Rare, elliptical craters reveal new clues about strange Saturn moons Tethys and Dione

Rare, elliptical craters spotted on two of Saturn's moons reveal new clues about the satellites' age and formation, a new study shows. 

Using data from NASA's Cassini mission, researchers from the Southwest Research Institute (SwRI) measured elliptical craters on Saturn's moons Tethys and Dione. Whereas circular craters are fairly common and form under various impact conditions, elliptical craters are rarer and are believed to form from slow and shallow impacts, according to a statement from the SwRI. 

"Our work aims to answer the broader question of how old these moons are," Sierra Ferguson, lead author of the study and a postdoctoral researcher at SwRI, said in the statement. "To get at this question, my colleagues and I mapped elliptical craters on the surfaces of these moons to determine their size, direction and location on the moon." 

Related: Amazing Saturn photos from NASA's Cassini orbiter

The shape and orientation of these elliptical craters are indicative of the trajectories of the objects that may have impacted Tethys and Dione. In turn, understanding the direction potential impactors may have come from allowed the researchers to estimate the age of the two moons. 

On Dione, Saturn's fourth-largest moon, the researchers spotted a pattern among the elliptical craters situated along the moon's equator. These craters appeared to be oriented in an east-west direction, whereas craters closer to the moon's poles exhibited more random orientations, according to the statement. 

The researchers identified similar east-west elliptical craters at the equator of Tethys, Saturn's fifth-largest moon, along with distinct populations of high-latitude craters resembling those on Neptune's moon Triton that are believed to be caused by objects drawn in by the ice giant's massive gravity. Therefore, similar objects may have caused the high-latitude craters observed on Tethys, the researchers said. 

"We initially interpreted this pattern to be representative of two distinct impactor populations creating these craters," Ferguson said in the statement. "One group was responsible for creating the elliptical craters at the equator, while another, less concentrated population may be more representative of the regular background population of impactors around Saturn.

The equatorial craters may have formed from either independent disks of debris orbiting each moon or a single debris disk that affected both moons, the researchers suggested. The team plans to compare their findings at Saturn to other planets, such as Uranus, to confirm how the craters formed. 

"Using Triton as a guide, Tethys could reasonably be billions of years old," Ferguson said in the statement. "This age estimate is dependent on how much material was available for impacting the surface and when it was available." 

The team needs more data to confirm the findings, but the new research can provide insight into the formation conditions of these moons. 

"Was this a system that was completely chaotic, with materials hitting these satellites every which way, or was there a neat and orderly system?" Ferguson said. "This is the first step toward a new perspective on the cratering history of these moons and their origin and evolution." 

Their findings were published online June 10 and will appear in the September 2022 issue of the journal Earth and Planetary Science Letters. 

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