Stickney (crater)

• 1. Introduction

  ›

  Nine kilometers across, Stickney crater takes up a substantial proportion of Phobos, the inner moon of Mars. The impact that formed it should have destroyed the moon it now dominates. For decades after its discovery, two questions hung over this scar. How did Phobos survive? And what do the grooves radiating outward from the crater reveal about what happened? The crater also carries the name of a mathematician almost no one knows. Her quiet persistence made the discovery of Phobos possible.

  1411

• 2. A Mathematician's Name

  ›

  Asaph Hall discovered Phobos, but he was candid about how close he came to giving up. His wife's support, he said, was what kept him searching. Chloe Angeline Hall, née Stickney, was a mathematician in her own right. Asaph Hall was explicit about his debt to her: her support was critical to his discovery of the moon. The International Astronomical Union honored that connection in 1973. The decision rested on images returned by the Mariner 9 spacecraft, which gave scientists their first detailed view of the crater. Carl Sagan chaired the nomenclature committee that made the decision. The crater predates Mariner 9 by billions of years. How many billions depends on a question about Phobos that the naming committee of 1973 could not have answered.

  123

• 3. Timing an Ancient Impact

  ›

  If Phobos has been orbiting Mars for 4.3 billion years, then Stickney formed roughly 4.2 billion years ago. A shorter orbital history of 3.5 billion years points to a crater around 2.6 billion years old. The uncertainty belongs to Phobos itself: the question of when the moon settled into its current orbit has not been resolved. For no more than 1,000 years after the Stickney collision, an enormous cloud of ejecta orbited Mars. The debris had been blasted free of Phobos's weak gravity by the impact. Some of it fell back onto Phobos, punching secondary craters into the surface. The majority of Phobos craters smaller than 600 meters in diameter were formed by that returning bombardment. The collision also left a network of grooves and crater chains across Phobos's surface. Explaining how they formed has proved far more difficult than counting them.

  45610

• 4. A Fractured, Surviving Moon

  ›

  A 2016 study by Syal and colleagues found that the high porosity of Phobos was critical in preventing its destruction during the collision. Without that property, the impact would have shattered the moon. Models of the moon's interior suggest the rock directly beneath Stickney may be denser and less porous than the surrounding material. But models of Phobos's interior vary on this point, and the picture is not yet complete.

  Grooves and crater chains radiate outward from Stickney, and several competing explanations exist for how they formed. One hypothesis holds that the Stickney impact generated stresses inside Phobos powerful enough to nearly split the moon in two. On this reading, those propagating stresses are what created the surface markings. Other researchers point to material ejected from impacts on Mars as the source. A third hypothesis invokes the tidal forces that Mars exerts on Phobos. A fourth suggests that boulders set loose by the Stickney collision rolled across the moon's surface and left the grooves behind. None of these explanations has been ruled out.

  Inside Stickney, the walls carry a lineated texture. That pattern was left by landslides, material falling inward from the crater walls and collecting on the floor. On the south-western edge of the crater, a patch of rock displays a distinct blue spectral coloration. Scientists believe this marks a relatively thin layer of rock. It formed from a combination of material originating within Stickney and from Limtoc, a smaller crater nested inside it. The question of what lies beneath Stickney's floor, whether dense rock or the same porous material that saved Phobos from destruction, remains open.

  11127915