Q How are moons created?
— Rebekah Neuman, Madison, Wis.
A Jim Lattis, director of UW Space Place at University of Wisconsin-Madison:
Anything that orbits a planet is a moon, and moons can form in several ways.
For example, Jupiter has four large moons called the Galilean moons. Those moons look like a small solar system with Jupiter acting the role of the sun. From the innermost to the outermost, the Galilean moons show a variation in size and composition in a way that’s similar to the planets of our solar system.
Those moons look like they formed along with Jupiter, with the whole system created together.
Our own Earth’s moon, on the other hand, did not form along with the Earth. The current theory, supported by evidence, is that our moon resulted from a collision that happened very early in the solar system.
The object that would eventually become Earth collided with another object. This second object was destroyed in the collision, and the collision debris turned into our moon.
Sometimes a moon is created when a planet captures it and pulls it into its orbit. Mars’ two very small, asteroid-like moons are possibly asteroids that Mars captured from the nearby asteroid belt.
Titan, the largest of Saturn’s moons, is particularly interesting. It’s a very complicated world with a temperature that allows for fluids, including methane and other hydrocarbons, to exist in frozen, liquid and vapor form.
This is similar to what’s happening on Earth with water. We have ice, liquid water and water vapor all interacting in our weather and climate system.