Pan (moon)
Pan is a small icy moon of Saturn orbiting inside the Encke Gap in the planet's A ring. Its gravity maintains this approximately 325 km-wide opening and produces recurring structures in the neighbouring ring material. Pan has a volumetric mean diameter of about 28 km and completes one orbit in 13.8 hours.[4][1][3]
The moon is distinguished by an equatorial ridge that surrounds its central body and gives it a broad, projecting rim. The Cassini spacecraft obtained detailed images of this structure during a close flyby in March 2017.[5]
Size and physical characteristics
Pan has a volumetric mean radius of 14 km, but its shape differs markedly from a sphere. Estimates based on early Cassini images give an equatorial diameter of approximately 33 km and a polar dimension of about 21 km.[1][6]
Its mass is estimated at approximately 4.2 × 1015 kg, although the uncertainty is large at about 50%. The tabulated mean density is 0.37 ± 0.21 g/cm³. The central value is consistent with a porous ice-rich body, but the substantial uncertainty in mass limits the precision of any estimate of its internal void space.[1]
Using the central values for mass and radius, a spherical estimate gives a surface gravity of about 0.0014 m/s² and an escape velocity of approximately 6 m/s, or 23 km/h. These values are approximate because they are affected by the uncertain mass, irregular shape and rotation of the moon.[1]
Orbit and the Encke Gap
Pan orbits at a mean distance of approximately 133,600 km from Saturn's centre. Its orbit is nearly circular and lies close to the plane of the rings. The tabulated orbital period is 0.575051 Earth days, or approximately 13 hours 48 minutes.[3]
The Encke Gap is a narrow opening within the A ring. Pan acts as a shepherd moon: during encounters with ring particles, its gravity changes their motion and prevents the main ring material from filling the gap.[4]
The gravitational influence of the small moon extends far beyond its own diameter. Regular disturbances along the gap's edges and in the adjoining ring allowed the existence of Pan to be predicted before it was identified in spacecraft photographs.[7]
Gravitational wakes in the A ring
Ring particles inside Pan's orbit move faster than the moon and overtake it. Their paths are altered during each close passage. Material outside the orbit also responds to Pan's gravity, producing ordered bands of denser material known as gravitational wakes.[8]
Cassini images show that the spacing between these bands increases with distance from the gap. The structures preserve a record of the moon's earlier passages. An image obtained on 20 May 2005 revealed disturbances created about four months before the observation, when Pan had occupied a different part of its orbit.[8]
Equatorial ridge and surface
Pan's relatively rounded central body is surrounded by a projecting equatorial ridge. The ridge is smoother than the central terrain, where grooves and fractures are visible. Its outline varies around the moon and is polygonal in some regions.[9]
Early estimates placed the ridge within approximately 15–20° of latitude on either side of the equator and assigned it about 10% of the moon's total volume. Its height above an idealised smooth central body was estimated at roughly 1.5 km along much of the equator and as much as 4 km in one direction.[6]
The neighbouring moon Atlas has a more prominent equatorial expansion relative to the size of its central body: the corresponding estimate assigns about 27% of its volume to the ridge. Despite their broadly similar appearance, the ridges of the two moons differ in proportions and outline.[6]
Accumulation of ring material
Pan gradually acquires particles from the surrounding rings. Together with Daphnis, it belongs to the moons whose surfaces have been altered most strongly by this process.[2]
Porous material collects around a denser central body and forms an equatorial belt. In this model, the characteristic ridge grows through accretion: the capture of particles from the neighbouring ring material.[2]
Models of ridge growth on Pan, Atlas and Daphnis show that the final shape depends on the distribution of particles and the conditions under which they strike the moon. Accretion from an asymmetric gap can produce several projecting regions like those observed around Pan's equator.[9]
Discovery and name
Mark Showalter discovered Pan in 1990 on Voyager 2 images taken in 1981. Before it was seen directly, researchers had identified wavy edges in the Encke Gap and attributed them to the gravity of an unknown moon.[4][7]
In 1985, Jeffrey Cuzzi and Jeffrey Scargle published evidence for a possible moon within the gap. Analysis of the ring disturbances refined its predicted orbit in 1986. When the Voyager image archive became available on compact discs, Showalter searched the calculated positions and found the moon as a bright point in several frames.[7]
The object received the provisional designation S/1981 S13 and was later named Pan after the Greek god of nature and shepherds. It is also designated Saturn XVIII.[4][10]
Cassini observations
On 7 March 2017, Cassini made its closest flyby of Pan during the mission, passing at a minimum distance of approximately 24,572 km. The images resolved the fine structure of the equatorial ridge and features on the central body.[5]
Colour differences in the composite image of Pan were enhanced to reveal variations in the surface material that would be too subtle for the human eye to distinguish directly.[11]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Planetary Satellite Physical Parameters // NASA JPL. Pan: GM = 0.00028 ± 0.00014 km³/s² from the SAT415 solution. Mass calculated using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.0 2.1 2.2 NASA’s Cassini Finds Saturn’s Rings Coat Tiny Moons // NASA/JPL-Caltech. 28 March 2019.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Planetary Satellite Mean Elements // NASA JPL. Pan: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Pan // NASA Science.
- ↑ 5.0 5.1 Cassini Reveals Strange Shape of Saturn’s Moon Pan // NASA/JPL-Caltech. 9 March 2017.
- ↑ 6.0 6.1 6.2 Saturn’s Saucer Moons // NASA/JPL/Space Science Institute. 6 December 2007. PIA08405.
- ↑ 7.0 7.1 7.2 Showalter M. You’ve Met Saturn’s Ravioli-Shaped Moon, Pan. Now Learn How It Was Discovered // The Planetary Society. 13 March 2017. Account by the discoverer.
- ↑ 8.0 8.1 Resonant Effects // NASA/JPL/Space Science Institute. 1 July 2005. PIA07533.
- ↑ 9.0 9.1 Quillen A. C., Zaidouni F., Nakajima M., Wright E. Accretion of Ornamental Equatorial Ridges on Pan, Atlas and Daphnis // Icarus. 2021. Vol. 357. Article 114260.
- ↑ Planetary Satellite Discovery Circumstances // NASA JPL.
- ↑ Pan Color PIA21449 // NASA/JPL-Caltech/Space Science Institute. Image description on Wikimedia Commons.
