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Object profile · TETHYS

Tethys

Ice-rich moon split by a giant chasm

A bright ice-rich moon carrying both the immense Odysseus impact basin and the globe-spanning Ithaca Chasma.

Atlas data reviewed
Editorial stage
Published
Visibility
Public
Cassini grayscale mosaic of Tethys showing its bright cratered disk and the long Ithaca Chasma fracture system.Direct observation
Nine Cassini clear-filter images assembled into a full-disk view of Tethys.

Official grayscale data mosaic. Geometry was assembled from multiple frames; this is not one natural-color exposure.

NASA/JPL/Space Science Institute ↗
Vastward explanation

What this world is

Tethys is almost all water ice, yet at this temperature that ice can hold enormous craters and cliffs. Ithaca Chasma stretches roughly 2,000 kilometres across the moon.

Why it looks this way now

Cassini directly resolved Odysseus, Ithaca Chasma and subtle color differences. Whether the chasm formed as an interior ocean froze and expanded or during the giant impact remains unsettled.

02

Where it sits

A mid-sized Saturnian moon whose orbit also anchors the small Trojan moons Telesto and Calypso near stable Lagrange points.

How to read this mapTwo-scale location · Solar System to the Saturn moons

Swipe sideways to inspect the full map

Two-scale location · Solar System to the Saturn moonsThe left panel places Saturn in planetary order; the right places Tethys among the profiled Saturn moon orbits.Scale one · Solar SystemSaturnPlanet 6 from the SunSunScale two · Saturn moon systemSaturnTethysProfiled orbit 3 of 9Mean distance · 295,000 km
Blue marks the current moon and orbit

Tethys is 3 of 9 profiled Saturn moons by distance, at a mean center distance of about 295,000 km.

Muted lines and dots show system context

The left panel locates Saturn, dashed lines mark the scale change, and muted orbits and dots represent other profiled moons on the right. Enceladus is the adjacent profiled moon inside; Dione is adjacent outside.

The figure uses two linked scales and compares only the same-system moons currently profiled by Vastward; orbit spacing, body sizes and marker positions are compressed for teaching.
03

Read the numbers

Reference values retain their units, context, evidence state and published uncertainty.

Physical measurements

Mean radiusglobal mean
531.1kmCalculated measurementThe mean radius is about 0.306 times the radius of Earth's Moon.

Irregular moons have no single true radius; the mean radius is an equal-volume comparison.

Published uncertainty: ± 0.6 km

JPL tabulated uncertainty for the adopted mean radius.

Massglobal mean
6.174959E20kgCalculated measurementMass is not read from a scale. Motion under gravity constrains GM, from which mass is derived.

Calculated from JPL GM with a conventional gravitational constant, then rounded for display.

Approximate value

Derived from JPL's gravitational parameter GM and a conventional gravitational constant, then rounded.

Mean densityglobal mean
0.9840g/cm³Calculated measurementIts bulk density is slightly below liquid water, showing that water ice dominates and only a small rock fraction is required.

This is a whole-body average and cannot by itself prove the composition of every interior layer.

Published uncertainty: ± 0.0033 g/cm³

JPL tabulated uncertainty for the adopted mean density.

Reference gravitysurface
0.146112m/s²Calculated measurementRepresentative surface gravity is about 1.49% of Earth's.

Shape, terrain and local mass distribution make real gravity vary by location.

Approximate value

Calculated from the adopted GM and mean radius. Irregular shape and local terrain can change the actual value.

Temperaturesurface
-187°CDirect observationAt about -187 °C, surface water ice behaves mechanically more like rock than familiar household ice.

This is a broad average; local lighting and terrain change the actual surface temperature.

Approximate value

NASA reports an average surface temperature near 86 K (-187 °C). Day, night, latitude and terrain still vary.

Orbit and rotation

Average distancesystem reference
295,000kmCalculated measurementThis is a representative center-to-center distance between Tethys and Saturn.

The real distance changes around an elliptical orbit.

Approximate value

Rounded orbital distance from the moon center to the planet center. The real orbit is not a perfect circle.

Rotation periodrelative to distant stars
1.887802Earth daysCalculated measurementTidal locking makes one rotation take nearly the same time as one orbit around the planet.

