How Did Mars’ Biggest Moon Come to Look Like the Death Star?


Was a comet responsible for the formation of the Stickney Crater, seen here on the right of Phobos?

Thanks to an enormous ding on its surface known as the Stickney Crater, Mars’ moon Phobos is said to look like the Death Star from the Star Wars movies. The crater is presumed to have been caused by comet impact, but exactly how a 9km wide crater could have formed on a 22km-wide moon without destroying it has remained something of a mystery.

Now researchers at Lawrence Livermore National Laboratory think they have the answer. Using computer simulations, they found that the most likely culprit was a comet with a 250m diameter, travelling at close to six kilometers per second.

Previous studies using 2D simulations lacked the resolution to model the Stickney Crater successfully and failed to account for the fact that Phobos is made of much less dense material than its host planet.

“We’ve demonstrated that you can create this crater without destroying the moon if you use the proper porosity and resolution in a 3D simulation,” said researcher Megan Bruck Syal. “There aren’t many places with the computational resources to accomplish the resolution study we conducted.”

As well as solving a longstanding mystery, this research also acts as a proof of concept exercise for the team’s modelling software, Spheral, which they use to simulate various methods of deflecting potentially hazardous Earth-bound asteroids. “Something as big and fast as what caused the Stickney Crater would have a devastating effect on Earth,” Syal said. “If NASA sees a potentially hazardous asteroid coming our way, it will be essential to make sure we’re able to deflect it. We’ll only have one shot at it, and the consequences couldn’t be higher. We do this type of benchmarking research to make sure our codes are right when they will be needed most.”