HR: 13:55h
AN: P33C-02    [Abstracts]
TI: Mars' internal viscosity structure and response to short-period loads
AU: * Ghent, R R
EM: ghentr@geology.utoronto.ca
AF: University of Toronto, Dept. of Geology, 22 Russell St., Toronto, ON M5S 3B1, Canada
AU: Bills, B G
AF: NASA Goddard Space Flight Center, Planetary Geodynamics Laboratory, Greenbelt, MD 20771, United States
AU: Bills, B G
AF: Scripps Institution of Oceanography, Institute for Geophysics and Planetary Physics, UC San Diego, La Jolla, CA 92039, United States
AU: Nimmo, F
AF: UC Santa Cruz, Earth and Planetary Sciences, Santa Cruz, CA 95064, United States
AU: Leverington, D W
AF: Texas Tech University, Department of Geosciences, Lubbock, TX 79409, United States
AB: Observations of the rate of secular evolution of the orbit of Phobos indicate that the interior of Mars is strongly dissipative relative to Earth. In this work, we report on possible models for Mars' interior viscosity structure that can satisfy the observed disspiation rate. We further examine the response of such a body to short-period surface mass loads arising from seasonal variations in polar cap masses. The simplest model we consider is that of a homogeneous Maxwell viscoelastic body, characterized by density ρ, rigidity μ, and viscosity ν. The latter two parameters are adjusted to fit the tidal response. For this model to reproduce the degree 2 Love number estimates and the secular acceleration of Phobos, we require μ = (4.6 + 2.0) 10¹⁰ Pa and ν = (8.7 + 0.6) 10¹⁴ Pa s, with a corresponding Maxwell relaxation time of just over 5 hours. A further constraint is that Mars has significant long-lived topography, which we accommodate in layered models via an outer elastic shell that supports loads for long times. In order to produce the observed tidal effects, the effective viscosity of the mantle and core must then have correspondingly lower viscosities. We explore a number of different scenarios for accommodating this requirement.
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An additional set of constraints on Mars' internal structure can be derived from the response to annual surface loads, associated with the seasonal transport of mass into and out of the polar caps. Previous treatments of this phenomenon have considered that the surface upon which dust and volatiles are deposited is perfectly rigid. We note that at least the very longest wavelength components of this process are likely to reflect a finite yielding of the surface in response to seasonally varying loads. As a result, the gravitational, topographic, and rotational responses will together provide joint constraints on the surficial mass transport, and the internal structure. The observed time varying gravitational signal, for example, represents contributions from the volatile masses on the surface and in the atmosphere, and the crustal deformation induced by these loads. We discuss a range of internal structure models which are consistent with these constraints.
DE: 5417 Gravitational fields (1221)
DE: 5430 Interiors (8147)
DE: 5450 Orbital and rotational dynamics (1221)
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting