Pan moon is one of the ring shepherds of Saturn. Aside from the interesting role it plays, it is also a favorite of many because of its unmistakable shape. Let us dig deeper into the nature of this moon.
What Is a Ring Shepherd Moon?
The gas giant Saturn stands out because of its most defining features— its rings!
Aside from that it also has the most number of moons known to date, with 82 natural satellites in total. Fifty-three of these moons are already confirmed and were given names. The discovery of the remaining 29 is yet to be confirmed.
The moons of this ringed planet are diverse and unique. They are of different shapes and sizes. Some of them are just tiny moonlets while the others are gigantic. One of its moon, Titan, lives up to its name as it is even bigger than the closest planet to the Sun, Mercury.
Some of the moons of Saturn orbit within, or just beyond, its rings. We call them the ring shepherd moons. These ring shepherds are tiny moons with weird shapes. Most of them are characterized by some bulge or equatorial ridges.
Why are the shepherd moons called such? Though it has nothing to do with sheep, these moonlets are named because of the effects they have on the famous rings of Saturn. They help keep these rings in shape.
The rings of Saturn are made up of so many different materials. These materials are believed to be pieces of ice and rock from shattered celestial bodies such as asteroids, comets, or even moons. Because these materials are suspended in space, they can also move around Saturn. This is where the shepherd moons come in. Since these moonlets are orbiting within or near the rings, they shepherd the loose ring particles and put them in place.
The shepherding moons are also responsible for creating the gaps between the rings. These gaps are the path that these little moons take as they orbit around Saturn. As they make their way through these paths, they usher the loose materials back to the rings. Acting like shepherds, indeed!
Shepherd moons with the core ratio of one-third to one-half its size now are believed to be from a parental moon that disintegrated a long time ago. These cores were already dense that they were able to accumulate materials from the rings. The buildup of materials increased the size of these satellites. The equatorial ridges on the surface of shepherd moons also likely formed because of accretion.
The shepherd moons of Saturn are Pan, Daphnis, Atlas, Prometheus, and Pandora. Pan lies within the Encke gap (A ring) while Daphnis is in the Keeler gap (A ring). Atlas is also in A ring while Prometheus and Pandora are both in the F ring. Other inner satellites that act as ring shepherds are the co-orbital moons Janus and Epimetheus.
The Pan Moon
Pan Moon: Facts and Figures
- Discoverer – Mark Robert Showalter
- Discovery date – July 16, 199
- Designation – Saturn XVIII
- Pronunciation – /ˈpæn/
- Named after – Pān (God of the shepherds in Greek Mythology)
- Provisional designation – S/1981 S 13
- Adjectives – Pandean
- Semi-major axis – 133584.0±0.1 km
- Eccentricity – 0.0000144±0.0000054
- Orbital period – 0.575050718 d (13.801217 h)
- Inclination – 0.0001°±0.0004°
- Dimensions – 34.4 × 31.4 × 20.8 km
- Mean radius – 14.1±1.3 km
- Equatorial circumference – 88.6km
- Volume – 11,742 km³
- Mass – (4.95±0.75)×1015 kg
- Mean density – 0.42±0.15 g/cm³
- Surface gravity – 0.0001–0.0018 m/s2
- Escape velocity – Approximately 0.006 km/s
- Rotation period – Synchronous (tidally locked)
- Axial tilt – zero
- Albedo – 0.5
- Temperature – Approximately 78 K
Pan Moon Features
Pan is designated as Saturn XVIII. It is a small moon in the Encke Gap of Saturn’s A ring. This bizarrely shaped satellite is one of the shepherd moons of this ringed planet. It is considered the innermost moon of Saturn that was given a name.
Pan keeps the 322-km (200-mile) wide Encke Gap clear of stray debris and ring materials. It does so by giving gravitational “kicks” to loose particles that come its way. Waves are created because of those kicks which in turn intersect with each other, thus creating “wakes.” These “wakes” keep the materials together, maintaining the shape of Saturn’s rings.
Pan was discovered in 1990 by Mark Showalter. He is a scientist at the SETI Institute and the discoverer of three planetary rings and a total of six moons, including Pan.
We can easily distinguish Pan because of its walnut-like appearance. It is also said to be the reddest of Saturn’s inner moons.
