Showing papers by "Sean C. Solomon published in 2004"

Correction to “Localized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution”

Abstract: [1] In the paper ‘‘Localized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution’’ by Patrick J. McGovern, Sean C. Solomon, David E. Smith, Maria T. Zuber, Mark Simons, Mark A. Wieczorek, Roger J. Phillips, Gregory A. Neumann, Oded Aharonson, and James W. Head (Journal of Geophysical Research, 107(E12), 5136, doi:10.1029/ 2002JE001854, 2002), the thickness of the lithosphere and lithospheric heat flow for a number of regions of Mars and as functions of time were inferred on the basis of gravity/topography admittance spectra. Observed admittances, derived from spherical harmonic expansions localized with the scheme of Simons et al. [1997], were compared with those predicted from models for the flexural response to lithospheric loading [e.g., Turcotte et al., 1981]. Gravity was calculated according to the finite-amplitude scheme of Wieczorek and Phillips [1998]. Estimates for the thickness of the elastic lithosphere Te at the time of loading for each region were converted to equivalent thermal gradient dT/dz and heat flux q by means of an elastic-plastic stressenvelope formalism [McNutt, 1984]. Here we describe a correction required in the calculation of the modeled gravity anomalies; we report new estimates of Te, load density rl, dT/dz, and q from corrected model admittances; and we discuss the implications of the new results. [2] The source of the required correction is a difference in reference radius values. As defined by McGovern et al. [2002], the planetary shape was taken to equal the radius from the center of mass of Mars to the Martian surface expressed as a spherical harmonic expansion and referenced to the mean equatorial radius Req = 3396 km:

Mercury: Determination of Internal Structure and Evolution

Abstract: Introduction: One of the most enigmatic questions with regard to the formation of terrestrial planets is the origin of Mercury’s large bulk density (~5400 kg/m) [e.g., 1-4]. This high bulk density suggests a larger metal:silicate ratio than observed in the other terrestrial planets. However, the bulk density of the planet alone does not uniquely constrain the composition; additional information is required. We model the internal structure of Mercury, giving particular attention to the core, in order to investigate the ability of anticipated future measurements of the planet’s normalized polar moment of inertia (C/MR) and ratio of the mantle moment of inertia to that of the planet (Cm/C) to constrain the planet’s structure and composition. Approach: Measurable quantities such as the values for a planet’s mass and moments of inertia are directly related to its internal density structure. Under the assumption of a spherically-symmetric planet these quantities are related by the following equations [e.g., 5]:

Shear Wave Splitting Beneath the Galapagos Archipelago

Abstract: Shear-wave splitting measurements in the Galapagos archipelago show a rapid change from consistently oriented anisotropy to no measurable anisotropy. At the western edge of the archipelago delay times are 0.4– 0.9 s and fast polarization directions are 81– 109°E. These directions are consistent with anisotropy resulting from shear of the asthenosphere by the overlying plate; there is no indication of fossil lithospheric anisotropy in the plate spreading direction. In contrast, beneath the center of the archipelago the upper mantle is isotropic or weakly anisotropic. The isotropic region coincides approximately with a volume of anomalously low upper mantle velocities, suggesting that the presence of melt may weaken the effects of fabric on anisotropy or that melt preferred orientation generates a vertical fast polarization direction. Alternatively, the complex flow field associated with a near-ridge hotspot may result in apparent isotropy.