The Stickney Impact of Phobos: A Dynamical Model

E. Asphaug, H. J. Melosh

Research output: Contribution to journalArticlepeer-review

139 Scopus citations


Phobos, the largest moon of Mars, has dimensions typical of minor asteroids, approximately 19 × 22 × 27 km. It is probably the best studied small planetary body in the Solar System, imaged in detail by both the Viking orbiters and the partially successful Phobos 2 probe, and the subject of ongoing Earth-based spectral observation. One large crater, Stickney, dominates an entire hemisphere and places fundamental constraints on cratering and disruption models for small planetary bodies. A complex fracture pattern intersects the surface as a series of ∼100-m wide regolith-filled grooves; these probably originated with the cratering event and therefore tell us a great deal about the fragmentation process at a size scale far beyond the reaches of the laboratory. The tidal effects of Mars (Phobos is within the Roche limit) and the complexity of Phobos' geometry (it is often described as a potato) make detailed model comparisons difficult; what we seek to accomplish here is an order-of-magnitude understanding of impact and fracture physics at the size scale of planetesimals, together with a fundamental explanation for the observed current state of Phobos. We model the Stickney impact in axial symmetry using the hydrocode SALE 2D with the Tillotson equation of state and the Grady-Kipp fragmentation rheology. Given the crater diameter (11.3 km, slightly larger than the mean radius of Phobos), scaling relationships predict the impactor size corresponding to a given impact velocity. The corresponding impactor is then established as the initial condition for the hydrocode run, so that at t = 0 the impactor is just touching the target at a velocity vi. The flow field established in the impacted hemisphere excavates Stickney, although only ∼20% of the crater ejecta escapes. The nonescaping fraction, if blanketed uniformly over Phobos, adds ∼60 m or more of new regolith. Surface particle velocities away from the crater average ∼0.7 m/sec. Unconsolidated surface materials (crater rims, for instance) are shaken by the impact with typical ballistic transport distances of ∼50 to 100 m, further degrading the surface. The interior of Phobos fractures on a size scale of several kilometers-consistent with the spacing of the surface grooves. The Stickney event did not, however, result in a porosity sufficient to explain the unusually low body density. The fracture grooves correlate with the impact geometry, suggesting homogeneity (as opposed to a rubble pile) prior to the impact. If so, the low density of Phobos is compositional in nature.

Original languageEnglish (US)
Pages (from-to)144-164
Number of pages21
Issue number1
StatePublished - 1993

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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