Amalthea: A Dusty Red World at the Heart of Jupiter's Rings

Amalthea, inner moon of Jupiter seen by Galileo — Amalthea and its reddish surface, pockmarked with craters and pronounced reliefs.
Image source: NASA Voyager 1 (new window)

Scientific summary

This article presents Amalthea, Jupiter's third closest moon, discovered in 1892 by Edward Barnard. With a mean radius of 83.5 km and an irregular shape, it orbits at 181,000 km (2.5 Jovian radii), within Jupiter's inner rings. Its exceptionally low density (0.86 g/cm³) suggests a porous composition of ice and silicates. Its intense red color may originate from sulfur compounds from Io or interplanetary dust. Micrometeorite impacts eject dust that feeds the Gossamer ring. Exposed to intense radiation from Jupiter's magnetosphere, its surface undergoes slow erosion and electrostatic charging. The Galileo probe measured its mass and density, revealing a porous world with extreme topography (Mount Pan, Gaea crater).

What is Amalthea and why is this small moon of Jupiter so intriguing?

Amalthea is a small moon of Jupiter discovered in 1892 by Edward Barnard. With a mean radius of 83.5 km and an irregular shape, it orbits just 181,000 km from Jupiter (2.5 Jovian radii), within its tenuous rings. Its surprisingly low density (0.86 g/cm³, less than water) indicates a porous structure composed of ice and silicates. Its intense red color intrigues planetary scientists: it may originate from sulfur compounds emitted by Io's volcanism, or from a layer of interplanetary dust trapped on its surface. Amalthea plays a dynamic role in the Jovian system: micrometeorite impacts eject dust that feeds the Gossamer ring, contributing to the diffuse luminosity of Jupiter's environment. Its proximity to the planet exposes it to an intense bombardment of energetic particles trapped in the magnetosphere, causing slow erosion and fluctuating electrostatic charges. Measured by the Galileo probe, its low gravity (1.8×10⁻³ m/s²) and extreme topography (Mount Pan, Gaea crater) make it a key object for understanding the formation and evolution of small bodies in planetary systems.

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A small irregular moon discovered by Edward Barnard

Discovered in 1892 by Edward Emerson Barnard (1857-1923), Amalthea is the third moon of Jupiter by distance and one of the most intriguing of its inner satellites. With an average radius of about 83 km, it has an irregular shape resembling a giant rock deformed by the planet's gravity. Its intense red color intrigues planetary scientists: it could be linked to sulfur compounds from Io's eruptions or a thin layer of interplanetary dust trapped on its surface.

An orbit within Jupiter's inner rings

Amalthea orbits Jupiter at a distance of about 181,000 km, or just 2.5 Jovian radii from the planet's center. It lies within Jupiter's tenuous ring system and directly contributes to their supply. Micrometeorites constantly striking its surface eject dust that integrates into the Gossamer ring (set of fine particles orbiting a planet). This dynamic interaction between gravity, dust, and plasma creates a complex environment where material density varies with Jovian magnetic latitude.

A surprisingly low density

Measurements taken by the Galileo probe revealed that Amalthea has an average density of only 0.86 g/cm³, less than that of water. This value indicates a composition mainly of ice mixed with porous and fractured material. With a surface gravity of only \(1.8 \times 10^{-3}\) m/s², an astronaut could easily jump hundreds of meters high. This low density supports the hypothesis of a primitive origin, perhaps an icy fragment captured in the early days of the Jovian system.

Extreme reliefs and interaction with Jovian radiation

Amalthea shows pronounced reliefs: Mons Pan and Gaea Crater reach several kilometers in depth. These structures indicate violent impacts and weak internal cohesion. Its orbit, very close to Jupiter, also exposes it to intense bombardment by energetic particles trapped in the planet's magnetosphere (region where a planet's magnetic field traps charged particles). The incident radiation causes fluctuating electrostatic charge and slow erosion of surface materials.

A paradoxical glow in Jupiter's shadow

Despite its small size, Amalthea strongly reflects sunlight, sometimes appearing as a reddish point during Jovian occultations. Its surface, composed of irregular materials, scatters light according to a modified Rayleigh law (scattering law inversely proportional to wavelength). The light reflected by Amalthea faintly illuminates the upper layers of the inner rings, thus contributing to the diffuse luminosity observed in Jupiter's environment.

