Reorientation of planets with lithospheres: The effect of elastic energy

Manga

2010

J. Geophys. Res.

[1] We use a revised partitioning of the planet figure into equilibrium and nonequilibrium contributions that takes into account the presence of an elastic lithosphere to study the Martian gravity field and shape. The equilibrium contribution is associated with the present rotational figure, and the nonequilibrium contribution is dominated by Tharsis and a remnant rotational figure supported by the elastic lithosphere that traces the paleopole location prior to the formation of Tharsis. We calculate the probability density functions for Tharsis' size and location, the paleopole location, and the global average thickness of the elastic lithosphere at the time Tharsis was emplaced. Given the observed degree-3 spherical harmonic gravity coefficients, the expected Tharsis center location is 258.6 ± 4.2°E, 9.8 ± 0.9°N, where the uncertainties represent the 90% confidence interval. Given this Tharsis center location and the observed degree-2 spherical harmonic gravity coefficients, the expected paleopole location prior to the emplacement of Tharsis is 259.5 ± 49.5°E, 71.1 −14.4 +17.5°N , and the expected elastic lithospheric thickness at the time of loading is 58 −32 +34 km. Our estimated paleopole colatitude implies 18.9 −17.5 +14.4°o f true polar wander (TPW) driven by the emplacement of Tharsis, in disagreement with previous studies that invoke large TPW. The remnant rotational figure is visible in both the nonequilibrium degree-2 geoid (areoid) without Tharsis and the nonequilibrium degree-2 topography without Tharsis. The remnant rotational figure is also visible in the total nonequilibrium geoid without Tharsis, but it is not visible in the total nonequilibrium topography without Tharsis due to the strong signal of the north-south dichotomy. Shorter wavelength geological features become significantly more visible in the geoid with the removal of the long wavelength contributions of the equilibrium rotational figure, Tharsis, and the remnant rotational figure. Removal of the equilibrium rotational figure and Tharsis from the topography reveals a better defined north-south dichotomy boundary.Citation: Matsuyama, I., and M. Manga (2010), Mars without the equilibrium rotational figure, Tharsis, and the remnant rotational figure,

Section: Paleopole Location and Thickness Of The Elastic Lithospheresupporting

confidence: 55%

Section: Paleopole Location and Thickness Of The Elastic Lithospherementioning

confidence: 99%

Mars without the equilibrium rotational figure, Tharsis, and the remnant rotational figure

Manga

2010

J. Geophys. Res.

“…The inertia tensor perturbations which depend on the final rotational and tidal potentials can adjust to any reorientation, thus the final orientation of the planet is governed by only those contributions to the inertia tensor which are independent of the final rotational and tidal potentials [ Willemann , 1984; Matsuyama et al , 2006; Matsuyama and Nimmo , 2007]. Matsuyama et al [2007] illustrated this by explicitly minimizing the rotational energy for planetary bodies without tidal deformation. We refer to the inertia tensor containing only contributions which are independent of the final rotational and tidal potentials as the nonequilibrium inertia tensor, I ij NE (hence the superscript NE).…”

Section: Theorymentioning

confidence: 99%

Gravity and tectonic patterns of Mercury: Effect of tidal deformation, spin‐orbit resonance, nonzero eccentricity, despinning, and reorientation

Nimmo

2009

J. Geophys. Res.

We consider the effect of spin‐orbit resonance, nonzero eccentricity, despinning, and reorientation on Mercury's gravity and tectonic pattern. Large variations of the gravity and shape coefficients from the synchronous rotation and zero eccentricity values, J2/C22 = 10/3 and (b − c)/(a − c) = 1/4, arise because of nonsynchronous rotation and nonzero eccentricity even in the absence of reorientation or despinning. Reorientation or despinning induces additional variations. The large gravity coefficients J2 = (6 ± 2) × 10−5 and C22 = (1 ± 0.5) × 10−5 estimated from the Mariner 10 flybys cannot be attributed to Caloris alone since the required mass excess in this case would have caused Caloris to migrate to one of Mercury's hot poles. Similarly, a large remnant bulge due to a smaller semimajor axis and spin‐orbit resonance can be dismissed since the required semimajor axis is unphysically small (<0.1 AU). Reorientation of a large remnant bulge recording an epoch of faster rotation (without significant semimajor axis variations) can explain the large gravity coefficients. This requires initial rotation rates ≳20 times the present value and a positive gravity anomaly associated with Caloris capable of driving ∼10°–45° equatorward reorientation. The required gravity anomaly can be explained by infilling of the basin with material of thicknesses ≳7 km or an annulus of volcanic plains emplaced around the basin with an annulus width ∼1200 km and fill thicknesses ≳2 km. The predicted tectonic pattern due to these despinning and reorientation scenarios, including some radial contraction, is in good agreement with the lobate scarp pattern observed by Mariner 10. We also predict that lobate scarps will follow a NE–SW orientation in the eastern hemisphere and a positive gravity anomaly (of a few hundred mGal) associated with Caloris.

“…A rotating fluid planet in hydrostatic equilibrium, subject to a superimposed nonequatorial surface load, reorients to place that mass excess on the equator. However, if a planet has a lithosphere with elastic strength at spherical harmonic degree two, the equilibrium location of the surface load is not at the equator, but at an intermediate latitude less than that of loading (Willemann, 1984;Ojakangas and Stevenson, 1989;Matsuyama et al, 2006Matsuyama et al, , 2007.…”

Section: Reorientation Of Mars By Surface Loadingmentioning

confidence: 99%

“…True Polar Wander (TPW) is the movement of a planet's entire lithosphere with respect to the spin axis in response to changes in mass distribution. Here we apply TPW theory (Gold, 1955;Goldreich & Toomre, 1969;Willeman, 1984;Matsuyama et al, accumulation. Late-stage TPW on Mars is a long-standing hypothesis (Murray & Malin, 1973;Schultz & Lutz, 1988;Tanaka, 2000;Fishbaugh & Head, 2000), but both TPW theory, and Martian geological and geodetic data, have been refined in recent years and so permit a quantitative analysis.…”

mentioning

confidence: 99%

True Polar Wander driven by late-stage volcanism and the distribution of paleopolar deposits on Mars

Kite

Earth and Planetary Science Letters

Manga

et al. 2009

Self Cite

Abstract.The areal centroids of the youngest polar deposits on Mars are offset from those of adjacent paleopolar deposits by 5-10°. We test the hypothesis that the offset is the result of true polar wander (TPW), the motion of the solid surface with respect to the spin axis, caused by a mass redistribution within or on the surface of Mars. In particular, we consider the possibility that TPW is driven by late-stage volcanism during the late Hesperian to Amazonian. There is observational and qualitative support for this hypothesis: in both North and South, observed offsets lie close to a great circle 90° from Tharsis, as expected for polar wander after Tharsis formed. We calculate the magnitude and direction of TPW produced by mapped late-stage lavas for a range of lithospheric thicknesses, lava thicknesses, eruption histories, and prior polar wander events. We find that if Tharsis formed close to the equator, the stabilizing effect of a fossil rotational bulge located close to the equator leads to predicted TPW of <2°,