Origin of Phobos grooves: Testing the Stickney Crater ejecta model

Abstract

The model of Wilson and Head (1989, 2005, 2015) interprets the grooves of Phobos as the product of sliding, rolling, and bouncing ejecta boulders from the Stickney Crater impact. To test the model of Wilson and Head (1989, 2005, 2015) we apply a dynamical physics simulation to a three-dimensional shape model of Phobos and systematically assess six specific objections to the rolling boulder model. Our simulation calculates the motions of Stickney Crater ejecta boulders under the influence of Mars gravitation, the post-Stickney-impact desynchronized rotation of Phobos and altered orbit of Phobos, a spherically symmetrical gravity field of Phobos, and a collision system that simulates the effects of surface friction. We also take the orbital history of Phobos into account and test the rolling boulder model at three semimajor axes greater than the present day (10,000 km, 12,000 km, and 14,000 km). At a Phobos semimajor axis of 12,000 km, we find that boulder motions are generally consistent with observed grooves – specifically 1) Test boulders travel in linear/parallel patterns that are consistent with observed grooves, including grooves that are not radially aligned with Stickney Crater; 2) test boulders drift into suborbital flights over the trailing hemisphere of Phobos and do not travel on the surface of Phobos in this region; 3) grooves inside Stickney Crater are produced by Stickney ejecta boulders that return to Stickney Crater; 4) boulders that travel around Phobos >180° from Stickney Crater crosscut the motions of other boulders that travel >180° from Stickney; 5) at boulder ejection velocities ≲6 m/s, test boulders moving in contact with Phobos typically follow the contours of local terrain. We also find that 6) a spike of Stickney secondary impacts likely destroyed missing groove-producing boulders and removed grooves with widths ≲80 m; 7) with respect to the motion of test boulders, the two possible pre-Stickney-impact tidal-lock orientations of Phobos produce the same boulder motion effects; and 8) where our 12,000 km semimajor axis testing model is consistent with observed grooves, this supports a ∼150 Ma prediction for the age of Phobos grooves and Stickney Crater.