By Paula A. Heckert, PH.D. | Decoded Science
Both Jupiter and Saturn have very extensive systems of moons. There are, however, differences. Jupiter has four fairly large moons, called the Galilean moons, that are comparable in size to Earth’s moon.
Saturn, on the other hand, has one large moon, Titan, and a number of middle-size moons. Both Jupiter and Saturn also have dozens of small moons.
Titan is comparable in mass to Jupiter’s Galilean moons, but because Jupiter is more massive than Saturn Titan is more massive compared to Saturn than the Galilean moons are compared to Jupiter. Titan, in fact, has more mass relative to the central planet than all Jupiter’s Galilean moons combined.
Jupiter and Saturn: Similar Yet Different
As planets Jupiter and Saturn are fairly similar, so why do their moon systems differ so much? Astronomers think that the moon systems of Jupiter and Saturn both formed by the same accretion process around the planets as the planets formed. This accretion process is similar to the process that formed the solar system of planets orbiting the Sun.
Erik Asphang of UC Santa Cruz and Andreas Reufer of the University of Bern has proposed a new theory to explain this difference, the origin of Saturn’s middle-sized moons, and Titan’s larger mass relative to its planet.
Asphang and Reufer proposed this theory at the American Astronomical Society’s Division for Planetary Sciences meeting on October 19, 2012. Their theory will also be published in the journal Icarus.
Saturn Formation Theory
Asphang and Reufer came up with their theory by performing computer simulations of the formation of Saturn’s system of moons. They postulated various scenarios and calculated the resulting moon system of these scenarios. When a calculated moon system matched Saturn’s actual moon system, they found the scenario that likely formed Saturn’s moon system.
The key is collisions: Asphang and Reufer suggest that Saturn started with a moon system similar to Jupiter’s. Saturn initially had several large moons. These moons were however less massive than Titan or Jupiter’s Galilean moons because Saturn is less massive than Jupiter. The proposed collisions might have occurred either late in the period when Saturn’s moons were forming or after a stable moon system had formed.
Their calculated simulations show that if Saturn’s large moons collided in a giant impact and merger with another similar sized body, the outer layers of the smaller satellite release spiral arms composed of icy materials.
The icy spiral arms then coalesce into middle-sized moons with sizes and compositions similar to Saturn’s middle-sized moons. The merged moons stick together to form a single larger moon.
After a series of such collisions, Saturn was left with one very massive moon relative to the planet, Titan, and over a half dozen middle-sized moons.
In this context icy materials include both water ice and ices of other common materials that are either liquid or gaseous on Earth.
The idea of colliding moons may sound far-fetched to some – moons and planets are far enough apart that they should not collide. However, the current best theory for Earth’s Moon’s origin, the giant impact model, involves similar collisions between the forming Earth and an object about the size of Mars.
In the early solar system there was much more debris and small objects floating around, hence collisions would have been much more frequent.
This theory is not yet perfect, but at this point it is a very plausible explanation for the unusual features of Saturn’s moon system. According to Asphang, new data from NASA’s Cassini mission to Saturn will help test this new hypothesis.