system. With that said, scientists found a way to classify them accordingly.
Moons of Saturn by Orbital Groups
- Ring moonlets
- Ring shepherds
- Inner large moons
- Trojan moons
- Outer large moons
- Irregular moons
- 8.1. Inuit group
- 8.2. Gallic group
- 8.3. Norse group
Ten moon groups can be formed by looking at the orbital characteristics of Saturn’s moons. One of these groups is the Alkyonides. What are these objects exactly?
An Alkyonide is one of the three tiny moons of Saturn that lie between the bigger satellites Mimas and Enceladus. The moons that make up this trio are Methone, Pallene, and Anthe. So far, Methone is the only one that has been seen up close.
Into Saturn’s Rings
Saturn is not the only planet in our solar system with rings. While the terrestrial planets, Mercury, Venus, Earth, and Mars have none, the outer worlds have their own sets of this marvelous sight. Rings are found on Jupiter as well as on the ice giants Uranus and Neptune. However, none of them is as intricate as the Saturnian system.
The ring system of Saturn is, in itself, a complex world. It may seem like a solid structure but it is actually made up of particles of water ice and some rocky material. These particles can be as small as dust or as large as houses or even mountains. They orbit around Saturn at unique speeds.
Starting from the closest one to the planet, the rings are D, C, B, A, F, G, and E Ring. A very faint addition called the Phoebe Ring was recently discovered just beyond the E Ring.
A, B, and C are considered the main rings. They are the densest and most prominent regions of the system. Rings D, G, and E are called dusty rings. They are more diffuse and contain smaller particles as compared to the main rings. The F ring has the characteristics of both groups.
Did You Know?
Although the rings of Saturn are reflective due to the fact that they are mostly made of water ice, we cannot see them with the naked eye.
Saturn’s rings were observed for the first time in 1610 by Italian astronomer Galileo Galilei using his telescope.
Saturn’s Alkyonides orbit within the E ring, the planet’s farthest ring. It is very wide but also very tenuous. Saturn’s innermost large moons, Mimas, Enceladus, Tethys, and Dione, also inhabit this faint ring.
Saturn’s Alkyonides have rings alongside their orbits. While Pallene’s ring is a complete circle, the rings of Methone and Anthe are more like arc structures. They are extremely faint and challenging to see at times.
The names of the three moons are based on the story of the Alkyonides (Alcyonides), which their group is named after. This tale in Greek mythology centers on the daughters of a giant named Alkyoneus (Alcyoneus).
The Methone Moon
All About the Name
- Pronunciation – /mɛˈθoʊniː/
- Behind the name – Methōnē (Daughter of Alkyoneus)
- Adjective/s – Methonean
- Other designation – Saturn XXXII
- Discoverer –
- S. Charnoz,
- C. Porco,
- Cassini Imaging Team
- Discovery date – June 1, 2004
- Parent Planet – Saturn
- Orbital period – 1.009573975 day/s
- Average Orbit Distance – 194 440 ± 20 km
- Mean Orbit Velocity – 50,400.3 km/h
- Orbit Eccentricity – 0.0001
- Equatorial Inclination – 0.007° ± 0.003° (to Saturn’s equator)
- Dimensions – (3.88 ± 0.04) × (2.58 ± 0.08) × (2.42 ± 0.04) km
- Equatorial radius – 1.6 km
- Equatorial circumference – 10.1 km
- Volume – 17 km3
- Mass – 8,992,010,598,583 kg
- Surface area – 32.17 km2
- Density – 0.5 g/cm3
- Surface gravity – 0.00023 m/s2
- Escape velocity – 3 km/h
- Rotation – Synchronous (Tidally locked)
- Axial tilt – Zero
Methone is an egg-shaped moonlet orbiting the gas giant Saturn. This natural satellite is also known as Saturn XXXII. It was discovered in June 2004 by the Cassini Imaging Team. Before it was awarded its name, it was given the provisional designation S/2004 S 1.
Did You Know?
Methone’s provisional designation reads, “The first new satellite of Saturn discovered in 2004.”
The temporary designation is dropped when a moon is finally confirmed and is awarded a permanent name.
One of the most remarkable features of Methone is its smooth surface. The images sent by NASA’s Cassini spacecraft did not reveal any craters on its unblemished appearance. Its neighbor, Pallene, and the small moon, Aegaeon, are thought to be similarly smooth.
Two theories are proposed on how Methone and the other Alkyonides came to be. Since the three moons are between the bigger Mimas and Enceladus, there is a probability that they might be fragments from those moons. The other explanation is that the Alkyonides, together with Mimas and Enceladus, may all have the same origin, and are remains of a larger group.
Methone’s albedo is different in some regions but its color is uniform throughout. The brighter area of the moon has an albedo of about 0.70 while a contrasting region on its leading point is about 0.13 darker.
