In the sea of natural satellites orbiting the ringed world lies a group of three small moons known as the Alkyonides. One of them is the moon Pallene. Curious about it? Let us read on to learn about it.
The Cassini-Huygens Mission
The Cassini-Huygens mission of NASA, ESA (European Space Agency), and ASI (Italian Space Agency) is by the far the most extensive exploration ever done in the Saturnian system. Of its nearly 20-year journey, it spent 13 years exploring Saturn as well as its rings and moons. It also provided us a peek into the system of Jupiter.
Cassini’s most precious targets were Titan and Enceladus as they possess conditions that may support life. At the end of its mission, the spacecraft plunged into Saturn to prevent damaging these potentially habitable bodies.
Before the mission ended on September 15, 2017, Cassini had observed thin atmospheres around Rhea and Dione. This remarkable feat also provided close-up looks on the extraordinary surfaces of moons such as the sponge-like Hyperion and the ridged Iapetus. It discovered seven moons, including Daphnis within one of Saturn’s rings, and the new group of moons called the Alkyonides.
What is an Alkyonide Moon?
The Alkyonides are a group of three tiny moons orbiting the gas giant Saturn. Remembering this bunch is easy since it only has three member moons: Pallene, Methone, and Anthe. They are named after the daughters of the giant Alkyoneus in Greek mythology.
These moons are relatively newly discovered. Pallene and Methone were discovered in 2004. Anthe’s discovery was three years later, in 2007, though searching through older images revealed that it was actually photographed in 2004 as well.
The Alkyonides are a tight-knit group, orbiting Saturn at more or less similar distances. Locating these small bodies in the sea of Saturnian moons can be quite challenging. However, we can use other more prominent moons as references in looking for the small Alkyonides.
Pallene, Methone, and Anthe are in the broad yet diffuse E ring of Saturn. They lie between the orbits of the bigger moons Mimas and Enceladus. These moons affect the orbits of the Alkyonides, causing them to vary.
Scientists have proposed two theories on how the Alkyonides came to be:
Pallene, Methone, and Anthe are probably fragments from either of their bigger neighbors, Mimas and Enceladus.
The Alkyonides, Mimas, and Enceladus are, possibly, remains of a big swarm of astronomical bodies that had wandered near the Saturnian system a long time ago.
The Pallene Moon
Pallene Moon: Facts and Figures
All About the Name
- Pronunciation – /pəˈliːniː/
- Behind the name – Pallēnē (Daughter of the Giant Alkyoneus)
- Adjective/s – Pallenean
- Other designation – Saturn XXXIII
- Provisional designation/s – S/1981 S 14; S/2004 S 2
- Discoverer – Cassini Imaging Team/ Voyager)
- Discovery date – June 1, 2004/ August 23, 1981
- Parent Planet – Saturn
- Orbital period – 1.153745829 day/s
- Average Orbit Distance – 212, 280 ± 5 km
- Mean Orbit Velocity – 48,158.2 km/h
- Orbit Eccentricity – 0.0040
- Equatorial Inclination – 0.1810 ± 0.0014° (to Saturn’s equator)
- Dimensions – 5.76 × 4.16 × 3.68 ± 0.14 km
- Equatorial radius – 2.5 km
- Equatorial circumference – 15.7 km
- Volume – 65 km3
- Mass – 32,970,705,528,138 kg
- Surface area – 78.54 km2
- Surface gravity – 0.00035 m/s2
- Escape velocity – 5 km/h
- Rotation – Synchronous (tidally locked)
- Axial tilt – Zero
Pallene Moon Features
Pallene is a tiny moon that is part of the Alkyonides group together with Methone and Anthe. It was discovered in 2004 by the Cassini Imaging Team, but later on, it was realized that the moon was photographed way back in 1981. Pallene is also called Saturn XXXIII.
Pallene resides in Saturn’s second furthest ring along with the other Alkyonides. The three of them are nestled between the round-shaped Mimas and Enceladus. These more massive moons perturb the trio and cause changes in their orbits. Methone and Anthe, even the moon Aegaeon, are affected by Mimas while Enceladus influences Pallene.
Properties of this moon including temperature, albedo or reflectivity, and geological features, among others, are yet to be determined.
The Cassini space probe revealed a lot about Saturn’s moons and the system as a whole. Through its priority were the moons Titan and Enceladus, it had made two flybys to the small Pallene moon. Its closest approach was on 16 October 2010 when it was 36,000 km away. Another flyby was made on 14 September 2011 at a distance of 44,000 km.
Pallene was discovered by S. Charnoz, C. Porco, and the Cassini Imaging Team on June 1, 2004. Cassini was 16.5 million km at the time, making the moon look like a faint dot in the background of brighter objects. It was given the provisional designation S/2004 S 2.
The team later learned that the moon had actually been seen in an older photograph taken by the Voyager 2 probe way back on August 23, 1981.
The small moon appeared only once in the 1981 images. Though data was limited during that time, its approximated orbit around Saturn was 200,000 km. It was given the temporary designation S/1981 S 14.
The Cassini image of 2004 is often acknowledged for Pallene’s discovery. Some sources also cite and recognize the findings of Voyager 2 in 1981, referring to them as the first and the second discovery of Pallene.
