Tethys (moon)
Tethys is an icy moon of Saturn with a diameter of approximately 1,062 km, making it the planet's fifth-largest satellite. It is distinguished by its exceptionally low density and two immense landforms: the impact crater Odysseus and the canyon system Ithaca Chasma. Both occupy a substantial part of the small moon's surface.[1][4]
Images and measurements from Cassini revealed that the surface is not uniform, even where it appears almost uniformly white in ordinary visible light. Tethys has faint reddish arcs and a region with an unusual pattern of daytime and night-time temperatures.[5][6]
Size and physical characteristics
Tethys has a mean radius of 531.1 km and a diameter of approximately 1,062 km, about 3.3 times smaller than the Moon. With a mass of approximately 6.18 × 1020 kg, its mean density is only 0.984 g/cm³, close to that of liquid water.[1]
This combination of size and mass indicates that water ice dominates its composition and that the proportion of rocky material is small. Tethys is much closer to an almost entirely icy body than Titan or Enceladus, both of which contain substantially more rock.[2]
Surface gravity is approximately 0.146 m/s², about 67 times weaker than Earth's. Impact craters and fractures cover the icy terrain, and many large ancient features have softened outlines.[1][7]
Orbit and rotation
Tethys orbits approximately 295,000 km from the centre of Saturn. Its orbit is nearly circular, and one revolution takes 45 hours 18 minutes. In one Earth week, the moon travels around Saturn almost four times.[3]
Its rotation is synchronous, so the same hemisphere always faces the planet.[2] The leading hemisphere, which faces forward along the orbit, and the trailing hemisphere experience different particle environments. The contrast appears in the colour and thermal properties of their surfaces.[8]
Odysseus crater
Odysseus is the largest impact structure on Tethys. It measures approximately 450 km across, more than 40% of the moon's diameter. In Cassini images the crater occupies an enormous part of the leading hemisphere.[4]
Despite its size, Odysseus is comparatively shallow. Its rim and central region have gradually flattened. The softened relief is attributed to the slow deformation of ice under its own weight over geological time.[9]
Ithaca Chasma
Ithaca Chasma is a vast system of troughs crossing much of Tethys. Its total length is commonly estimated at approximately 2,000 km, while individual sections reach 100 km in width and several kilometres in depth.[10][11][2]
One hypothesis links Ithaca Chasma to expansion of the interior: water beneath a solid outer shell froze, increased in volume and stretched the crust. Another connects the fractures with the impact that formed Odysseus.[2]
Later craters cover both the chasma and surrounding terrain. Cassini mosaics show softened slopes and the effects of prolonged impact erosion. The fracture system is therefore ancient, although its precise age and origin remain unresolved.[7]
Reddish arcs
Colour images taken in April 2015 revealed narrow, curved bands on Tethys. They are several kilometres wide and extend for hundreds of kilometres. Images from several spectral filters were combined to emphasise the faint colour differences.[5]
The surface appears much more uniform in natural colour, while the reddish arcs become clearer in enhanced-colour images. Possible explanations include impurities in the ice, material from the interior or fractures too narrow to resolve confidently in existing photographs.[5]
The bands themselves are directly observed, but their reddish colour does not by itself identify their composition or establish a particular geological process.[5]
Thermal properties of the surface
Cassini infrared measurements revealed an area on the leading hemisphere shaped like the video-game character Pac-Man. The discovery was announced in 2012. In data obtained on 14 September 2011, part of this region was approximately 15 K colder during the day than the surrounding terrain.[6]
High-energy electrons in Saturn's magnetosphere are thought to alter the structure of the loose surface ice and compact it. The modified material warms more slowly during the day and cools more slowly at night.[6]
Within the anomalous region, the ground's thermal inertia is approximately three times greater than outside it, while the proportion of reflected sunlight changes little. The anomaly is therefore primarily a difference in the physical properties of the surface layer.[8]
Ice particles from the E ring, which is supplied by the plumes of Enceladus, also fall onto Tethys. Its surface is consequently altered by particle bombardment while being coated with new fine-grained ice.[6][8]
Telesto and Calypso
Two small moons share the orbit of Tethys: Telesto and Calypso. Telesto travels approximately 60° ahead of Tethys, while Calypso follows by roughly the same angle.[12]
They occupy the triangular L4 and L5 Lagrange regions of the Saturn–Tethys system. Such bodies are known as Trojan satellites. Telesto and Calypso orbit Saturn together with Tethys while maintaining this stable configuration.[12][13]
Discovery and exploration
Giovanni Domenico Cassini discovered Tethys on 21 March 1684. The moon is named after Tethys, a Titaness in Greek mythology and the wife of Oceanus. John Herschel proposed the name, and many surface features were later named after characters and places in Homer's Odyssey.[2]
Voyager 1 in 1980 and Voyager 2 in 1981 returned the first close images, revealing Odysseus and Ithaca Chasma. From 2004 to 2017, Cassini mapped the moon in much greater detail and added spectral and thermal measurements.[14][7]
A mosaic of the southern region taken on 24 September 2005 shows ancient softened formations alongside younger craters with sharp rims and bright floors.[7]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Planetary Satellite Physical Parameters // NASA JPL. Tethys: SAT441 solution. Mass calculated from GM using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 Tethys // NASA Science.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Planetary Satellite Mean Elements // NASA JPL. Tethys: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 Epic Odysseus // NASA Science. 12 March 2010.
- ↑ 5.0 5.1 5.2 5.3 Red Arcs on Tethys // NASA Science. 29 July 2015.
- ↑ 6.0 6.1 6.2 6.3 Cassini Finds a Video Gamers' Paradise at Saturn // NASA/JPL-Caltech. 26 November 2012.
- ↑ 7.0 7.1 7.2 7.3 Tethys in Full View // NASA/JPL/Space Science Institute. Cassini mosaic PIA07738.
- ↑ 8.0 8.1 8.2 Howett C. J. A. et al. Maps of Tethys' Thermophysical Properties // Icarus. 2019. DOI: 10.1016/j.icarus.2018.12.018.
- ↑ Thomas P. J., Squyres S. W. Relaxation of Impact Basins on Icy Satellites // Journal of Geophysical Research. 1988. Vol. 93. NASA NTRS archive.
- ↑ Tethys' Great Rift // NASA Science. 2005.
- ↑ Tethys' Ithaca Chasma // NASA Science. 2004.
- ↑ 12.0 12.1 Telesto // NASA Science.
- ↑ Calypso // NASA Science.
- ↑ Cassini-Huygens // NASA Science.