Geological Society of America

T340 Oceans, Fire, and Ice of the Outer Solar System

Wednesday, 4 November 2015
1:30 PM - 5:30 PM
Room 344

SURFACE EVOLUTION FROM ORBITAL DECAY ON PHOBOS AND TRITON

Paper number: T340-1
Program: Technical Sessions
Time: 1:30 PM – 1:50 PM

Authors

Terry Hurford Presenting Author
NASA Goddard Space Flight Center, Greenbelt, MD
Erik Asphaug
Arizona State University, Tempe, AZ
Joseph Spitale
Planetary Science Institute, Tucson, AZ
Douglas Hemingway
University of California, Santa Cruz, CA
Alyssa Rhoden
Southwest Research Institute, Boulder, CO
Wade Henning
NASA Goddard Space Flight Center, Greenbelt, MD
Bruce Bills
California Institute of Technology, Pasadena, CA
Simon Kattenhorn
Conoco Phillips Company, Houston, TX
Matthew Walker
University of California, Los Angeles, Los Angeles, CA

Abstract

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.