Amalthea
Amalthea is the largest of Jupiter's four small inner moons, all of which orbit closer to the planet than Io. It has an irregular shape, a reddish surface and a low mean density. Dust ejected from Amalthea supplies one of Jupiter's faint rings.[3][1]
Size and physical characteristics
Amalthea has a volumetric mean radius of about 83.5 km. A sphere of the same volume would be 167 km in diameter, but the moon itself is markedly elongated and has no single diameter comparable to that of a nearly spherical planet. Images reveal craters, highlands and deep topographic irregularities.[1][5]
The current JPL table gives a mass of approximately 2.47 × 1018 kg and a density of 1.01 ± 0.12 g/cm³, close to the density of water. This is substantially lower than the density of solid rock and points to extensive porosity and possibly a significant proportion of ice.[1][2]
Surface gravity calculated from the mass and mean radius is approximately 0.024 m/s², about one four-hundredth of Earth's. Local gravity on the elongated, rotating moon varies with position; this figure describes its overall scale.[1]
Orbit and rotation
Amalthea orbits approximately 181,000 km from the centre of Jupiter. Its nearly circular path lies close to the planet's equatorial plane. One revolution takes slightly less than 12 hours, so it circles Jupiter about twice during one Earth day.[4][3]
Its rotation is synchronous, with the same side always facing Jupiter. Together with Metis, Adrastea and Thebe, Amalthea forms the group of small inner Jovian moons. All four lie inside Io's orbit and differ from the large Galilean moons in their small size and irregular shape.[3]
Surface and topography
Amalthea has a distinctly reddish surface. Voyager and Galileo images show brighter patches against the generally dark terrain. The moon's weak gravity allows large irregularities and its strongly nonspherical form to persist.[5][6]
A bright streak appears in two Galileo images acquired in August and November 1999. The different viewing directions helped establish its position relative to the surrounding terrain. Surface details approximately 3.8 km across can be distinguished in the images.[6]
The red colour is not evidence that the surface is hot. It is a property of the way the surface material reflects sunlight; visible colour and thermal measurements describe different characteristics of the moon.[5]
Internal structure
During Galileo's close flyby on 5 November 2002, Amalthea's gravity altered the spacecraft's motion. These measurements produced a mean-density estimate of 0.86 ± 0.10 g/cm³. Such a low value indicates a porous body with internal voids and probably a substantial quantity of water ice.[7][2]
A later reference value maintained by JPL gives a mean density of about 1.01 g/cm³. Amalthea therefore cannot be stated with confidence to be less dense than water. Its low density remains consistent with voids and ice, but the separate proportions of rock and ice cannot yet be determined.[1][2]
Ice in the interior would not imply an exposed icy surface. Amalthea looks unlike the bright ice-covered moons: its upper layer is dark and reddish. Its interior material has never been sampled directly.[2][5]
Connection with Jupiter's rings
Amalthea supplies particles to the Amalthea gossamer ring, the inner of Jupiter's two broad gossamer rings. Micrometeoroid impacts eject material from the moon, and some particles remain in orbit around Jupiter. The outer gossamer ring is similarly associated with Thebe.[8]
Galileo's dust detector recorded particles directly during crossings of the rings. Their distribution agrees with an origin at the nearby moons. Particles with radii of approximately 5 μm, far thinner than a human hair, contribute especially strongly to the ring's visible scattering area.[8]
Origin
Amalthea's porous structure and possible ice content constrain models of its formation. Conditions close to the young Jupiter may have been too hot for large quantities of ice to survive. The moon may therefore have formed in a colder region and subsequently moved inward, or assembled at a later stage.[2]
One model proposes that Amalthea formed farther from Jupiter and was later transported inward when migrating Io captured it in an orbital resonance. Their coupled motion could have carried the small moon through the gas disc surrounding the young planet. This mechanism works only under a limited range of disc conditions and remains a proposed explanation rather than a confirmed history.[9]
Discovery and exploration
Edward Emerson Barnard discovered Amalthea on 9 September 1892. It received the designation Jupiter V, as the fifth known moon of the planet. Its name refers to Amalthea of Greek mythology and was proposed by Camille Flammarion.[3]
Voyager and Galileo images established the moon's shape and volume. During its encounter on 5 November 2002, Galileo passed approximately 160 km above the surface. This was the spacecraft's final close moon flyby and provided the essential measurement of Amalthea's mass.[7][2]
On 7 March 2024, JunoCam aboard the Juno spacecraft photographed Amalthea against Jupiter. The moon appears as a small bright point beside the planet's immense atmospheric structures. Galileo's images remain the most detailed views of its surface.[5]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Planetary Satellite Physical Parameters // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Icy Jupiter Moon Throws a Curve Ball at Formation Theories // NASA Jet Propulsion Laboratory. 31 May 2005.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Amalthea // NASA Science.
- ↑ 4.0 4.1 4.2 4.3 4.4 Planetary Satellite Mean Elements // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 5.0 5.1 5.2 5.3 5.4 NASA's Juno Mission Spots Jupiter's Tiny Moon Amalthea // NASA Science. 13 May 2024.
- ↑ 6.0 6.1 Bright Streak on Amalthea, PIA02532 // NASA Science; NASA/JPL. 24 April 2000.
- ↑ 7.0 7.1 Anderson J. D. et al. Amalthea's Density Is Less Than That of Water // Science. 2005. NASA Technical Reports Server.
- ↑ 8.0 8.1 Krüger H. et al. Galileo In-Situ Dust Measurements in Jupiter's Gossamer Rings // Icarus. 2009. Preprint arXiv:0803.2849.
- ↑ Brunton I. R., Batygin K. On the Origin and Dynamical Evolution of Jupiter's Moon Amalthea // Accepted in The Astrophysical Journal; arXiv:2507.17464. 2025.