Jupiter’s Innermost Moons

“Tiny specks, even in the cameras’ best magnification. Captured asteroids, undoubtedly; minor worldlets that had been seized by the king and forced to circle his majesty until one day they approached too close and were ground into dust by Jupiter’s enormous gravitational power.”

- Ben Bova, Jupiter (2001)

This is a portrait of the 4 innermost moons of Jupiter. From left to right, arranged in order of decreasing distance from Jupiter, are Thebe, Amalthea, Adrastea and Metis. These images were taken by the Galileo spacecraft between November 1996 and June 1997. Image credit: NASA/Cornell University.

Jupiter is best known for its 4 large Galilean moons - Io, Europa, Ganymede and Callisto. Each measuring thousands of kilometres in size, the 4 Galilean moons are unique worlds in their own right. Ganymede, for example, is larger in size than the planet Mercury. In addition to the 4 Galilean moons, dozens and dozens of tiny moons circle Jupiter. Situated closer to Jupiter than Io are 4 tiny moons - Metis, Adrastea, Amalthea and Thebe. They comprise the innermost moons of Jupiter. Metis is the innermost of the 4 moons, and from its surface, Jupiter would span a whopping 67.9 degrees. Metis, Adrastea, Amalthea and Thebe have respective orbital periods of 7h 4m, 7h 9m, 11h 57m and 16h 11m. In fact, both Metis and Adrastea orbit Jupiter in less than a Jovian day.

Observations of Metis, Amalthea and Thebe by the Galileo spacecraft show their leading sides to be brighter than their trailing sides by 25 to 30 percent. The asymmetry is presumably due to the high velocities (~20 to 30 km/s) at which these moons whiz around Jupiter, resulting in a higher frequency of meteoroid impacts on their leading sides. These impacts excavate brighter subsurface material, supposedly water-ice, resulting in a more reflective surface. The tiny moons are believed to be composed primarily of water-ice with some amount of porosity. Ejecta from these moons, along with a number of as-yet-unseen inner moonlets, are thought to be the source of Jupiter’s tenuous ring system.