Telesto (moon)
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Telesto is a small icy moon of Saturn that travels approximately 60° ahead of Tethys in the same orbital region. Together with Calypso, which follows behind Tethys, it forms a pair of Trojan moons. Telesto has a mean diameter of approximately 25 km.[1]
Cassini images revealed an irregular body with an unexpectedly smooth-looking surface. Fine-grained icy material softens the relief, although individual craters, projections and possible large blocks remain visible.[3]
Size and physical characteristics
Telesto's mean radius is approximately 12.4 km, giving a mean diameter of 24.8 km. Its actual outline differs noticeably from a sphere. Telesto is slightly larger than Calypso, whose mean radius is approximately 10.7 km.[1][6]
Observations constrain mainly the moon's exterior shape and surface material. Spectra show abundant water ice, but they do not determine the proportions of ice, rocky impurities and void space throughout its interior.[2]
Orbit and position relative to Tethys
Telesto orbits Saturn at a mean distance of approximately 295,000 km from the planet's centre. One revolution takes 1.887802 Earth days, or about 45 hours 18 minutes. The orbit is nearly circular, with a mean eccentricity of approximately 0.001.[4]
The moon occupies the region around the L4 Lagrange point of the Saturn–Tethys system. Telesto and Tethys are in a 1:1 orbital resonance: their mean orbital periods are practically identical, while the smaller moon librates around a position 60° ahead of Tethys.[7]
Calypso occupies the opposite Trojan region near L5, approximately 60° behind Tethys. Telesto nevertheless orbits Saturn rather than Tethys itself.[1][7]
Surface and smoothed topography
Unlike many Saturnian moons, Telesto does not display a dense network of sharply defined impact craters. Cassini images show a surface covered by fine-grained icy material that softens the outlines of depressions.[3]
The small number of clearly visible craters does not mean that the moon avoided impacts. Loose material can bury and smooth impact depressions, while some of the debris ejected during collisions falls back and renews the outer layer.[8]
Surface composition from JWST
Telesto's spectrum is dominated by water-ice absorption bands near 1.5, 2, 3 and 4.5 μm. They were recorded by the NIRSpec instrument on the James Webb Space Telescope and agree with earlier measurements by Cassini's VIMS spectrometer.[2]
A narrow peak near 3.1 μm and an additional absorption feature near 1.65 μm are more prominent on Telesto than on Epimetheus, indicating stronger signatures of crystalline ice. The spectrum also permits some amorphous or very fine-grained ice, and the precise mixture remains uncertain.[2]
Exchange of impact ejecta
After an impact, the slowest particles fall back to Telesto or Calypso. Faster material can escape the small moon and move through their shared orbital region under the gravity of Saturn and Tethys.[8]
These trajectories allow material to be exchanged between the Trojan moons. The fastest debris can cross the region of Tethys, which captures most of it in the model, while a small fraction can reach other moons. The calculations describe possible transport paths rather than a directly measured stream between Telesto and Calypso.[8]
Discovery and Cassini observations
Telesto was discovered in 1980 through ground-based observations by Bradford Smith, Harold Reitsema, Stephen Larson and John Fountain. Its provisional designation was S/1980 S13, and it is also known as Saturn XIII. The moon is named after Telesto, a daughter of Oceanus and Tethys in Greek mythology.[5][1]
On 11 October 2005, Cassini photographed Telesto from a distance of approximately 14,500 km. The image clearly shows its smooth covering, individual craters and projecting surface features.[3]
Another close observation took place on 27 August 2009. Images from multiple encounters provided a more complete view of Telesto's shape and topography.[9]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Telesto // NASA Science.
- ↑ 2.0 2.1 2.2 2.3 Hedman M. M. et al. Water-Ice Dominated Spectra of Saturn’s Rings and Small Moons From JWST // Journal of Geophysical Research: Planets. 2024. Vol. 129. Article e2023JE008236.
- ↑ 3.0 3.1 3.2 3.3 Smooth Surface of Telesto; PIA07702 // NASA/JPL/Space Science Institute. 14 February 2006.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Planetary Satellite Mean Elements // NASA JPL. Telesto: SAT441; epoch 1 January 2000, 12:00 TDB; referenced to the local Laplace plane.
- ↑ 5.0 5.1 5.2 Planetary Satellite Discovery Circumstances // NASA JPL.
- ↑ Calypso // NASA Science.
- ↑ 7.0 7.1 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.
- ↑ 8.0 8.1 8.2 Dobrovolskis A. R., Alvarellos J. L., Zahnle K. J., Lissauer J. J. Exchange of Ejecta Between Telesto and Calypso: Tadpoles, Horseshoes, and Passing Orbits // Icarus. 2010. Vol. 210. P. 436–445.
- ↑ Saturn’s Moon Telesto; PIA12704 // NASA/JPL/Space Science Institute. Image description on Wikimedia Commons.
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