Calypso (moon)
Calypso is a small icy moon of Saturn that travels approximately 60° behind Tethys in the same orbital region. It is one of Tethys's two Trojan moons; the other, Telesto, travels ahead of the larger moon. Calypso has a mean diameter of approximately 21 km.[1]
Cassini images revealed an irregular body with subdued depressions and a bright surface covering. Spectroscopic observations place Calypso among the Saturnian moons whose surfaces are particularly rich in water ice.[3][2]
Size and shape
Calypso has a mean radius of approximately 10.7 km, corresponding to a diameter of 21.4 km. Its outline is irregular and shaped by large projections and depressions rather than a smooth spherical surface.[1][3]
Some depressions may be overlapping impact craters. Loose surface material softens their outlines, so the images do not show the dense pattern of sharply defined crater rims seen on many other Saturnian moons.[1][3]
Orbit and Trojan motion
Calypso orbits Saturn at a mean distance of approximately 295,000 km from the planet's centre. One revolution takes 1.887803 Earth days, or about 45 hours 18 minutes. Its orbit is nearly circular, with a mean eccentricity of approximately 0.001.[4]
The moon occupies the region around the L5 Lagrange point of the Saturn–Tethys system. Calypso and Tethys are in a 1:1 orbital resonance: their mean periods are practically identical, while Calypso librates around a position 60° behind the larger moon.[5]
Telesto occupies the leading L4 region. A similar pair accompanies Dione: Helene travels ahead of it and Polydeuces follows behind. These moons provide examples of small bodies sharing an orbital resonance with a much more massive neighbour.[5]
Surface and icy covering
Calypso's visible surface has subdued topography. Detailed images from 2010 show a less prominent pattern of craters than is typical of many small Saturnian moons. Telesto has a similarly smooth appearance.[3]
Its spectrum is dominated by water ice, with comparatively little material that alters the surface colour. Cassini's VIMS measurements place Calypso and Enceladus among the Saturnian moons with the purest exposed ice.[2]
Spectroscopy constrains the composition of Calypso's outer covering but does not reveal what materials lie deeper inside the moon.[2]
Reflectivity
Calypso has a very bright surface. Particles from the E ring, supplied by material erupting from Enceladus, are a possible source of its icy coating.[1]
The moon is substantially brighter than Tethys even though their orbits lie at almost the same distance from Saturn. A similar contrast exists between Helene and Dione. The difference remains after the elongated shapes of the small moons, which affect their apparent brightness, are taken into account.[6]
The ordinary flux of E-ring particles is insufficient to explain the brightness of Calypso and Helene. They may receive additional dust because of differences in particle motion, or their surfaces may have been renewed comparatively recently. The reason these two moons are coated with exceptionally bright material remains uncertain.[6]
Discovery and name
Calypso was discovered in March 1980 through ground-based observations by Dan Pascu, P. Kenneth Seidelmann, William Baum and Douglas Currie. Its provisional designation was S/1980 S25, and it is also known as Saturn XIV.[1][7]
The moon is named after Calypso, the nymph from Greek mythology best known from the story of Odysseus.[1]
Cassini observations
On 13 February 2010, Cassini obtained detailed images of Calypso. One of the frames was taken from a distance of approximately 21,000 km.[3]
It shows the moon's leading hemisphere, the side facing in the direction of orbital motion. The published image was enlarged by a factor of two and contrast-enhanced. The observation allowed the moon's irregular shape to be compared with the unusually subdued appearance of its surface.[3]
Cassini images reveal Calypso's shape and topography, while measurements by the VIMS spectrometer establish the composition of its icy surface material.[8][2]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Calypso // NASA Science.
- ↑ 2.0 2.1 2.2 2.3 2.4 Filacchione G. et al. Saturn’s Icy Satellites and Rings Investigated by Cassini–VIMS. III: Radial Compositional Variability // Icarus. 2012. Vol. 220. P. 1064–1096.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Calypso Close Up // NASA/JPL/Space Science Institute. 26 March 2010. PIA12598.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Planetary Satellite Mean Elements // NASA JPL. Calypso: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 5.0 5.1 Robutel P., Rambaux N., El Moutamid M. Influence of the Coorbital Resonance on the Rotation of the Trojan Satellites of Saturn // Celestial Mechanics and Dynamical Astronomy. 2012. Vol. 113. P. 1–22.
- ↑ 6.0 6.1 Hedman M. M. et al. Photometric Analyses of Saturn’s Small Moons: Aegaeon, Methone, and Pallene Are Dark; Helene and Calypso Are Bright // The Astronomical Journal. 2020. Vol. 159. Article 129.
- ↑ Saturn Moons // NASA Science. Table of moons and discovery circumstances.
- ↑ N00151485 Calypso Crop // NASA/JPL/Space Science Institute. Image description on Wikimedia Commons.
