Phobos

Moon· orbits Mars

Phobos is the larger, inner moon of Mars, orbiting so low and fast that it crosses the Martian sky twice a day and is slowly spiralling inward toward eventual breakup. Because it rides so low and quick, from the Martian surface Phobos is regularly seen passing directly in front of the Sun — a transit, or partial solar eclipse, that rovers such as Perseverance have photographed (the moon is too small to hide the Sun completely).

Open in 3D viewSee Phobos in the live, interactive 3D map

Key facts

Mass

1.07 × 10¹⁶ kg

0.000000145 Moon masses

Radius

11.1 km

1% of our Moon's radius

Surface gravity

0.00579 m/s²

0% of Earth's gravity

Day length

7.65 hours

Orbit around Mars

7.6 hours

0.32 Earth days

Science

Physical

Mass: 1.07 × 10¹⁶ kg · 0.000000145 Moon masses; JPL / IAU mean radii; spacecraft-tracked GM where measured
Radius: 11.1 km · 1% of our Moon's radius; JPL SSD Planetary Satellite Physical Parameters (mean radius)
Density: 1,871 kg/m³ · 1.9× water; Computed from mass and radius
Surface gravity: 0.00579 m/s² · 0% of Earth's gravity; Computed from mass and radius
Escape velocity: 11.3 m/s · 0% of Earth's escape velocity; Computed from mass and radius

Orbit around Mars

Distance from Mars: 9,375 km; From JPL orbital elements
Orbital period: 7.65 hours; From JPL orbital elements
Eccentricity: 0.015; From JPL orbital elements
Inclination to Mars's Laplace plane: 1.10 °; From JPL orbital elements · The Laplace plane is the one a moon's orbit precesses about: close in it is the planet's equatorial plane, the plane of the rings, and further out it tilts toward the planet's own orbit.

Rotation

Rotation period: 7.65 hours
Tidal locking: Tidally locked to its parent

Temperature

Mean: 233 K (-40 °C)

Composition

Bulk composition: Dark, carbonaceous-chondrite-like rock with a porous, possibly rubble-pile interior. Its bluer terrain shows a faint absorption band, confirmed in 2026 from Mars Express and ExoMars TGO data, that suggests iron-bearing (mafic) silicates. Its origin (captured asteroid vs. re-accreted debris) is still debated.

Radiation & temperature

Insolation: 0.404 S⊕ · 40% of the sunlight Earth receives; Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
Geometric albedo: 0.07; Geometric (V-band) albedo — a very dark surface.
Equilibrium temperature: 218 K (-55 °C); Computed from albedo + distance from the Sun (airless black body); uses the geometric albedo as an estimate
Measured mean temperature: 233 K (-40 °C)
Greenhouse warming: +15 K; Measured temperature minus the airless equilibrium — the greenhouse effect

Discovery & history

Discovered: August 18, 1877
By: Asaph Hall
Method: Visual discovery (telescopic)
Name: Phobos ('fear'), a son of the war-god Ares (Mars) in Greek myth.

Asaph Hall found both of Mars's tiny moons over a few nights in August 1877. Phobos, the larger, inner moon, orbits so close that it crosses the Martian sky three times a day and is slowly spiralling inward — in tens of millions of years it will break apart into a ring or strike the planet.