Atlas (moon)
Atlas is a small icy moon of Saturn orbiting immediately beyond the outer edge of the A ring. A broad ridge around its equator gives the moon an unusually flattened shape: from the side it resembles a disc with a thickened centre. Atlas has a volumetric mean diameter of approximately 30 km and completes one orbit in about 14.5 hours.[4][1][3]
Observations by Cassini linked the moon's shape to the accumulation of ring material. Ice and dust particles settle on small moons near the rings and form loose surface layers that are particularly prominent around the equator.[2]
Size and physical characteristics
Atlas has a volumetric mean radius of 15.1 km and a mass of approximately 5.54 × 1015 kg. Its volume equals that of a sphere 30.2 km across, but the equatorial diameter reaches approximately 39 km, while the distance between the poles is only about 18 km. The moon is therefore more than twice as wide as it is thick.[1][5]
Its mean density is approximately 0.39 g/cm³, far below the density of solid water ice. This is consistent with a porous interior containing a large volume of voids.[1] Cassini's close observations also indicate that the surface material of the small ring moons is highly porous.[2]
A spherical estimate gives a surface gravity of approximately 0.0016 m/s², about 6,000 times weaker than Earth's. The calculated escape velocity is only about 7 m/s, or 25 km/h. The body's flattened shape and rotation affect local conditions at the surface.[1]
Equatorial ridge
Atlas's defining feature is a broad equatorial ridge surrounding its central body. Estimates based on images obtained between 2005 and 2007 place its extent at approximately 20–30° of latitude on either side of the equator.[5]
The ridge rises an estimated 3–5 km above an idealised smooth central body and accounts for approximately 27% of the moon's total volume. Its outline varies with longitude and is more rounded in some regions. These measurements describe the large-scale shape reconstructed from early Cassini images.[5]
A similar ridge surrounds Pan, which orbits inside the Encke Gap in the A ring. Relative to the volume of the central body, Atlas's equatorial expansion is even more pronounced.[5]
Formation and surface material
Small moons within and near Saturn's rings collect dust and ice particles. Loose material accumulates around a denser central body. When most of it settles close to the equator, it gradually builds a broad ridge and produces Atlas's distinctive shape.[2]
Not all of the coating originates in the nearest rings. Observations also connect the surface material with icy particles ejected by Enceladus and distributed throughout the Saturn system.[2]
The equatorial ridges of Atlas, Pan and Daphnis may all have grown by collecting ring particles. Their shapes depend on the moon's orbit, the distribution of material near the edge of a ring and changes in the body's orientation. Uneven delivery of particles from the nearby gap may have produced a depression in the ridge of Atlas.[6]
Orbit
Atlas orbits at a mean distance of approximately 137,700 km from Saturn's centre. Its orbit is nearly circular and lies close to the plane of the rings. The tabulated orbital period is 0.604602 Earth days, or about 14 hours 31 minutes.[3]
Prometheus orbits farther from Saturn, with a semi-major axis of approximately 139,400 km. The difference between their mean orbital distances is about 1,700 km.[3] Their mutual gravity produces resonant perturbations in the motion of Atlas.[7]
Orbital resonances
The 54:53 resonance with Prometheus is particularly important for Atlas. Two coupled components of this resonance produce chaotic variations in its orbit. The main features of the motion can be reproduced using only the gravity of Prometheus and Saturn's oblateness, without modelling every other moon in detail.[7]
Atlas is also affected by the 70:67 resonance with Pandora. The domains of the Prometheus–Atlas and Pandora–Atlas resonances overlap near the moon's present orbit. Dynamical models characterise the resulting motion as weakly to moderately chaotic rather than strongly unstable.[8]
Discovery and exploration
Atlas was discovered in 1980 by Richard Terrile and the Voyager 1 science team in images obtained during the spacecraft's encounter with Saturn. Its provisional designation was S/1980 S28, and it is also known as Saturn XV. The moon was named after Atlas, the Titan of Greek mythology who held up the sky.[4][9]
On 12 April 2017, Cassini passed approximately 11,000 km from Atlas and obtained the closest images of the moon during the mission. The narrow-angle camera resolved the flattened central body and the broad equatorial ridge in detail.[10][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. Atlas: GM = 0.00037 ± 0.00001 km³/s² from the SAT415 solution. Mass calculated using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.0 2.1 2.2 2.3 2.4 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 3.7 Planetary Satellite Mean Elements // NASA JPL. Atlas: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 4.2 4.3 Atlas // NASA Science.
- ↑ 5.0 5.1 5.2 5.3 Saturn’s Saucer Moons // NASA/JPL/Space Science Institute. 6 December 2007. PIA08405.
- ↑ 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.
- ↑ 7.0 7.1 Renner S. et al. Origin of the Chaotic Motion of the Saturnian Satellite Atlas // The Astronomical Journal. 2016. Vol. 151. Article 122.
- ↑ Ceccatto D. T., Callegari N. Jr., Guimarães G. T., Gimenez K. Orbital Dynamics of Atlas (S XV): Its Current Orbit and the Recent Past // Planetary and Space Science. 2025. Vol. 255. Article 106016.
- ↑ Saturn Moons // NASA Science. Table of moons.
- ↑ Cassini Timeline: “Flying-Saucer” Moon Atlas // NASA Science. Event of 12 April 2017.
- ↑ Atlas, 2017, Cassini N00279648 // NASA/JPL-Caltech/Space Science Institute. Wikimedia Commons.
