2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 340-1
Presentation Time: 1:30 PM


HURFORD, Terry1, ASPHAUG, Erik2, SPITALE, Joseph3, HEMINGWAY, Douglas4, RHODEN, Alyssa2, HENNING, Wade5, BILLS, Bruce6, KATTENHORN, Simon7 and WALKER, Matthew8, (1)Planetary Systems Lab, NASA Goddard Space Flight Center, Greenbelt, MD 20771, (2)School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, (3)Planetary Science Institute, Tucson, AZ 85719, (4)University of California, Santa Cruz, CA 95064, (5)NASA Goddard Space Flight Center, Greenbelt, MD 20771, (6)Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, (7)Geological Technology, Conoco Phillips Company, Houston, TX 77079, (8)Earth, Planetary and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, terry.a.hurford@nasa.gov

Phobos, the innermost satellite of Mars, displays an extensive system of grooves that are mostly symmetric about its sub-Mars point. Phobos is steadily spiraling inward due to the tides it raises, and will suffer tidal disruption before colliding with Mars. We calculate the surface stress field of the de-orbiting satellite and show that the first signs of tidal disruption are already present on its surface. Most of Phobos’ prominent grooves have an excellent correlation with computed stress orientations. The model predicts an interior that has very low strength on the tidal evolution timescale, overlain by a ~10-100 m exterior shell that has elastic properties similar to lunar regolith.

Shortly after the Viking spacecraft obtained the first geomorphic images of Phobos, it was proposed that stresses from orbital decay cause grooves. But, assuming a homogeneous Phobos, it proved impossible to account for the build-up of failure stress in the exterior regardless of the value assumed for Phobos’ rigidity. Hence, the tidal model languished. Here, we revisit the tidal origin of surface fractures with a more detailed treatment that shows the production of significant stress in a surface layer, with a very strong correlation to the geometry of grooves.

Our model results applied to surface observations imply that Phobos has a rubble pile interior that is nearly strengthless. A lunar-like cohesive regolith outer layer overlays the rubble pile interior. This outer layer behaves elastically and can experience significant tidal stress at levels able to drive tensile failure. Fissures can develop as the global body deforms due to increasing tides related to orbital decay. Phobos may have an active and evolving surface; an exciting target for further exploration. The interior predictions of this model can be evaluated by future detailed studies performed by an orbiter or lander.

Finally, our results have direct implications for one other satellite in our Solar System, Triton. This icy satellite’s orbit is decaying and surface evolution may be driven by its orbital migration.