Within the orbit of Jupiter’s innermost large moon, Io, are four smaller moons: Metis, Adrastea, Amalthea, and Thebe. They are known as the Inner Group, or the Amalthea group after their largest member, which measures 270 kilometers (168 miles) at its longest.

Amalthea is the reddest object in the Solar System, possibly because of sulfur being ejected by Io’s volcanoes. Amalthea also appears to radiate more heat than it receives from the Sun. According to NASA, this may be because electric currents are induced in the moon's core as it orbits within Jupiter's powerful magnetic field. It might also be generated by the same kind of tidal forces that make Io the volcanic world it is. 

All four Amalthean moons orbit Jupiter at break-neck speed. Thebe goes around Jupiter in about 16 hours, and Amalthea in 12. Adrastea whips around the giant planet in only seven hours, and Metis in just five.

Adrastea and Metis orbit inside Jupiter's main ring and are likely the source of the ring’s material. Amalthea and Thebe shed material that creates Jupiter’s Gossamer ring.