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Tethys


Tethys is the ninth of Saturn's moons. Tethys' low density indicates that it is almost completely composed of water ice, similar to Dione and Rhea.

The western hemisphere is dominated by a huge impact crater, called Odysseus, whose 400 km diameter is nearly 2/5 of that of Tethys itself. That such an impact didn't shatter Tethys completely indicates that it may have been liquid or at least not very solid at the time. The crater is now quite flat (or more precisely, it conforms to Tethys' spherical shape), like the craters on Callisto, without the high ring mountains and central peaks commonly seen on the Moon and Mercury.

The second major feature seen on Tethys is a huge valley (called Ithaca Chasma) 100 km wide and 3 to 5 km deep which runs 2000 km or 3/4 of the way around Tethys' circumference.

Clearly then, Tethys has not always been frozen solid. At some point in its past it was probably liquid. The impact craters from that era have been smoothed out. As it froze and expanded, the surface must have cracked to accommodate the extra volume producing Ithaca Chasma. The smaller impact craters we see today are more recent.

A vast expanse of relative young plains also exists on Tethys.

Telesto and Calypso orbit in Tethys' Lagrange points (60 degrees ahead and behind Tethys in the same orbit).


Orbital Distance:   294,700 km (circular)
Eccentricity:       0.000
Local Year:         1.888 days (1d 21h 18m 26s)
Orbital Velocity:   11.350 km/s
Inclination:        1.09°
Local Day:          1.888 days (1d 21h 18m 26s)
Axial Tilt:         n/a

Diameter:           1,060 km
Composition:        Ice
Density:            1.21 g/cm3
Mass:               7.55 x 1017 tonnes (0.0001 earths)
Gravity:            0.018 G
Escape Velocity:    0.436 km/s
Area:               3.5 million km2
Surface Material:   Mostly H2O ice
Atmosphere:         None
Pressure:           None
Albedo:             0.90
Temperature:        -187°C

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