The Stickney Crater ejecta secondary impact crater spike on Phobos: Implications for the age of Stickney and the surface of Phobos

Abstract

A global and uniformly distributed spike of secondary impact craters on Phobos with diameters (D) <0.6km and a portion of craters up to D 2km were produced by Stickney Crater ejecta, including secondary craters within the surface area of Stickney Crater. The global exposure of Phobos to Stickney secondary impacts was facilitated by the desynchronized orbital/rotational period of Phobos that was produced by the impulse of the Stickney impact event. In our model we apply the Tsiolkovsky rocket equation to calculate the total available Stickney impact acceleration impulse delta-v (Δv) and further calculate the effective impulse by incorporating the energy conversion inefficiencies of the crater formation process. We also calculate the pre- and post-impact Phobos moment of inertia that further contributes to the desynchronizing effect. The majority of the Stickney ejecta that exited from Phobos was trapped in orbits around Mars until it later accumulated back onto Phobos over a period of <1000 years. However, Phobos de-spun back to a synchronous rotation after a much longer period of at least 5000 years. Therefore, a sufficient period of desynchronized rotation exposed the entire surface of Phobos to ejecta that returned from martian orbits. In view of how all or most craters observed inside Stickney Crater approximate the size/frequency distribution (SFD) of Stickney secondary impacts, it is infeasible to derive an age for Stickney Crater based on an assumption of background impacts (~2.8–4.2Ga according to Schmedemann et al. (2014)). In view of how crater-counting is unworkable for age-dating Stickney Crater we conclude an alternate age for Stickney Crater of 0.1–0.5 Ga that is constrained instead by the boulder evidence of Thomas et al. (2000), the boulder destruction rate analysis of Basilevsky et al. (2013, 2015), and the observed space weathering of Phobos regolith (Cipriani et al., 2011; Pieters et al., 2014). Assessing several implications of our model we 1) summarize the crater SFD and temporal nature of the Stickney secondary impact spike on Phobos, 2) predict the global equivalent thickness of deposits on Phobos from Stickney ejecta and subsequent secondary impact gardening, 3) examine the hypothesis that the Stickney impact was a trailing hemisphere event on Phobos that reoriented Phobos to its present-day synchronous “tidal lock” longitude, 4) set limits on the volume of low-velocity Stickney ejecta that is available to produce Phobos grooves from rolling boulders, and 5) estimate the crater SFD of a meteor spike on Mars from a trailing hemisphere Stickney impact.