Amalthea

Amalthea matters because it shows what a moon looks like at the ragged inner edge of a giant planet's realm. Orbiting closer to Jupiter than Io, it is far too small for gravity to round it out, so it stays an irregular, elongated shape only a couple hundred kilometers across. It was the last moon in the Solar System discovered by direct visual observation, in 1892, before photography took over the hunt.

What makes Amalthea physically intriguing is what Galileo found. Its surface is one of the reddest in the Solar System, probably stained by sulfur drifting in from volcanic Io, and measurements of its mass imply a density so low that the moon cannot be solid. Instead it is likely a porous rubble pile, full of empty space, perhaps even holding water ice despite sitting deep inside Jupiter's fierce radiation environment.

That is why Amalthea belongs on this scale. It bridges the gap between the great round Galilean moons and the tiny captured rocks farther out, showing that a planet's moon system includes irregular, fragile bodies as well as worlds. Amalthea is the reminder that not every moon is a sphere.