A new SwRI study describes how the unique populations of craters on two of Saturn’s moons could help determine the satellites’ age and formation conditions. SwRI postdoctoral researcher Dr. Sierra Ferguson surveyed elliptical craters on Saturn’s moons Tethys and Dione using data from NASA’s Cassini mission for this study, which was co-authored by SwRI Principal Scientist Dr. Alyssa Rhoden, Lead Scientist Dr. Michelle Kirchoff, and Lead Analyst Dr. Julien Salmon.
“Our research aims to provide an answer to the larger question of how old these moons are. To answer this question, my colleagues and I measured the size, direction, and location of elliptical craters on the surfaces of these moons “Ferguson explained.
Circular craters are extremely common and can be formed by a variety of impact conditions. However, elliptical craters are more common and form from slow and shallow impacts, making them particularly useful in determining an object’s age because shape and orientation also indicate the trajectory of the impactor.
“We can get an idea of what the impactors that made these craters looked like in a dynamical sense and from which direction they might have hit the surface by measuring the direction these craters point,” she said.
Ferguson was not expecting to find a pattern among the elliptical crater directions, but she eventually noticed one along the equator of Dione, one of Saturn’s small moons. There, elliptical craters were overwhelmingly oriented east/west, whereas directions near the moon’s poles were more random.
“At first, we thought this pattern represented two distinct impactor populations creating these craters,” she explained. “One group was responsible for the elliptical craters near the equator, while another, less concentrated population may be more representative of Saturn’s regular background population of impactors.”
Ferguson also mapped elliptical craters on Tethys, Saturn’s fifth largest moon, and discovered that while a similar size-frequency distribution of craters is unusual for objects orbiting the Sun, it coincides with estimates for the impactor population that appears to exist on Neptune’s moon, Triton. Ferguson’s findings emphasise the importance of considering planetocentric impactors when examining the age of objects in the Saturnian system because that population is thought to be planetocentric, or drawn in by the ice giant’s massive gravity. “Seeing these patterns was really amazing,” she said.
Ferguson believes the equatorial craters formed from separate discs of debris orbiting each moon, or from a single disc that affected both moons.
“Tethys could be billions of years old if we use Triton as a guide. This age estimate is based on the amount of material available for impacting the surface and when it was available “Ferguson explained. “Of course, more data will be needed to be certain, but this research tells us a lot. It can help us understand the conditions under which these moons formed. Was this a completely chaotic system, with materials hitting these satellites in every direction, or was there a neat and orderly system?”
Ferguson hopes to one day be able to compare her data from Saturn’s moons to that of Uranus, another ice giant. While current data is inconclusive, one of the flagship missions recommended by the Planetary Science Decadal Survey, which was published in April, is a mission to Uranus and its moons.
“This is the first step toward a new perspective on the cratering history of these moons, as well as their origin and evolution,” Ferguson said.