Did You Know?
Pallene and Methone have the same discovery dates. They were the first natural satellites discovered in the images sent by the Cassini spacecraft.
Only 18 moons of Saturn were known before the Cassini-Huygens mission launched in 1997— far less than the 82 moons that we know now!
Pallene’s mean radius is 2.5 km. It is larger than the other two Alkyonide moons, being 1.6 times bigger than Methone and nearly three times larger than Anthe. The dimensions of this small natural satellite are about 5.76 × 4.16 × 3.68 km. Its circumference is 15.7 km.
Being the fifth largest moon in the solar system, our planet’s Moon is 695 times bigger than Pallene.
Not much is known about the composition of Pallene yet.
Pallene orbits Saturn at a distance of about 212,000 km or 132,000 miles. It completes a journey around the ringed planet in 1.15 Earth days or about 27.7 hours.
A mean-longitude resonance with Enceladus perturbs Pallene. Because of this, its semi-major axis changes by about 4 km in amplitude. This effect, however, is not as strong as Mimas’s influence on Methone which causes it to vary at an amplitude of roughly 20 km.
Pallene’s inclination is affected by this perturbation, varying between 0.178° and 0.184°. Similarly, its eccentricity changes between 0.002 and 0.006 as observed in different periods.
Pallene is responsible for the formation of a faint ring that circles Saturn. This moon-made structure is called the Pallene Ring. Aside from that, Pallene and the rest of the Alkyonides are believed to be also among the astronomical bodies contributing material to the wide E ring.
What Does Pallene Look Like?
There is still a lot to learn about the Pallene moon considering that it was just recently discovered. Observing its surface to look for clues about its composition is also quite a challenge because of its small size.
While a close-up image of its sister moon Methone shows a smooth and egg-like body, available images that we have of Pallene do not reveal much about the satellite.
The best image that we have of this moon hints that it may be ellipsoidal in shape. Both its dayside and nightside are shown. It was taken during Cassini’s flyby on October 16, 2010, when the spacecraft was 36,000 kilometers or 22,000 miles from the moon.
Did You Know?
When talking about natural satellites, dayside means the face of the moon where we can see the moonglow, as it receives sunlight, while nightside is the part that is in darkness.
Just like Pallene, our Moon is tidally locked to the Earth. That means its dayside will always face our planet, while the nightside will always face away from the Earth. It is a myth that the far side of the moon lies in perpetual darkness. It does get light from the Sun, we just can’t see it from Earth. Take a Solar Eclipse for example, where the Sun is blotted out by the moon, the far side of the moon is illuminated then.
The Pallene Ring
The Pallene Ring is a new addition to the band of icy particles surrounding the planet. Particles expelled from the moon’s surface during meteoroid impacts are the source of the ring’s formation. It was discovered by the Cassini probe in 2006.
The ring is co-orbital to the eponymous moon. It was seen in the Cassini images as a faint and narrow structure that is about 2,500 km wide. It is roughly 211,000 –213,500 km distant from Saturn’s center.
The other Alkyonides also have ring structures coinciding with their orbits. However, they are incomplete and partial rings unlike Pallene’s. They are the Methone Ring Arc and the Anthe Ring Arc. They are believed to be caused by the same kinds of impacts that led to the Pallene Ring’s formation.
Behind the Name
Pallene had two provisional designations before it was awarded the name as we know it today. It was S/1981 S 14 in the Voyager 2 images and S/2004 S 2 in the images of the more recent Cassini probe.
The permanent name of this moon was tentatively approved by the International Astronomical Union (IAU) in 2005 and was ratified the next year. The name was based on Greek mythology’s Alkyonides, the seven daughters of the Giant Alkyoneus (Alcyoneus).
The Alkyonides of Greek Mythology
Mother Gaea (Gaia) conceived the Giants (Gigantes) from Uranus’ blood. He was castrated by his son Cronus when the Titans took over in ruling the world.
One of the best-known Giants is Alkyoneus. He had seven beautiful daughters called the Alkyonides: Pallene, Methone, Anthe, Alkippe, Asteria, Drimo, and Phthonia.
As if history was repeating itself, the Titans’ rule did not last long. Zeus and the Olympians overpowered Cronus. It resulted in many battles, including the Gigantomachia or Gigantomachy.
Alkyoneus fought along with other Giants against the Olympians in this war. Unfortunately, the Giant was killed by the hero Heracles.
The Alkyonides were heartbroken about their father’s death that they jumped into the sea. The sea goddess Amphitrite rescued them just in time and turned them into halcyons.
- Saturn moons (selected destination): https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens#Itinerary
- Anthe moon; discovery; what it looks like:: https://en.wikipedia.org/wiki/Anthe_(moon)#cite_ref-rhodanthe_2-0
- Pallene moon: https://en.wikipedia.org/wiki/Pallene_(moon)
- Methone moon: https://en.wikipedia.org/wiki/Methone_(moon)
- Pallene Moon: https://solarsystem.nasa.gov/moons/saturn-moons/pallene/in-depth/
- Pallene Ring: http://ciclops.org/view.php?id=2276&js=1
- Alkyoneus vs Hercules: https://en.wikipedia.org/wiki/Alcyoneus