Object profile · UMBRIEL
Umbriel
Dark, ancient major moon of Uranus
The darkest major Uranian moon, preserving an old cratered surface and a bright ring whose origin is still unknown.
- Atlas data reviewed
- Editorial stage
- Published
- Visibility
- Public
Direct observationOfficial grayscale spacecraft observation. Background specks include image noise, and the view covers only part of the southern hemisphere.
NASA/JPL ↗What this world is
Umbriel reflects little sunlight, so old craters blend into a dark surface. One bright ring near the equator stands out, but scientists do not yet know what made it.
Why it looks this way now
Voyager 2 directly showed heavy cratering and the bright ring. A 2023 thermal model permits a briny ocean below the ice, but the surface itself offers no direct ocean measurement.
Where it sits
The middle member of the five major Uranian moons, orbiting between Ariel and Titania.
Swipe sideways to inspect the full map
Umbriel is 3 of 5 profiled Uranus moons by distance, at a mean center distance of about 265,986 km.
The left panel locates Uranus, dashed lines mark the scale change, and muted orbits and dots represent other profiled moons on the right. Ariel is the adjacent profiled moon inside; Titania is adjacent outside.
Read the numbers
Reference values retain their units, context, evidence state and published uncertainty.
Physical measurements
- Mean radiusglobal mean
- 584.7kmCalculated measurementThe mean radius is about 0.337 times the radius of Earth's Moon.
Irregular moons have no single true radius; the mean radius is an equal-volume comparison.
Published uncertainty: ± 2.8 km
JPL tabulated uncertainty for the adopted mean radius.
- Massglobal mean
- 1.27504E21kgCalculated 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
- 1.5230g/cm³Calculated measurementSimilar in size to Ariel but slightly less dense, showing that size alone does not fix interior composition.
This is a whole-body average and cannot by itself prove the composition of every interior layer.
Published uncertainty: ± 0.041 g/cm³
JPL tabulated uncertainty for the adopted mean density.
- Reference gravitysurface
- 0.24892m/s²Calculated measurementRepresentative surface gravity is about 2.54% 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
- -188°CScientific modelThis system-level sunlit reference hides large changes between illuminated and dark terrain.
Sparse thermal coverage and Uranus' long seasons make a single global temperature misleading.
Approximate value
Approximate sunlit upper reference near 85 K for Uranian moon terrain. It is not a measured global mean and does not represent polar night.
Orbit and rotation
- Average distancesystem reference
- 265,986kmCalculated measurementThis is a representative center-to-center distance between Umbriel and Uranus.
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
- 4.144177Earth 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
- 4.144177Earth 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.
Atmosphere and inside
Atmosphere
Umbriel has no persistent atmosphere. Its surface is exposed directly to space weathering and impacts.
Surface and interior
Umbriel likely contains rock and water-rich material below an ancient ice-rich crust. A residual ocean is plausible in one thermal model, not confirmed.
- 1Modeled rock and water-rich interior
Density is consistent with both rocky and water-rich material, but their exact arrangement is unknown.
Scientific model - 2Possible residual briny ocean
Thermal modeling permits a salty liquid layer, not a directly imaged or sampled ocean.
Scientific model - 3Ancient dark ice-rich crust
Voyager images directly show heavy cratering, low reflectivity and the unexplained bright ring.
Direct observation
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.
- 01Direct observation
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.
- 02Calculated measurement
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.
- 03Direct observation
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.
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.
Missions and instruments
Voyager 2
Voyager 2 returned the only close Umbriel images, establishing its dark ancient surface and enigmatic bright ring.
Official mission ↗- 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.
- Infrared Interferometer Spectrometer and RadiometerSeparates light by wavelength to constrain composition, gases and temperature.
Official source trail
Links below are the exact records used for this profile. Access dates are retained with the content.
- 01
NASA Science · official-page
Umbriel
- Source updated
- No source update date published
- Access checked
- 2026-07-31
- 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
- 03
NASA Science Photojournal · image
Umbriel at Closest Approach
- Source updated
- No source update date published
- Access checked
- 2026-07-31
- 04
NASA Science · official-page
Voyager mission
- Source updated
- No source update date published
- Access checked
- 2026-07-31
- 05
NASA Technical Reports Server · research-paper
Uranus Satellites: Surface Properties
- Source updated
- No source update date published
- Access checked
- 2026-07-31
- 06
NASA Jet Propulsion Laboratory · official-page
New Study of Uranus' Large Moons Shows 4 May Hold Water
- Source updated
- No source update date published
- Access checked
- 2026-07-31
Continue exploring
Atlas is a starting point. Use these relationships to move into explanation, experiment and mission thinking.