Prometheus (moon)
Prometheus is a small icy moon of Saturn that orbits along the inner side of the F ring. Its volumetric mean diameter is approximately 86 km, although the body itself is strongly elongated. Prometheus's gravity repeatedly pulls material out of the ring, creating bright streamers and dark channels observed by the Cassini spacecraft.[1][2][4]
Its influence extends beyond the structures visible in photographs. Cassini radio-occultation measurements indicate that orbital resonances with Prometheus help confine the F ring's dense core, which consists of a discontinuous chain of short arcs containing relatively large particles.[5]
Size and physical characteristics
Prometheus has a volumetric mean radius of 43.1 km and a mass of approximately 1.60 × 1017 kg. Its volume equals that of a sphere about 86 km across, but its greatest dimension is approximately 148 km because of its elongated and irregular shape.[1][6]
The moon is approximately 3.3 times less massive than Epimetheus and almost 12 times less massive than Janus. Its mean density is only 0.48 g/cm³, about half the density of solid water ice.[1]
The low density and bright icy surface are consistent with a body containing a large volume of voids. Prometheus is therefore not regarded as a compact block of solid ice; porosity is an important part of models of its internal structure.[1][2]
A spherical estimate gives a surface gravity of approximately 0.0058 m/s², about 1,700 times weaker than Earth's. The calculated escape velocity is only 22 m/s, or about 80 km/h. Local values vary because the moon is strongly elongated.[1]
Orbit
Prometheus orbits approximately 139,400 km from Saturn's centre. It lies outside the main A ring but inside the narrow F ring. One revolution takes approximately 14.8 hours.[3][2]
The orbit is nearly circular and lies close to the plane of the rings. Even its small eccentricity matters, however: changes in the relative orientation of the moon's orbit and the ring particles' orbits alter the distance and geometry of successive encounters.[3][5]
Pandora travels outside the F ring. Unlike Janus and Epimetheus, Prometheus and Pandora do not exchange orbits; their mutual motion is governed by a different set of orbital resonances.[7]
Surface and topography
Prometheus has an elongated outline marked by projections, ridges and depressions. Its surface contains impact craters as large as approximately 20 km, although it appears less densely cratered overall than Pandora, Janus and Epimetheus.[2]
Close views show rugged terrain beside smoother areas. On a body this small, the largest impact structures occupy a substantial fraction of the surface and affect the moon's overall shape rather than only its local topography.[8]
On 26 December 2009, Cassini photographed Prometheus from a distance of approximately 59,000 km. The image resolves its elongated form, large depressions and differences between the terrain of separate regions.[9]
Channels and streamers in the F ring
As Prometheus passes the F ring, it draws particles towards itself and changes their orbits. Elongated concentrations of material form streamers, while areas of lower particle density become channels. Orbital motion then stretches and deforms both structures.[4]
Repeated encounters produce a sequence of diagonal bands. This is a changing pattern in moving ring material: the particles continue around Saturn, and each channel evolves after the moon has moved away.[4]
Cassini images also reveal fan-shaped structures near some channels. They are associated with disturbances caused by small bodies or dense clumps embedded in the ring. Prometheus creates the large-scale channels, while objects within the ring add smaller local features.[6]
Confinement of the F-ring core
The F ring has a dense core that is in places less than 1 km wide. Much of the ring's mass is concentrated in a discontinuous chain of arcs containing particles with radii greater than a few millimetres. Micrometre-sized dust dominates ordinary photographs but accounts for only a small fraction of the total mass.[5]
These arcs can be confined by a resonance with Prometheus, in which their orbital motion remains aligned with a particular part of the moon's orbit. The interaction can preserve narrow concentrations for decades or longer. Small chaotic changes in the moon's orbit may disrupt this balance and rearrange the arcs.[5]
A 2025 model combined resonant confinement with collisions among the ring particles. In this model, Prometheus confines part of the material, while aligned motion and collisions can oppose the ring's ordinary spreading through a process described as negative diffusion.[10]
Interaction with Pandora
Prometheus and Pandora gravitationally perturb one another. Their complex motion arises from the 121:118 mean-motion resonance, which is divided into several overlapping components. This overlap produces chaotic variations in their orbital longitudes and mean motions.[7]
The effect became apparent when observations from different periods were compared: the moons' positions gradually diverged from predictions based on Voyager-era measurements. Accurate calculations therefore require dynamical models that include their mutual attraction and other perturbations in the Saturn system.[7]
Prometheus ringlet
Cassini detected a faint ringlet close to the orbit of Prometheus. Its mean orbital radius ranges from approximately 139,300 to 139,400 km, consistent with particles held in a 1:1 co-orbital resonance with the moon.[11]
The ringlet's eccentric orbit slowly turns at almost the same rate as the F ring rather than at the rate expected near Prometheus. Collisions between ringlet particles and F-ring material may contribute to this behaviour, but the interaction has not yet been fully explained.[11]
Discovery and exploration
Prometheus was discovered by the Voyager 1 science team in October 1980. Its provisional designation was S/1980 S27, and it is also known as Saturn XVI. The moon is named after Prometheus, the son of the Titan Iapetus who brought fire to humanity in Greek mythology.[2]
Cassini studied both the moon and the ring structures that form near it. Image sequences traced the development of channels, streamers and clumps after successive passages of Prometheus. Radio occultations separately detected dense parts of the ring core that are difficult to distinguish among the bright dust in ordinary images.[4][5]
The spacecraft obtained a detailed close view on 6 December 2015. These images supplemented earlier observations of the moon's shape and craters and allowed different regions of its surface to be compared.[8]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Planetary Satellite Physical Parameters // NASA JPL. Prometheus: gravitational parameter from the SAT415 solution. Mass calculated from GM using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Prometheus // NASA Science.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Planetary Satellite Mean Elements // NASA JPL. Prometheus: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 4.2 4.3 Dance of Particles in Saturn’s F Ring // NASA/JPL. 20 July 2010.
- ↑ 5.0 5.1 5.2 5.3 5.4 Cuzzi J. N. et al. Saturn’s F ring is intermittently shepherded by Prometheus // Science Advances. 2024. Vol. 10, issue 19. Article eadl6601. DOI: 10.1126/sciadv.adl6601.
- ↑ 6.0 6.1 Multiple F-Ring “Fans” // NASA/JPL/Space Science Institute. 20 July 2010.
- ↑ 7.0 7.1 7.2 Goldreich P., Rappaport N. Origin of Chaos in the Prometheus–Pandora System // Icarus. 2003. Vol. 166, issue 2. P. 320–327.
- ↑ 8.0 8.1 Prometheus Up Close // NASA/JPL-Caltech/Space Science Institute. 2015.
- ↑ Prometheus 12-26-09a.jpg; description of the Cassini image // NASA/JPL/Space Science Institute.
- ↑ Alrebdi A., Esposito L. W. Saturn’s F Ring Is Confined by Prometheus and Negative Diffusion // Icarus. 2025. Vol. 441. Article 116709.
- ↑ 11.0 11.1 Hedman M. M., Carter B. J. A Curious Ringlet That Shares Prometheus’ Orbit but Precesses like the F Ring // Icarus. 2017. Vol. 281. P. 322–333.
