Tethys, Saturn's Moon: Ithaca Chasma, Frozen Ocean and Fractured Crust

Icy surface of Tethys: Saturn's moon — The surface of **Tethys** is marked by two remarkable geological structures: the Odysseus impact basin (≈ 450 km in diameter) and the Ithaca Chasma fracture, about 2,000 km long, which runs across three-quarters of its circumference.
Image source: NASA/JPL/Space Science Institute (new window)

Scientific summary

This Astronoo article presents Tethys, one of Saturn's icy moons. Discovered in 1684 by Cassini, it has a radius of 531 km and a density of 0.984 g.cm⁻³, close to that of water, indicating it is mainly composed of ice. Its highly reflective surface (albedo >0.8) is traversed by Ithaca Chasma, a giant 2000 km fracture born from the solidification of an internal ocean. The article details its synchronous rotation, orbital resonance with Mimas and Enceladus, and its extreme environment where the average temperature reaches 86 K. This world without atmosphere or erosion preserves intact the memory of the early solar system.

What are the physical and geological characteristics of Tethys that make this icy world a privileged witness to the history of the solar system?

Tethys, one of Saturn's moons, presents itself as a true "silent world frozen in ice". Its composition, dominated by water ice, and its low density, close to that of liquid water, make it a unique body. The absence of an atmosphere and the extreme temperatures, around -200 °C, have frozen its geological evolution. Thus, every impact crater and every fracture, like the immense Ithaca Chasma, have been preserved intact for billions of years. This exceptional preservation offers scientists a "intact memory of the earliest times" of the solar system. The study of its surface, revealed by the Cassini probe, allows us to trace ancient processes, such as the solidification of an internal ocean that caused the expansion and fracturing of its crust. Furthermore, its synchronous rotation and orbital resonances with other moons, such as Enceladus, testify to the complex gravitational interactions that shape the Saturnian system.

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Tethys, an icy fragment of the Saturnian system

A moon discovered by Cassini in 1684

Tethys is one of Saturn's large icy moons, discovered in 1684 by Giovanni Domenico Cassini (1625-1712). An Italian astronomer who became French, Cassini was the first director of the Paris Observatory and used astronomical telescopes of his own design to observe the Saturnian system. Between 1671 and 1684, he successively discovered Iapetus, Rhea, Tethys and Dione, bringing the number of known moons of Saturn to four. The name Tethys, chosen by John Herschel in 1847, refers to the Titaness Tethys, goddess of the sea springs in Greek mythology. Three centuries after its discovery, the Cassini-Huygens probe (1997-2017) made it possible to study this icy moon in detail, providing the first close-up images of its surface and confirming hypotheses about its composition and geology.

Composition and properties: a world of ice and rock

With a mean radius R = 531 km and density ρ = 0.984 g.cm⁻³, Tethys is mainly composed of water in solid form, mixed with a small fraction of silicate rocks. With a radius slightly smaller than that of Dione (561 km) and larger than that of Enceladus (252 km), Tethys sits in the middle of Saturn's icy moons. Its density, the lowest of the group along with Iapetus (1.03 g.cm⁻³), testifies to a particularly high proportion of ice. Its very bright surface (albedo greater than 0.8) makes it one of the most reflective objects in the solar system, a characteristic it shares with Europa, Jupiter's moon, whose albedo reaches 0.67. This high reflectivity is attributed to the purity of the surface ice, which is not contaminated by dark silicates, and to the surface texture, made rough by micrometeorite bombardment, which effectively scatters light.

Ithaca Chasma: when a frozen ocean fractures the crust

Images from the Cassini probe (1997-2017) revealed an almost frozen landscape: ancient craters, frozen plains and a giant fracture called Ithaca Chasma, over 2000 km long, crossing nearly three-quarters of the moon's circumference. Ithaca Chasma reflects an ancient expansion of Tethys's crust, probably linked to the solidification of an internal ocean, whose increase in volume caused the formation of fractures on the surface.

A fragile but stable gravitational equilibrium

Tethys's rotation is synchronous (synchronous rotation: rotation period equal to orbital period): it always shows the same face to Saturn, like the Moon does to Earth. Its nearly circular orbit at an average distance of \(294\,660\ \mathrm{km}\) from the planet suggests gravitational stabilization by orbital resonance (orbital resonance: integer ratio between the periods of two bodies) with the neighboring moons Mimas and Enceladus. This resonance produces weak internal tidal stresses, maintaining a balance between complete solidification and slight plasticity of the ice mantle.

