Amalthea
Moon· orbits Jupiter
Amalthea is a small, irregular, reddish inner moon of Jupiter — a porous, low-density body that appears to be a loosely bound rubble pile, reddened by sulfur from Io.
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Key facts
Mass
2.08 × 10¹⁸ kg
0.0000283 Moon masses
Radius
83.5 km
5% of our Moon's radius
Surface gravity
0.0199 m/s²
0% of Earth's gravity
Day length
12.00 hours
Orbit around Jupiter
12.0 hours
0.50 Earth days
Science
Physical
- Mass
- 2.08 × 10¹⁸ kg · 0.0000283 Moon masses
- JPL / IAU mean radii; spacecraft-tracked GM where measured
- Radius
- 83.5 km · 5% of our Moon's radius
- JPL / IAU mean radii; spacecraft-tracked GM where measured
- Density
- 853 kg/m³ · 0.85× water
- Computed from mass and radius
- Surface gravity
- 0.0199 m/s² · 0% of Earth's gravity
- Computed from mass and radius
- Escape velocity
- 57.7 m/s · 1% of Earth's escape velocity
- Computed from mass and radius
Orbit around Jupiter
- Distance from Jupiter
- 181,400 km
- From JPL orbital elements
- Orbital period
- 12.00 hours
- From JPL orbital elements
- Eccentricity
- 0.003
- From JPL orbital elements
- Inclination to Jupiter's Laplace plane
- 0.40 °
- 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
- 12.00 hours
- Tidal locking
- Tidally locked to its parent
Composition
- Bulk composition
- A porous, low-density mix of water ice and rock (a likely rubble pile), its surface reddened by sulfur from Io.
Radiation & temperature
- Insolation
- 0.0354 S⊕ · 4% of the sunlight Earth receives
- Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
- Geometric albedo
- 0.09
- Geometric (V-band) albedo — a dark surface; Bond albedo not directly measured.
- Equilibrium temperature
- 118 K (-155 °C)
- Computed from albedo + distance from the Sun (airless black body); uses the geometric albedo as an estimate
Discovery & history
- Discovered
- September 9, 1892
- By
- Edward Emerson Barnard
- Method
- Visual discovery (telescopic)
- Name
- Amalthea, the nymph (or she-goat) who nursed the infant Zeus in Greek myth.
Amalthea was the last moon in the Solar System found by direct visual observation — Edward Barnard glimpsed it at Lick Observatory in 1892, the first new Jovian moon since Galileo's four in 1610. Reddish and irregularly shaped, it is dusted with sulfur erupted from nearby Io.
Images
Images from open archives · credit shown per image