Excavating Stickney crater at Phobos

Stickney crater, at 9 km across, dominates the morphology of ~22 km Phobos, the larger of the two moons of Mars. The Stickney impact event had global repercussions for Phobos, including extensive resurfacing and fracturing of the moon. Understanding the initial conditions and dynamical consequences of the collision is necessary to test competing hypotheses for the origin of peculiar grooved terrain that striates much of the surface. Previous modeling of the impact event was unable to replicate Stickney without globally fragmenting the satellite. Here we describe high‐resolution numerical simulations that successfully generate Stickney crater while maintaining the large‐scale structure of Phobos. Target porosity, which is estimated to be significant, aids in keeping the moon intact. Damage follows patterns centered on Stickney that are inconsistent with the observed alignment of grooved terrain on Phobos. Low‐velocity boulders are ejected at shallow angles in sufficient numbers to support a rolling‐boulder origin for grooved terrain. Plain Language Summary Mars' larger moon, Phobos, is famously scarred by Stickney crater, which stretches nearly halfway across its 22‐km diameter. The formation of this megacrater is successfully reproduced for the first time, using a Phobos‐shaped target and high‐resolution, 3‐D impact simulations. Target porosity and numerical resolution are found to play key roles in preventing global devastation during the event. The simulations provide new insight into the origins of hundreds of mysterious, parallel grooves that encircle the satellite. These features are shown to not be related to impact fracture. Instead, they may be related to low‐velocity crater ejecta processes or unrelated to the Stickney event entirely. Full modeling of the Stickney impact places constraints on the history and current state of Phobos, which is of particular interest for future robotic and human exploration. Key Points Stickney‐forming impacts are modeled using a Phobos‐shaped target for the first time High‐resolution impact simulations successfully generate Stickney crater without catastrophically disrupting Phobos Resulting damage to Phobos suggests that the satellite's grooved terrain is unrelated to Stickney‐induced fracture