Our own Planetary Science Institute announced a new timeline for the moons of Saturn. Generally, the age of rocky bodies in our solar system are determined by the cratering rates of the surface – how many craters are on the surface and how often those bodies are hit with impacts big enough to leave those craters. For the Moon and Mars, we know these numbers; however, for the moons of Saturn, we do not. So Institute scientist Sam Bell took a different approach.

First, the accepted theory states that craters on these moons should be caused by objects that orbit the Sun. Second, in accordance with that assumption, is that moons closer to Saturn will show evidence of more cratering due to Saturn’s gravitational pull bringing more objects closer in to hit those moons. 

However, Bell found the evidence contradicted that second assumption. He explains: The crater densities of the oldest surfaces of Mimas, Tethys, Dione, Rhea, and Iapetus are all relatively similar. It would be too much of a coincidence for the ages of the oldest surfaces on each moon to vary by the exact amounts necessary to produce broadly similar crater densities. As a result, it seems much likelier that the impactors actually come from objects orbiting Saturn itself, moonlets that would be too small to detect with current technology.

This new chronology changes how we think about the ages of Saturn’s moons. If everything that impacted them had to orbit the Sun, then the moons could not be less than four billion years old. With the impactors now orbiting Saturn, we create room in the chronology for younger moons, which goes along with observations of how their orbits have evolved. And as Bell notes: The assumption of impactors orbiting the Sun results in the conclusion that the surface of Titan is probably at least 4 billion years old, even though Titan shows clear evidence of active weathering. With the new chronology, Titan could be quite young, which is much more consistent with observations of lakes, riverbeds, dunes, and mountains.

Saturn continues to fascinate, and there is still more to learn about this beautiful planet and its system. 

More Information

Planetary Science Institute press release 

“Relative Crater Scaling Between the Major Moons of Saturn: Implications for Planetocentric Cratering and the Surface Age of Titan,” Samuel W. Bell, 2020 May 26, Journal of Geophysical Research Planets