The Cassini spacecraft and its Huygens probe—which descended onto the surface of Titan, Saturn's largest moon, in 2005—measured the composition of both planet and moon. Cassini found that Saturn is more enriched with heavy elements than Jupiter. Measurements of Titan's atmospheric composition suggest that the moon didn't form from the solar nebula along with Saturn. Instead, Titan was assembled later, from a disk of gas and dust that circled an already formed Saturn.

When Cassini first entered the Saturnian system in 2004, it flew past Phoebe, one of Saturn's strangest—and most primitive—moons. Phoebe is covered in dark material and follows a tilted orbit in a direction opposite to Saturn's other major moons. Cassini's analysis showed that Phoebe is similar in composition to Pluto—and therefore most likely originated from the Kuiper Belt, a collection of icy objects beyond the orbit of Neptune. Phoebe's chemical composition has probably remained the same since it formed along with the sun and planets, offering insight into the solar system's past.

Juno will focus on Jupiter itself, unraveling what the gas giant's chemical composition and interior have to say about the solar system's history. Although it won't land on any of its moons, Juno will study their influence on Jupiter and its magnetic field.