The outer moons of Jupiter are predominantly retrograde because of a phenomenon called "capture" or "resonance."
When a moon forms in a planetary system, it typically follows a prograde (or forward) orbit around its parent planet. However, Jupiter's outer moons, which are located far from the massive planet, are influenced by several factors.
One significant factor is the gravitational interaction with other moons. These moons can exert gravitational forces on each other, leading to a phenomenon known as orbital resonance. Orbital resonance occurs when the orbital periods of moons are related by a ratio of small integers.
Jupiter's powerful gravity and the presence of many large moons create a complex gravitational environment. As the outer moons interact with each other over time, their orbits can become resonant, which causes their orbits to be modified. This gravitational interaction tends to result in the moons' orbits becoming tilted and elongated, eventually leading to a retrograde motion.
Additionally, the influence of the Sun's gravity on these outer moons plays a role. The Sun's gravitational pull can disrupt the moons' orbits, leading to changes in their inclination and causing them to adopt retrograde orbits.
It's important to note that not all outer moons of Jupiter are retrograde. Some exhibit prograde motion, but retrograde moons are more prevalent in the outer regions due to the specific dynamics of Jupiter's moon system.