VASTWARDCelestial Atlas
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Object profile · PHOBOS

Phobos

Irregular natural satellite of Mars

A battered, inward-spiraling moon that may someday break into a ring or strike Mars.

Atlas data reviewed
Editorial stage
Published
Visibility
Public
HiRISE view of irregular Phobos with Stickney crater and a dusty, grooved surface.Direct observation
Mars Reconnaissance Orbiter HiRISE observation of Phobos.

Official spacecraft observation. Perspective and lighting exaggerate relief; the image is not a scale comparison with Mars.

NASA/JPL-Caltech/University of Arizona ↗
Vastward explanation

What this world is

Phobos is not a smooth mini-planet. It is an irregular, dusty world whose orbit is shrinking by about 1.8 metres per century.

Why it looks this way now

Images directly show Stickney crater, grooves and mobile dust. Its exact origin remains unsettled: capture and impact-debris models each face constraints.

02

Where it sits

The inner and larger Martian moon, circling so close that it rises and sets several times during a Martian day.

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

Swipe sideways to inspect the full map

Two-scale location · Solar System to the Mars moonsThe left panel places Mars in planetary order; the right places Phobos among the profiled Mars moon orbits.Scale one · Solar SystemMarsPlanet 4 from the SunSunScale two · Mars moon systemMarsPhobosProfiled orbit 1 of 2Mean distance · 9,376 km
Blue marks the current moon and orbit

Phobos is 1 of 2 profiled Mars moons by distance, at a mean center distance of about 9,376 km.

Muted lines and dots show system context

The left panel locates Mars, 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; Deimos 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
11.08kmCalculated measurementThe mean radius is about 0.0064 times the radius of Earth's Moon.

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

Published uncertainty: ± 0.04 km

JPL tabulated uncertainty for the adopted mean radius.

Massglobal mean
1.061834E16kgCalculated 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
1.8720g/cm³Calculated measurementLess dense than compact rock, consistent with substantial porosity.

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

Published uncertainty: ± 0.076 g/cm³

JPL tabulated uncertainty for the adopted mean density.

Reference gravitysurface
0.005773m/s²Calculated measurementRepresentative surface gravity is about 0.059% 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
-4°CDirect observationThe displayed value is the measured sunlit high; darkness only kilometers away can be far colder.

Phobos loses heat quickly. NASA reports roughly -4 °C on the sunlit side and -112 °C on the dark side.

Approximate value

NASA reports a measured sunlit high near -4 °C and a dark-side low near -112 °C. This entry shows the high, not a global mean.

Orbit and rotation

Average distancesystem reference
9,376kmCalculated measurementThis is a representative center-to-center distance between Phobos and Mars.

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
0.31891Earth 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
0.31891Earth 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

Phobos has no persistent atmosphere. Material released from the surface escapes easily.

Surface and interior

A porous interior and impact-broken outer regolith are supported, but a detailed layer-by-layer interior is not directly measured.

  1. 1
    Porous rock-rich interior

    Bulk density and orbit data are consistent with a porous body, but do not uniquely reveal how rock, voids and possible ice are arranged.

    Scientific model
  2. 2
    Impact-broken regolith

    Repeated impacts have ground the outer material into loose blocks and fine dust.

    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

Active

Mars Reconnaissance Orbiter

HiRISE resolved Phobos at high detail and tracked surface features from Mars orbit.

Official mission ↗
Instruments
  • HiRISE cameraImages the limb, terrain, color differences and time-dependent surface changes.
  • Spacecraft radio scienceMeasures Doppler and range changes that constrain trajectory, gravity and mass.
  • CRISM imaging 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

    Phobos

    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

    Martian Moon Phobos

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

    NASA Science · official-page

    Mars Reconnaissance Orbiter

    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 Phobos back inside Mars'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