Stickney Crater's age splits Phobos' two origin stories by 1.6 billion years

Both origin stories include the Stickney impact. A collision big enough to leave a crater 9 km across ^[2] would melt and compact the rock under it whether the target was debris from Mars or a captured asteroid, so a buried density contrast by itself does not say which. The scenarios disagree about when. If Phobos formed from material blasted off Mars, the Stickney collision may date to roughly 4.2 billion years ago ^[7]; if Phobos was captured, it could have come much later, around 2.6 billion ^[8]. The gap is about 1.6 billion years ^[9].

The gravity modeling is aimed at the interior instead. Haser and Andert's 2026 paper in Monthly Notices of the Royal Astronomical Society reports current estimates of a porous interior that may contain water ice ^[10], and Haser's EGU paper adds a denser concentration of material near Phobos' equatorial region ^[11]. What the team wants to know is whether the Stickney event left a concentrated region of denser material beneath the crater ^[12]. Haser, a doctoral student in planetary science at Germany's Universitat der Bundeswehr Munchen, told Universe Today in Vienna that the Stickney event is one of the most important episodes in Phobos' history ^[16].

Scale is the problem the interior model has to solve. Stickney's 9 km diameter is about 41 percent of the moon's 22.2 km mean diameter ^[19], and an impact large enough to cut that scar would be expected to destroy a body that small ^[20]. Haser's explanation for survival runs against a lumpy interior. "You would assume that such an impact would have shattered Phobos, unless it has a very low homogeneous density, like a sponge that can absorb that kind of impact," he said ^[13]. He also described the heat at the site: "And at that impact region, there must be very high temperature that melted and compressed the stone beneath it" ^[14]. That compression would leave a denser zone below Stickney and a subtle gravitational signature ^[15].

The published account of the work does not include a predicted amplitude for that signature ^[21]. The amplitude is what would decide whether an orbiter's tracking data could pull the anomaly out of the rest of the field. This was a conference presentation of a model ^[1].

Japan's upcoming MMX sample return mission could provide the measurements and physical evidence needed to settle the question, according to the summary of the work published by ScienceDaily ^[17]. The part of that package which bears on the 4.2-versus-2.6 split is the returned rock, because ages come from laboratory dating rather than from gravity. Haser and Andert write that spectral properties, together with models of asteroid capture, support the capture scenario ^[6]; the competing account has a large object striking Mars, feeding a debris disc, and producing both Phobos and Deimos ^[5].

So the gravity work can deliver a better interior: porosity, ice, where the mass sits. The date the two hypotheses dispute will have to come from the returned rock.