Being in a synchronous rotation with Saturn, only one side of Methone always faces the planet while the other stays in darkness. Our Moon also has this relationship with the Earth.
Did You Know?
Though Europa, one of Jupiter’s moons, is in synchronous rotation with the planet, its nightside is not in perpetual darkness.
It is believed that the ocean-filled moon glows as a result of radiation blasted by Jupiter.
Methone was discovered on June 1, 2004, together with the Alkyonide moon Pallene. These moons were the first of the many discoveries made based on images sent by the Cassini-Huygens mission.
Methone was only visited twice by the Cassini orbiter probe. The unmistakably oblong shape and smooth surface of this moon were revealed in great detail to the Cassini Imaging Team during one of the spacecraft’s flybys to the tiny satellite.
It was on the 20th of May 2012 that the closest approach to Methone took place. The Cassini spacecraft was about 4,000 km from the moon during that visit. Images taken on that flyby remain the best view that we have of Methone.
The moon Methone has a mean radius of 1.6 km. Among the Alkyonides, it is 1.6 times smaller than Pallene yet 1.8 times larger than the moon Anthe.
The estimated density of Methone is very low. This, together with its smooth surface, gives us a clue of its composition. The moon is likely made up of some icy fluff which allows for mobility, hence its crater-free appearance.
The orbit of Methone is affected by the larger moon Mimas making it vary by about 20 km. Its inclination and eccentricity also show variations of some degree.
This small satellite orbits roughly 194,000 km or 120,546 miles away from Saturn. It completes a trip around the planet in about 24 hours, just like the Earth.
Methone, Pallene, and Anthe are believed to be among the particle-contributing bodies of the E Ring. This is yet to be confirmed as much is still unknown about them.
What Does Methone Look Like?
The first thing that would come to an observer’s mind when looking at photographs of Methone is how closely it resembles an egg. That was what the Cassini Imaging Team also thought when they had the first close-up look of the moon in May 2012.
The Cassini probe images revealed no craters on the surface of the moon during its closest approach. It was indeed very smooth just like an egg!
Since Methone is a small celestial body, it does not have enough gravity to pull itself into a spherical shape, unlike Saturn’s largest moons Titan and Rhea. A great size does not guarantee a spherical shape, though. We can think of the moon Hyperion for example. This satellite is one of the biggest moons with a non-round shape.
Did You Know?
Another thing that makes Hyperion unique is its sponge-like appearance and its chaotic rotation. This type of rotation was also observed in two of Pluto’s moons, Hydra and Nix.
The Methone Ring Arc
A partial ring structure was discovered in the orbit of Methone in September 2006. It is called the Methone Ring Arc. It is made up of dust from the small Alkyonide moon.
The material forming the arc is believed to be blown off the moon as a result of impacts from small meteoroids (micrometeoroids). This ring’s longitudinal extent covers approximately 10 degrees, half the size of the Anthe Ring Arc.
This partial ring may be one of the reasons why Methone’s surface is smooth. It might be that particles from it settle on the moon’s surface and cover its craters. The same event can be linked to other moons of Saturn like Pan and Atlas. However, their accumulated material is more concentrated around the equator, resulting in equatorial ridges.
Behind the Name
The moon was given the provisional designation of S/2004 S 1. Just about a year after its discovery, the International Astronomical Union (IAU) approved the name Methone for this moon on January 21, 2005. The name was based on Greek mythology’s Alkyonides. It was ratified in 2006.
Alkyoneus and the Alkyonides
Alkyoneus was one of the Gigantes or the Giants. His daughters are called the Alkyonides. There are seven of them, namely Methone, Pallene, Anthe, Alkippe, Asteria, Drimo, and Phthonia.
Alkyoneus’s story appears in several different versions. One of them was told by mythographer Apollodorus.
Being one of the Giants, Alkyoneus fought the Olympian gods in the Gigantomachy. It was there that he became known as the traditional opponent of the famous hero Heracles.
One of the giant’s strengths is that he was immortal as long as he remains in his homeland. Unfortunately, Heracles was able to drag Alkyoneus out of his native land after shooting him with his arrow. The giant died which broke his daughters’ hearts.
The Alkyonides jumped into the sea upon hearing the news of their father’s death. The goddess of the sea, Amphitrite, later on, turned them into halcyons.
By the Numbers | Methone – NASA Solar System Exploration
In Depth | Saturn – NASA Solar System Exploration
Astrophile: Saturn’s egg moon Methone is made of fluff | New Scientist
- Rings of Saturn: https://kids.britannica.com/students/assembly/view/93046
- Methone moon; discovery image: https://en.wikipedia.org/wiki/Methone_(moon)
- Egg: http://www.replica.co.uk/item.php?item=eg007
- Alkyoneus vs Heracles: https://en.wikipedia.org/wiki/Alcyoneus