amalthea

Orbiting within Io's orbit, which is the innermost of the four largest moons of Jupiter (called the Galilean moons), are four smaller moons named Metis, Adrastea, Amalthea, and Thebe.

All of the moons within this grouping are oddly shaped, lacking either the mass and/or fluidity of composition to pull themselves into a reasonably spherical shape. The Galileo spacecraft has revealed some surface features, including impact craters, hills, and valleys.

Amalthea is the largest within this grouping with a mean radius of about 83.5 ± 2.4 km. Amalthea orbits 181,400 km from its parent planet, Jupiter. Amalthea takes 0.498 Earth days to complete one orbit.

Amalthea is the reddest object in the solar system and it appears to give out more heat than it receives from the sun. This may be because, as it orbits within Jupiter's powerful magnetic field, electric currents are included in the moon's core. Alternatively, the heat could be from tidal stresses.