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Object profile · MIMAS

Mimas

Small cratered major moon of Saturn

A scarred ice-rich moon dominated by Herschel crater, with a possible young ocean hidden beneath an outwardly quiet shell.

Atlas data reviewed
Editorial stage
Published
Visibility
Public
Cassini grayscale mosaic of Mimas with the giant Herschel crater dominating the visible hemisphere.Direct observation
Six Cassini clear-filter images assembled into a high-resolution view of Herschel crater and surrounding fractures.

Official grayscale data mosaic. It combines several exposures and is not a natural-color single photograph.

NASA/JPL/Space Science Institute ↗
Vastward explanation

What this world is

Herschel crater is about one-third of Mimas' diameter. The impact was so large that fractures on the opposite side may record the shock that nearly broke the moon apart.

Why it looks this way now

Cassini directly imaged the cratered shell and measured Mimas' motion. A 2024 dynamical study fits those motions with a geologically young internal ocean; no camera saw that ocean and no water sample came from it.

02

Where it sits

The innermost of Saturn's rounded major moons, orbiting just outside the main rings in less than one Earth day.

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 Mimas among the profiled Saturn moon orbits.Scale one · Solar SystemSaturnPlanet 6 from the SunSunScale two · Saturn moon systemSaturnMimasProfiled orbit 1 of 9Mean distance · 186,000 km
Blue marks the current moon and orbit

Mimas is 1 of 9 profiled Saturn moons by distance, at a mean center distance of about 186,000 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. This is the innermost profiled moon in the system; Enceladus 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
198.2kmCalculated measurementThe mean radius is about 0.114 times the radius of Earth's Moon.

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

Published uncertainty: ± 0.4 km

JPL tabulated uncertainty for the adopted mean radius.

Massglobal mean
3.75094E19kgCalculated 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
1.1501g/cm³Calculated measurementOnly modestly denser than water, consistent with an ice-rich body containing rock and pore space.

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

Published uncertainty: ± 0.007 g/cm³

JPL tabulated uncertainty for the adopted mean density.

Reference gravitysurface
0.063729m/s²Calculated measurementRepresentative surface gravity is about 0.65% 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.

Temperatureverified station extreme
-181°CDirect observationCassini found adjacent daytime regions differing by about 15 K, showing that equal sunlight does not guarantee equal surface temperature.

The value represents the warmer daytime region in one Cassini thermal map; the neighboring cold region was about -196 °C.

Approximate value

Cassini CIRS measured typical daytime temperatures near 92 K (-181 °C) in the warmer region and about 77 K (-196 °C) in the colder region. The displayed value is not a global mean.

Orbit and rotation

Average distancesystem reference
186,000kmCalculated measurementThis is a representative center-to-center distance between Mimas 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
0.942422Earth 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
0.942422Earth 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

Mimas has no persistent atmosphere. Its weak gravity cannot retain a substantial gas layer.

Surface and interior

Mimas looks inactive at the surface, while modern dynamics allow a rock-rich core, a possible young ocean and an intact cratered ice shell.

  1. 1
    Modelled rock-rich core

    Density and rotational dynamics allow a rock-rich center, but its exact shape and porosity are model-dependent.

    Scientific model
  2. 2
    Possible young subsurface ocean

    A 2024 orbital-dynamics model favors liquid water beneath the shell; this is an inference, not a directly imaged layer.

    Scientific model
  3. 3
    Cratered ice-rich shell

    Cassini images directly reveal an old, heavily cratered surface dominated by the enormous Herschel basin.

    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 Mimas, measured its thermal anomaly and rotational motion, and supplied the observations later used to test interior models.

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

    Mimas

    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

    Examining Herschel Crater

    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 ↗
  5. 05

    NASA Science · official-page

    Bizarre Temperatures on Mimas

    Source updated
    No source update date published
    Access checked
    2026-07-31
    Open official source ↗
  6. 06

    Nature · research-paper

    A recently formed ocean inside Saturn's moon Mimas

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

academyAvailable now

How do we detect a buried ocean?

Compare magnetic, gravity, plume and geology evidence while keeping ocean, habitability and detected life separate.

academy