Comparative table of Saturn's icy moons

From small shepherd moons like Pan and Atlas to icy giants like Rhea and Iapetus, passing through the enigmatic Titan, the Saturnian system hosts an exceptional diversity of icy bodies. Tethys, with its density close to that of water and its extremely bright surface, occupies a special place in this group, accompanied by its two Trojan moons, Telesto and Calypso, which share its same orbit.

Comparison of the main icy moons of Saturn by increasing distance
Moon	Mean distance from Saturn (km)	Radius (km)	Density (g/cm³)	Mean temperature (K)	Particularity
Pan	133 584	14	0.42	~78	Walnut-shaped moon, located in the Encke gap
Atlas	137 670	15	0.44	~78	Flying saucer shape, shepherd moon of the A ring
Prometheus	139 380	43	0.48	~78	Shepherd moon, creates filamentary structures in the rings
Pandora	141 720	41	0.50	~78	Shepherd moon, perturbs the F ring
Epimetheus	151 422	58	0.69	~78	Shared orbit with Janus (co-orbital)
Janus	151 472	89	0.63	~78	Shared orbit with Epimetheus (co-orbital)
Mimas	185 520	198	1.15	64	Herschel crater, marked relief
Methone	194 440	1.5	~0.5	~75	Small moon, orbit perturbed by Mimas
Anthe	197 700	1	~0.5	~75	Small moon, discovered by Cassini in 2007
Pallene	211 000	2.5	~0.5	~75	Orbit between Mimas and Enceladus
Enceladus	237 950	252	1.61	75	Active cryovolcanic plumes
Tethys	294 660	531	0.984	86	Ithaca Chasma, very bright surface
Telesto	294 660	12	~0.5	~86	Trojan moon of Tethys (L4 point)
Calypso	294 660	10	~0.5	~86	Trojan moon of Tethys (L5 point)
Dione	377 400	561	1.48	84	Ancient craters and linear fractures
Helene	377 400	16	~0.5	~84	Trojan moon of Dione (L4 point)
Polydeuces	377 400	2.6	~0.5	~84	Trojan moon of Dione (L5 point)
Rhea	527 040	764	1.23	99	Cratered surface, possible tenuous ring
Titan	1 221 870	2 575	1.88	94	Dense atmosphere, hydrocarbon lakes
Iapetus	3 560 820	734	1.03	110	Two-tone surface, synchronous rotation
Phoebe	12 944 000	107	1.63	73	Retrograde rotation, likely external origin

References

ESA – Cassini-Huygens mission (new window).

FAQ: Everything you need to know about Tethys, Saturn's icy moon

Who discovered Tethys and when?

Tethys was discovered in 1684 by the Italian astronomer Giovanni Domenico Cassini (1625-1712), who also discovered other moons of Saturn such as Iapetus, Rhea and Dione. The Cassini space probe, named in his honor, later studied the moon in detail between 1997 and 2017.

Why is Tethys's surface so bright?

Its surface is extremely reflective because it is almost entirely composed of crystalline water ice, with an albedo greater than 0.8. This means it reflects more than 80% of the sunlight it receives, making it one of the brightest objects in the solar system.

What is Ithaca Chasma and how did it form?

Ithaca Chasma is a giant fracture over 2000 kilometers long that runs across three-quarters of Tethys's circumference. Its formation is likely due to the solidification of an internal ocean of liquid water. As it solidified, the ice increased in volume, causing the expansion and fracturing of the moon's outer crust.

What is the temperature on Tethys's surface?

The surface temperature is extremely cold, ranging between 70 K and 90 K (about -200 °C) depending on solar exposure. This freezing temperature is a key factor contributing to the preservation of its geologically frozen surface.

Why is Tethys considered a "witness of cosmic time"?

Devoid of atmosphere and erosion, Tethys is a geologically inactive body. Every impact crater formed over billions of years has remained perfectly visible. It thus offers an "intact memory" of the conditions that prevailed in the early solar system, making it a valuable object of study for planetary scientists.