Tethys is a heavily cratered ball of nearly pure water ice, scarred by two outsized features: the 450-km impact basin Odysseus and a canyon system, Ithaca Chasma, that runs three-quarters of the way around the moon. Its density is slightly below that of liquid water - it is, in effect, a frozen ocean with a thin dusting of rock.
At 0.98 g/cm³ it is less dense than liquid water - a moon that would float, given a big enough ocean.
Ithaca Chasma stretches about 2,000 km, three-quarters of the way around the moon.
The impact basin Odysseus spans ~450 km, two-fifths of Tethys’ diameter.
When a moon freezes, it cracks
Water is strange: it expands when it freezes. If Tethys once had a liquid interior that slowly froze solid, the moon’s volume would have grown and split the rigid outer shell - one leading explanation for Ithaca Chasma, a crack up to 100 km wide and several kilometres deep stretching 2,000 km across the surface.
Physical characteristics
Diameter
1,062 km
Mass
6.170e+20 kg
Surface gravity
0.15 m/s²
Density
0.98 g/cm³
Tidally locked
Yes
Mean temperature
-187 °C
Atmosphere and composition
Atmosphere
None.
Composition
Almost pure water ice - there is very little rock in it at all.
Discovery
Discovered by
Giovanni Cassini
Year
1684
How
Aerial telescope at the Paris Observatory
Missions
Voyager 2NASA · Flyby probe · 1977 · active
Launched 16 days before its twin, Voyager 2 is the only spacecraft ever to visit all four giant planets - Jupiter, Saturn, Uranus and Neptune. It entered interstellar space in 2018 and is still returning data.
Cassini orbited Saturn for 13 years, delivering ESA’s Huygens probe to the surface of Titan in January 2005 - the first landing in the outer Solar System. It ended with a deliberate plunge into Saturn in 2017 to protect the potentially habitable moons Enceladus and Titan.