This is a sidereal rotation, not a local sunrise-to-sunrise solar day.

Approximate value

Rounded synchronous rotation period. The same hemisphere normally faces the primary planet.

Orbital periodrelative to distant stars
1.887802Earth daysCalculated measurementThis reports the time for one orbit, not a claim that the path is perfectly circular.

A rounded sidereal period is used for the beginner-facing display.

Approximate value

Rounded sidereal orbital period.

04

Atmosphere and inside

Atmosphere

Tethys is airless in the everyday sense and has no persistent substantial atmosphere.

Surface and interior

Tethys is an ice-dominated body with a scarred outer crust; a specific ancient ocean history remains a hypothesis rather than a present-day detection.

  1. 1
    Ice-rich interior with minor rock

    The very low bulk density strongly favors water ice with a smaller rock component, without uniquely fixing the internal arrangement.

    Scientific model
  2. 2
    Cratered and fractured ice crust

    Spacecraft images directly show Odysseus basin, Ithaca Chasma and terrains reworked by impacts.

    Direct observation
05

How we know

Images constrain size, orbital and radio tracking constrain GM, and spectra and thermal emission constrain materials. Every number retains its method and limitation.

  1. 01

    Spacecraft imaging and shape reconstruction

    Repeated views reveal the limb, terrain, crater record and the shape used to estimate size.

    Where this method stops

    Lighting, viewing angle and incomplete coverage can hide topography. A mosaic is not a single untouched photograph.

    Direct observation
  2. 02

    Radio tracking and orbital dynamics

    Engineers measure spacecraft motion and moon orbits, solve for GM, and then derive mass and gravity.

    Where this method stops

    The result depends on trajectory coverage and a dynamical model. Very small moons leave weaker gravitational signatures.

    Calculated measurement
  3. 03

    Spectroscopy and thermal sensing

    The spectrum and thermal glow constrain surface materials, gases and representative temperature.

    Where this method stops

    A spectrum samples the visible surface or atmosphere. It does not directly photograph deep interior layers.

    Direct observation
06

Evidence key

Direct observation
An instrument or sample recorded the phenomenon, with processing still disclosed.
Calculated measurement
Observed motion or signal is converted into a physical quantity using equations and reference constants.
Scientific model
A tested interpretation that fits observations but is not directly imaged or sampled.
Vastward explanation
Original beginner-facing synthesis, traceable to the source records but not itself a measurement.
Artist visualization
An interpretive image, not observational evidence.
07

Missions and instruments

Completed

Cassini-Huygens

Cassini mapped Tethys at far higher resolution than Voyager, resolving the chasm, impact terrain and subtle surface-color patterns.

Official mission ↗
Instruments
  • Imaging Science SubsystemImages the limb, terrain, color differences and time-dependent surface changes.
  • Spacecraft radio scienceMeasures Doppler and range changes that constrain trajectory, gravity and mass.
  • Visual and Infrared Mapping SpectrometerSeparates light by wavelength to constrain composition, gases and temperature.
08

Official source trail

Links below are the exact records used for this profile. Access dates are retained with the content.

  1. 01

    NASA Science · official-page

    Tethys

    Source updated
    No source update date published
    Access checked
    2026-07-31
    Open official source ↗
  2. 02

    NASA Jet Propulsion Laboratory, Solar System Dynamics · dataset

    Planetary Satellite Physical Parameters

    Source updated
    No source update date published
    Access checked
    2026-07-31
    Open official source ↗
  3. 03

    NASA Science Photojournal · image

    Tethys in Full View

    Source updated
    No source update date published
    Access checked
    2026-07-31
    Open official source ↗
  4. 04

    NASA Science · official-page

    Cassini mission

    Source updated
    No source update date published
    Access checked
    2026-07-31
    Open official source ↗
09

Continue exploring

Atlas is a starting point. Use these relationships to move into explanation, experiment and mission thinking.

Available now

Understand moon systems

Place Tethys back inside Saturn's moon system and connect orbit, tides and resonance.

academy
Relationship reserved

Compare the major moons

Compare size, density, gravity and orbit without treating similar colors as identical composition.

lab