Showing papers by "Sean C. Solomon published in 2002"

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

Abstract: From gravity and topography data collected by the Mars Global Surveyor spacecraft we calculate gravity/topography admittances and correlations in the spectral domain and compare them to those predicted from models of lithospheric flexure. On the basis of these comparisons we estimate the thickness of the Martian elastic lithosphere (T_e) required to support the observed topographic load since the time of loading. We convert T_e to estimates of heat flux and thermal gradient in the lithosphere through a consideration of the response of an elastic/plastic shell. In regions of high topography on Mars (e.g., the Tharsis rise and associated shield volcanoes), the mass-sheet (small-amplitude) approximation for the calculation of gravity from topography is inadequate. A correction that accounts for finite-amplitude topography tends to increase the amplitude of the predicted gravity signal at spacecraft altitudes. Proper implementation of this correction requires the use of radii from the center of mass (collectively known as the planetary “shape”) in lieu of “topography” referenced to a gravitational equipotential. Anomalously dense surface layers or buried excess masses are not required to explain the observed admittances for the Tharsis Montes or Olympus Mons volcanoes when this correction is applied. Derived T_e values generally decrease with increasing age of the lithospheric load, in a manner consistent with a rapid decline of mantle heat flux during the Noachian and more modest rates of decline during subsequent epochs.

Assessing the depth resolution of tomographic models of upper mantle structure beneath Iceland

Abstract: [1] Earlier inversions of body wave delay-time data recorded during the ICEMELT portable broadband experiment imaged a cylindrical plume-like low-velocity anomaly extending to at least 400 km depth beneath Iceland, but the depth extent of the anomaly resolvable by tomography has recently been called into question. We have performed several additional resolution tests to evaluate the depth resolution of tomographic models of the Icelandic upper mantle. The distribution of paths of body waves recorded by ICEMELT can distinguish among three different types of models: (a) a wide and shallow anomaly, (b) a narrow and deep anomaly, and (c) a narrow and shallow anomaly. While tomographic models contain an element of nonuniqueness, these tests illustrate that the depth resolution of passive seismic experiments spanning subaerial Iceland is adequate for distinguishing among alternative geodynamic models.

A Procedure for Determining the Nature of Mercury's Core

Abstract: We review the assertion that the precise measurement of the second degree gravitational harmonic coefficients, the obliquity, and the amplitude of the physical libration in longitude, C20, C22, θ, and Φ, for Mercury are sufficient to determine whether or not Mercury has a molten core. The conditions for detecting the signature of the molten core are that such a core not follow the 88-day physical libration of the mantle induced by periodic solar torques, but that it does follow the 250,000-year precession of the spin axis that tracks the orbit precession within a Cassini spin state. These conditions are easily satisfied if the coupling between the liquid core and solid mantle is viscous in nature. The alternative coupling mechanisms of pressure forces on irregularities in the core-mantle boundary (CMB), gravitational torques between an axially asymmetric mantle and an assumed axially asymmetric solid inner core, and magnetic coupling between the conducting molten core and a conducting layer in the mantle at the CMB are shown for a reasonable range of assumptions not to frustrate the first condition while making the second condition more secure. Simulations have shown that the combination of spacecraft tracking and laser altimetry during the planned MESSENGER orbiter mission to Mercury will determine C20, C22, and theta to better than 1% and Φ to better than 8%-sufficient precision to distinguish a molten core and constrain its size. The possible determination of the latter two parameters to 1% or less with Earth-based radar experiments and MESSENGER determination of C20 and C22 to 0.1% would lead to a maximum uncertainty in the ratio of the moment of inertia of the mantle to that of the whole planet, Cm/C, of ~2% with comparable precision in characterizing the extent of the molten core.

Shear wave splitting across the Iceland hot spot: Results from the ICEMELT experiment

Abstract: [1] We report on observations of upper mantle anisotropy from the splitting of teleseismic shear waves (SKS, SKKS, and PKS) recorded by the ICEMELT broadband seismometer network in Iceland. In a ridge-centered hot spot locale, mantle anisotropy may be generated by flow-induced lattice-preferred orientation of olivine grains or the anisotropic distribution of magma. Splitting measurements of teleseismic shear waves may thus provide diagnostic information on upper mantle flow and/or the distribution of retained melt associated with the Iceland mantle plume. In eastern Iceland, fast polarization directions lie between N10� W and N45� W and average N24� W; delay times between the fast and slow shear waves are generally 0.7–1.35 s. In western Iceland, in contrast, the fast polarization directions, while less well constrained, yield an average value of N23� E and delay times are smaller (0.2–0.95 s). We propose that splitting in eastern Iceland is caused by a 100- to 200-km-thick anisotropic layer in the upper mantle. The observed fast directions in eastern Iceland, however, do not correspond either to the plate spreading direction or to a pattern of radial mantle flow from the center of the Iceland hot spot. We suggest that the relatively uniform direction and magnitude of splitting in eastern Iceland, situated on the Eurasian plate, may therefore reflect the large-scale flow field of the North Atlantic upper mantle. We hypothesize that the different pattern of anisotropy beneath western Iceland, part of the North American plate, is due to the different absolute motions of the two plates. By this view, splitting in eastern and western Iceland is the consequence of shear by North American and Eurasian plate motion relative to the background mantle flow. From absolute plate motion models, in which the Eurasian plate is approximately stationary and the North American plate is moving approximately westward, the splitting observations in both eastern and western Iceland can be satisfied by a background upper mantle flow in the direction N34� W and a velocity of 3 cm/yr in a hot spot reference frame. This inference can be used to test mantle flow models. In particular, it is inconsistent with kinematic flow models, which predict southward flow, or models where flow is dominated by subduction-related sources of mantle buoyancy, which predict westward flow. Our observations are more compatible with the flow field predicted from global seismic tomography models, which in particular include the influence of the large-scale lower mantle upwelling beneath southern Africa. While the hypothesized association between our observations and this upwelling is presently speculative, it makes a very specific and testable prediction about the flow field and hence anisotropy beneath the rest of the Atlantic basin. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; 8155 Tectonophysics: Plate motions—general; KEYWORDS: Iceland, anisotropy, hot spot, plate motion, shear wave splitting, mantle flow

Coordinated seismic experiment in the Azores

Abstract: Memorandum of Understanding COSEA is devoted to coordinate the efforts of Portuguese, French, American and Swiss scientific institutions for the deployment, operation and maintenance of a temporary broadband seismic experiment in the Azores Islands and for the collection, distribution and scientific exploitation of data In collaboration with other running projects, this experiment has three goals: (1) study of the Azores plume: its structure, its origin at depth and its interaction with the Mid-Atlantic Ridge; (2) investigation of the structure and deep interactions of the plate boundaries between Eurasia, Africa and North America; (3) at a local scale, monitoring of seismic and volcanic activity for a better determination of hazard by combining broadband data with existing volcano and earthquake surveillance networks

An older face for Mars.

Abstract: Mars has a north–south divide in the age of its surface, as judged by the density of impact craters. Altimetry data, which by inference provide a subsurface view of the planet, reveal that the divide is only skin deep.

Insights into the Earliest History of Mars: A New Synthesis

Abstract: Motivated by the latest data from Mars Global Surveyor and from recent analyses of Martian meteorites, we offer a new synthesis of the relative timing of major events in the early geological history of Mars together with associated uncertainties. Additional information is contained in the original extended abstract.