That’s No Space Station: How Mars’ “Death Star” Moon Got its Crater

With its signature crater, the largest of Mars’ two moons, Phobos, is sometimes called the Death Star, calling to mind the “technological terror” prominent in the Star Wars films. The moon has not only spurred the public’s imagination, but that of astrophysicists as well. Many had wondered how the impact that created such a huge crater could have done so without destroying the entire body. At nine kilometers in diameter, the crater, Stickney, takes up a huge amount of the moon’s surface—for scale, the entire moon is only 70 kilometers around.

Phobos’ Stickney crater, imaged by NASA’s Mars Reconnaissance Orbiter in March 2008.
Image Credit: NASA/JPL-Caltech/University of Arizona.

However, a new study by the planetary defense team at Lawrence Livermore National Laboratory (LLNL) published in Geophysical Research Letters Oct. 8 documents a successful simulation of the formation of the Stickney crater. Previously, this was not possible—limited by the technology and data available at the time. Those past studies were unaware of Phobos’ relatively low density, owing to its porous crust.

Furthermore, those simulations were limited by the two-dimensional graphics and relatively low resolutions. “There aren’t many places with the computational resources to accomplish the resolution study we conducted,” Megan Bruck Syal, a study author, said.

The recent LLNL study confirms the plausibility of theories that a huge impact created the Stickney crater. “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,” Syal said.

The above animation shows one of several feasible scenarios, with a 200-meter object impacting at eight kilometers per second, in which the Stickney crater could have formed on Phobos.
Credit: Lawrence Livermore National Laboratory

“Something as big and fast as what caused the Stickney crater would have a devastating effect on Earth,” Syal said in the press release. She is a member of the LLNL planetary defense team, which has been studying ways to protect Earth from the devastating effect of a collision with a massive asteroid.

The study of this real Death Star has been treated as a case study to perfect the modeling of these “kinetic impacts.” Beyond the 3D imaging, the study helped test open-source code developed at LLNL called Spheral. The code is also used to simulate ways to deflect potentially hazardous asteroids from hitting Earth. “We do this type of benchmarking research to make sure our codes are right when they will be needed most,” Syal said.

Others involved in the LLNL Phobos study were second author Jared Rovny, a student visiting from Yale University, his mentor LLNL computational physicist Mike Owen, and Paul Miller, who leads the planetary defense team at LLNL.

Clinton Parks

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