Improved estimate of tidal dissipation within Mars from MOLA observations of the shadow of Phobos

Abstract

[1] We report on new observations of the orbital position of Phobos, the innermost natural satellite of Mars, and show that these observations provide an improved estimate of the rate of tidal dissipation within Mars. The observations were made with the Mars Orbiter Laser Altimeter instrument on the Mars Global Surveyor spacecraft. The secular acceleration in along-track orbital motion is conventionally expressed in terms of a quadratic term in mean orbital longitude, which yields s = (dn/dt)/2 = (136.7 ± 0.6) Â 10 À5 deg/yr 2 , where n is the mean motion. The corresponding fractional rate of change in orbital angular velocity is (dn/dt)/n = (6.631 ± 0.029) Â 10 À9 /yr, the highest measured for any natural satellite in the solar system. The energy dissipation rate is (3.34 ± 0.01) MW. Because Phobos is so close to Mars, there are nonnegligible contributions to the tidal evolution from harmonic degrees 2, 3, and 4. However, the elastic tidal Love numbers are observationally constrained only at degree two. The observed acceleration is consistent with that for a homogeneous Maxwell viscoelastic model of Mars with effective viscosity of (8.7 ± 0.6) Â 10 14 Pa s.

Seismological implications of a lithospheric low seismic velocity zone in Mars

“…Iron enrichment will only increase the magnitude of any seismic 127 LVZ (Mocquet et al, 1996). The temperature of the deep mantle is constrained by observations of 128 Phobos' orbital decay, which yields the bulk k 2 /Q ratio (see Bills et al, 2005) we produce a similar model with the same structure at all depths below 300 km, but with no seismic 160 lid and a positive velocity increase with depth from the crust-mantle boundary, similar to earlier 161 Martian models. We label the latter model the noLVZ model, recognizing that it is likely less 162 justifiable on physical grounds than the LVZ model.…”

Advancing tests of relativistic gravity via laser ranging to Phobos

“…Recent fits by Bills et al [84], Lainey et al [85], and Jacobson [72] give accelerations in the forward orbital longitude a dn/dt of ∼416 m/yr 2 . This secular acceleration would easily be detected by PLR giving refined accuracy.…”

Planetary Magnetic Dynamo Effect on Atmospheric Protection of Early Earth and Mars

“…The only in situ observations important for the interior are those of the martian gravity field, the tidal effect on an orbiter, and the polar moment of inertia, which are derived from radio tracking of orbiting and landed spacecraft (e.g., Folkner et al 1997;Konopliv et al 2006). These observations are the main constraints for interior models additionally based on analysis of SNC meteorites or chondrite data and extrapolation of the Earth's internal structure to the lower pressures of Mars' interior (Sohl and Spohn 1997;Sanloup et al 1999;Bertka and Fei 1998;Sohl et al 2005;. The question of whether the core is presently liquid or solid is still an open question although the measured tidal effect on the orbits of the MGS, Mars Odyssey, and MEX suggests an at least partially liquid core (Yoder et al 2003;Balmino et al 2006;Konopliv et al 2006;Duron et al 2007).…”