Tethys

Key facts

Diameter

1,062 km31% of Earth's Moon

Mass

6.17 × 10²⁰ kg

Surface gravity

0.146 m/s²0.015 g

Density

984 kg/m³Less dense than water

Distance from Saturn

294,619 km≈ 4.9 Saturn radii

Orbital period

45.3 hoursSame as its rotation (tidally locked)

Discovered

1684Giovanni Cassini

Surface

Bright, heavily cratered water ice

Surface temperature

About −187 °C (86 K)

Rotation

Tidally locked to Saturn

Co-orbital moons

Telesto (leading) and Calypso (trailing)

Named after

Tethys, a Titaness of Greek mythology

Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.

Surface and geology

Odysseus crater is about 450 kilometres wide, more than two-fifths of Tethys' diameter. Unlike younger craters, it is fairly shallow, its floor having slowly relaxed back towards the moon's natural curve over time, which suggests the ice was once warmer and softer than it is today.

Ithaca Chasma is a valley up to about 100 kilometres wide and several kilometres deep that stretches for roughly 2,000 kilometres, running north to south across much of the globe. Scientists have suggested it formed as the interior froze and expanded, cracking the crust, or that it is linked to the Odysseus impact, but neither idea has been proven.

Cassini also found strange reddish arcs, thin streaks that cross the surface in curving lines. Their cause is unknown; they may be exposed ice of a different composition or the result of chemical changes, and they appear to be relatively young.

Interior and composition

Tethys has a density very close to that of water ice, so it can contain only a small fraction of rock. It may be largely undifferentiated, without a distinct rocky core, though the evidence is limited. There is currently no strong sign of a present-day ocean, although its past heating, recorded in Ithaca Chasma and the relaxed floor of Odysseus, hints at a warmer youth.

Orbit and rotation

Tethys orbits Saturn in just under two days on a nearly circular path aligned with the planet's equator. It is tidally locked, so its leading hemisphere always faces forward along its orbit and is sandblasted by fine ice grains from the E ring, which keeps it bright.

Telesto sits about 60 degrees ahead of Tethys and Calypso about 60 degrees behind, at the gravitationally stable Lagrange points. Tethys also shares an orbital resonance with Mimas, which affects how both moons' orbits are tilted.

Origin

Tethys most likely formed from the icy disc around the young Saturn. Its unusually low rock content is a puzzle; one idea is that it accreted from ice-rich material stripped from the outer layers of a larger body or from an ancient ring, but this remains speculative.

Exploration

Giovanni Domenico Cassini discovered Tethys in March 1684, together with Dione. Spacecraft have since revealed its dramatic landforms in detail.

The moons of Saturn

Surprising facts about Tethys

• Tethys' average density is slightly lower than that of liquid water, so in a large enough bathtub it would float.
• Ithaca Chasma is about 2,000 km long, wide enough in places to swallow a small country, and runs across much of the moon's surface.
• The Odysseus impact basin is large enough to hold the entire moon Mimas inside it.
• Two small moons, Telesto and Calypso, share Tethys' orbit at stable points 60 degrees ahead and behind.
• Cassini spotted mysterious red streaks on Tethys that scientists still cannot fully explain.

Keep exploring

• Dione, moon of Saturn: ice cliffs and wispy streaks
• Mimas, moon of Saturn: the death star with a secret
• Rhea, moon of Saturn: saturn's ancient frozen giant
• Saturn: the planet guide
• Watch Tethys orbit Saturn in the interactive 3D solar system
• All the major moons of the Solar System

Sources

• NASA Science — Tethys
• JPL Solar System Dynamics — Planetary satellite physical parameters
• Wikipedia — Tethys