Space · Moons

Amalthea

A moon of Jupiter — Jupiter's reddest moon — the fifth-largest Jovian satellite and the first found after Galileo's quartet, after a 282-year gap.

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Quick facts

Parent planet

Jupiter

Diameter (mean)

167 km

Mass

2.08 × 10¹⁸ kg
2.83e-05 Moon masses

Mean orbital radius

181,400 km

Orbital period

0.498 Earth days

Discovery year

1892

Discoverer

Edward Emerson Barnard

Naming origin

Nymph who nursed infant Zeus

Surface conditions

Amalthea is an irregularly-shaped, elongated body 250×146×128 km across, the reddest object in the solar system — slightly redder than Mars, due to surface deposits of sulfur material likely escaped from Io. The surface is heavily cratered with two enormous impact features (Pan and Gaea) whose combined size exceeds Amalthea's mean radius, suggesting the moon has been catastrophically reshaped at least twice. Density is unusually low (less than water), implying a rubble-pile or ice-rich interior.

Amalthea is one of four moons that orbit within the orbits of the Galileans — the Amalthea ring system feeds material into Jupiter's faint main and gossamer rings. Galileo's flyby in November 2002 returned the most detailed images available.

Missions and observations

Every Jupiter-system mission has had an opportunity to image or characterize Amalthea. The list below is the Jupiter-system mission catalog; specific Amalthea encounters are documented in mission archives.

Mission	Year at Jupiter	Status
Pioneer 10 NASA	1973	Completed
Pioneer 11 NASA	1974	Completed
Voyager 1 NASA	1979	Completed
Voyager 2 NASA	1979	Completed
Ulysses NASA/ESA	1992	Completed
Galileo NASA	1995	Completed
Cassini-Huygens NASA/ESA/ASI	2000	Completed
New Horizons NASA	2007	Completed
Juno NASA	2016	Active
Europa Clipper NASA	2030	On the way
JUICE ESA	2031	On the way

Naming etymology

Amalthea was the nymph or goat (sources differ) who nursed the infant Zeus on Crete after his mother Rhea hid him from his father Cronus, who was eating his children. Amalthea's broken horn became the cornucopia, the horn of plenty. Edward Barnard discovered the moon in 1892 from the Lick Observatory in California — the first new Jovian moon found in 282 years, ending the Galilean monopoly. The IAU formally adopted 'Amalthea' in the 1970s; before then the moon was simply 'Jupiter V.'

Methodology & sources

Diameter, mass, and orbital parameters from JPL Solar System Dynamics — Physical Parameters. Discovery year and discoverer from the JPL Satellite Discovery Circumstances. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Stylized SVG hero composed from NASA / JPL imagery as visual reference; no photographs are reproduced.