N.B.:
Jupiter's inner moons (Metis, Adrastea, Amalthea, and Thebe) all orbit within the Jovian magnetosphere. Their interaction with coronal plasma generates luminous arcs and radio emissions detectable from Earth.

Main physical characteristics of Amalthea
Parameter	Average value	Unit	Comment
Mean radius	83.5	km	Irregular shape, very elongated (250 × 146 × 128 km)
Mass	2.08 × 10¹⁸	kg	Low gravity, close to \(1.8 \times 10^{-3}\) m/s²
Density	0.86	g/cm³	Indicates a porous structure composed of ice and silicates
Average distance from Jupiter	181,366	km	Located in the inner Gossamer ring
Orbital period	0.498	Earth days	Synchronized orbits, rotation locked on Jupiter

References

FAQ: Everything about Amalthea, Jupiter's Little Red Moon

Who discovered Amalthea and when?

Amalthea was discovered in 1892 by American astronomer Edward Emerson Barnard (1857-1923). It is the last satellite of Jupiter discovered by direct visual observation, before the era of space probes. Barnard used the 91 cm refractor at Lick Observatory in California to spot this small moon, whose brightness was drowned in Jupiter's glare.

What are Amalthea's physical characteristics?

Amalthea has remarkable characteristics:
• Mean radius: 83.5 km, with an irregular shape (250 × 146 × 128 km).
• Density: 0.86 g/cm³, less than water, indicating a porous structure of ice and silicates.
• Color: intense red, likely due to sulfur compounds from Io or interplanetary dust.
• Surface gravity: 1.8×10⁻³ m/s², extremely weak (a jump would reach several hundred meters).

Where is Amalthea located in the Jovian system?

Amalthea orbits at 181,000 km from Jupiter, about 2.5 Jovian radii. It lies within the inner rings of Jupiter, specifically in the Gossamer ring. Its position makes it Jupiter's third closest moon, after Metis and Adrastea, but before Thebe. Its orbit, very close to the planet, exposes it to intense radiation trapped in Jupiter's magnetosphere.

What is Amalthea's role in feeding Jupiter's rings?

Amalthea plays a crucial dynamic role in feeding Jupiter's rings. Micrometeorites striking its surface continuously eject dust that integrates into the Gossamer ring, the outermost and most diffuse part of Jupiter's ring system. This ejection and gravitational capture process creates a complex environment where material density varies with Jupiter's magnetic latitude. Light reflected by Amalthea also contributes to the faint diffuse luminosity observed in this region.

Why is Amalthea so red?

Amalthea's intense red color intrigues planetary scientists. Two main hypotheses are proposed:
• Sulfur compounds: the moon may be covered with sulfur and sulfur compounds from Io's volcanic eruptions, transported by magnetospheric plasma.
• Interplanetary dust: a thin layer of dust rich in organic matter or radiation-altered silicates may have accumulated on its surface.
This red color is a chemical marker that helps understand material exchange in the Jovian system.

What were the Galileo probe's discoveries about Amalthea?

The Galileo probe (1995–2003) provided the first precise measurements of Amalthea:
• Mass: 2.08 × 10¹⁸ kg, determined by gravitational perturbations on the probe's trajectory.
• Density: 0.86 g/cm³, confirming a porous structure and ice-rich composition.
• Extreme topography: Mount Pan and Gaea crater, reaching several kilometers in depth, indicating violent impacts and weak internal cohesion.
• Radiation interaction: the probe measured the effects of energetic particle bombardment on the moon's surface.

How does Amalthea interact with Jupiter's magnetosphere?

Amalthea orbits within Jupiter's magnetosphere, a region dominated by Jupiter's magnetic field and populated by energetic particles. Its interaction with the magnetic environment manifests as:
• Particle bombardment: the surface is constantly struck by high-energy ions and electrons, causing slow erosion.
• Electrostatic charging: the surface accumulates charges that can discharge suddenly, influencing dust behavior.
• Radio emission: Amalthea's interaction with coronal plasma generates luminous arcs and radio emission detectable from Earth, a phenomenon shared by the other inner moons (Metis, Adrastea, Thebe).