With a mean radius of about 14.1 km, Pan is 451.8 times smaller than Earth. That is roughly the same size as New York. Pan orbits Saturn at a distance of about 134,000 kilometers (83,000 miles). This distance varies by about 4 km because of the eccentricity of this moon’s orbit.
It takes about 13.8 hours for Pan to orbit its planet. That is just a little longer than Saturn’s rotation period which takes about 10.2 Earth hours. This little moon is in a synchronous rotation with Saturn. Being tidally locked, the same side of the moon faces the planet all the time.
The names of Saturn’s moons are originally based on the Titans of Greek and Roman mythology. However, as more and more of them are getting discovered, scientists also started to get inspiration from other mythological stories.
Pan is a Greek god that is described as half man-half beast, like a satyr or faun. His Roman equivalent is Faunus. He is the god of the flocks and shepherds, among many things.
What Does Pan Look Like?
Saturn’s moon Pan is just one of its many odd-shaped satellites. The ridge around Pan’s equator had sparked the imaginations of many. Some even compare to a flying saucer!
Other than its resemblance to a walnut fruit, the appearance of the moon Pan is also often compared to items of Italian and Spanish cuisine. People came up with clever ways to call it, comparing it to a ravioli and an empanada.
Below is a side-by-side comparison of empanadas, Pan, and ravioli. See the resemblance?
Some call Pan a “space empanada.” An empanada is a type of bread or pastry that is stuffed with potatoes, chicken, egg, and other types of fillings. Ravioli, on the other hand, is a type of pasta that is stuffed with fillings such as cheese. It is a traditional food that originated in Italy.
Needless to say, these are unique and easy ways of describing and remembering this equally unique celestial object.
Aside from accretion, new research suggests that Pan’s strange geography could be the result of moonlet collisions. This theory is called the pyramidal regime formation scenario. It suggests that Pan (and other moons) may have formed from the merger of colliding moonlets.
The discovery of the Pan moon is attributed to M. R. Showalter. Even though this discovery was in 1990, the existence of this moon was already predicted. Five years earlier, in 1985, Jeffrey D. Scargle and Jeffrey N. Cuzzi noticed some wavy edges in the Encke Gap. This is an indication that a celestial body in this gap must be the source of this gravitational disturbance.
Showalter and his team further supported the prediction by inferring the mass and orbit of this moon. Their analysis was based on the photos sent by the Voyager 2 probe years and years back. The team examined all the images of Voyager 2 and found the tiny moon in 11 of the images sent by the probe’s 1981 flyby. Because of that, Pan was given the alternative name S/1981 S 13 at first.
Close-up and more defined images of Pan were taken by NASA’s Cassini spacecraft. The Cassini-Huygens mission launched on October 15, 1997. It focused on the study of the planet Saturn as well as its system and moons. The closest approach of this spacecraft to Pan was on March 7, 2017. Cassini was only about 24,600 kilometers (15,268 miles) from the walnut-looking moon at this point.
Looking at a more detailed Pan, Carolyn Porco of the Space Science Institute even shared her excitement on Twitter saying, “Nearing its end, Cassini delights again.” Cassini’s imaging team also said that the rounded shape of Pan’s central region was formed through material accretion when the ring system was still thicker.
Cassini was already running out of fuel. But it braved through its Grand Finale, a final mission to observe Saturn and its rings up close. Months after taking close-up images of Pan and other moons, the Cassini aircraft had to end its mission.
After making its 22 last orbits around Saturn and its rings, it purposely plunged into the atmosphere of the planet. It did so to protect Saturn’s natural satellites, as some of them hold the potential to support life. Some of the strongest candidates are the large moons Titan and Enceladus. The Cassini mission concluded on September 15, 2017.
Behind the Name
The god Pan was associated with the wild, field, forest, shepherds, and rustic music. It is for that reason that this name was chosen since the then-newly discovered space empanada tended to the rings as a shepherd herded his flocks. This little moon was officially given the name on September 16, 1991.
The rustic god Pan calls Arcadia his home. He is said to be the son of Hermes and an unidentified wood nymph. Others say that his parents are Apollo and Penelope. The pan flute is the sacred symbol of this god while his sacred animal is the goat. Interestingly, Pan is the only god in Greek mythology who dies.
The name “Pan” was sometimes used in the past for an irregular satellite on the planet Jupiter. That was around 1955 to 1975 when the said moon was not yet formally named. This moon is now called Carme, also known as Jupiter XI.