Jean-Charles Marty

Regional variations in subsidence rate of oceanic plates: a global analysis

A global analysis of the dependence of seafloor depth on crustal age has been performed for each large oceanic plate independently. Each plate has been divided into regions bounded by major tectonic features. A total of 32 regions have been considered. Bathymetry corrected for sediment loading has been plotted as a function of crustal age for each region. Except for three regions (western Pacific between 10°S and 40°N, North American plate between 24°N and 38°N and African plate between 10°N and 24°N) where ocean floor older than 80–100 Ma flattens with age, in other regions, depth increases linearly with age1/2. Subsidence rate over each region has been computed by linear regression. Large variations (up to a factor 2) are reported from one region to another. Asymmetrical subsidence is also observed in a number of areas. However, the largest asymmetries are less than the regional variations in subsidence rate occurring on individual plates. Regional variations in subsidence rate appear linearly related to regional variations in ridge crest topography, shallow ridge subsiding quickly and deep ridge subsiding slowly. Hence seafloor depth subsides at a rate modulated by the value of the initial (axial) depth. This observation suggests that regional variations in subsidence rate reflect the thermal regime of the mantle beneath mid-ocean ridges rather than lateral temperature variations inside the asthenosphere involving small-scale convection.

A new global Earth's gravity field model from satellite orbit perturbations: GRIM5‐S1

A new model of the Earths gravity field, called GRIM5-S1, was prepared in a joint German-French effort. The solution is based on satellite orbit perturbation analysis and exploits tracking data from 21 satellites to solve simultaneously for the gravitational and ocean tide potential and tracking station positions. The satellite-only solution results in a homogeneous representation of the geoid with an approximation error of about 45 cm in terms of 5×5 degree block mean values, and performs globally better in satellite orbit restitution than any previous gravity field model. The GRIM5 normals, which were generated taking into account the latest computational standards, shall be the reference for use during the coming geopotential satellite mission CHAMP and should provide new standards in computing orbits of next altimetric missions like Jason and ENVISAT. The GRIM5-S1 normals also give the basis for the tracking/surface data combined solution GRIM5-C1.

ESA's satellite‐only gravity field model via the direct approach based on all GOCE data

Gravity field and steady state Ocean Circulation Explorer (GOCE) gravity gradient data of the entire science mission and data from LAGEOS 1/2 and Gravity Recovery and Climate Experiment (GRACE) were combined in the construction of a satellite-only gravity field model to maximum degree 300. When compared to Earth Gravitational Model 2008, it is more accurate at low to medium resolution, thanks to GOCE and GRACE data. When compared to earlier releases of European Space Agency GOCE models, it is more accurate at high degrees owing to the larger amount of data ingested, which was moreover taken at lower altitude. The impact of orbiting at lower altitude in the last year of the mission is large: a model based on data of the last 14 months is significantly more accurate than the release 4 model constructed with the first 28 months. The (calibrated) cumulated geoid error estimate at 100 km resolution is 1.7 cm. The optimal resolution of the GOCE model for oceanographic application is between 100 and 125 km.

EIGEN-6C: A High-Resolution Global Gravity Combination Model Including GOCE Data

GOCE satellite gradiometry data were combined with data from the satellite missions GRACE and LAGEOS and with surface gravity data. The resulting high-resolution model, EIGEN-6C, reproduces mean seasonal variations and drifts to spherical harmonic degree and order (d/o) 50 whereas the mean spherical harmonic coefficients are estimated to d/o 1420. The model is based on satellite data up to d/o 240, and determined with surface data only above degree 160. The new GOCE data allowed the combination with surface data at a much higher degree (160) than was formerly done (70 or less), thereby avoiding the propagation of errors in the surface data over South America and the Himalayas in particular into the model.

Regional variations in subsidence rate of lithospheric plates : implication for thermal cooling models

Abstract Although simple thermal models of lithospheric cooling predict to first order the general behaviour of observed seafloor depth with increasing age, important regional variations in seafloor subsidence, in the range 250–400 m Ma 1 2 , are reported for several lithospheric plates. Such variations cannot be accounted for by classical cooling models unless implausible variations in asthenospheric temperature of ∼550°C are assumed. Here we present an alternative cooling model, which assumes that at the ridge axis the temperature may deviate from the mean asthenospheric temperature. Such a model satisfactorily explains the data provided that the temperature deviation is ±100°C only.

Mars Long Wavelength Gravity Field Time Variations: A New Solution from MGS Tracking Data

Several modern solutions of the Mars gravity field have already been obtained from the Mars Gobal Surveyor (MGS) mission, by different NASA teams working at GSFC and at JPL, which also have shown that degree two and three lumped zonal coefficients exhibit time variations related to the seasonal cycle of carbon dioxide exchange between the planet surface and its atmosphere. A new solution of these time variations has been obtained by a third team working in Europe with a totally independent software. Five years of one and two way Doppler, and range tracking data collected by the Deep Space Network have been processed in three day arcs, taking into account all disturbing forces of gravitational and non gravitational origins; for each arc the state vector, drag and solar pressure model multiplying factors, and angular momentum dump parameters are adjusted. The zonal harmonics up to degree five and the k2 Love number are solved for. The zonals are estimated every ten days, or every thirty days in some variants, with a priori uncertainties either on their values or on their changes. The lumped C20 and C30 coefficients show similar patterns as in the anterior US solutions, also in accordance with the variations estimated from the output of a Global Circulation Model and from the HEND instrument on board Mars Odyssey. Annual and semiannual terms have been derived for comparisons and future evaluation in terms of global constraints put on such planetary mass transfer. Finally the values found for k2 (in the range 0.10 – 0.15 depending on the solution strategy) are discussed.

New Global Gravity Field Models from Selected CHAMP Data Sets

Several sets of data were carefully selected out of the first 12 months of CHAMP GPS and STAR micro-accelerometer observations to derive a new model of the Earth’s static gravity field. These data have been dynamically processed as 63 orbital arcs, each of one to one and a half day duration. The approach consists of two steps: the GPS satellite orbits and clocks are first determined, then the CHAMP orbital arcs are adjusted using undifferenced phase and pseudo-range SST observables, and the residuals of those are used for retrieving the gravity harmonic coefficients. Micro-accelerometer, attitude and manoeuver information are taken into account to determine the surface accelerations as well as the residual thrusts acting on the spacecraft. Temporal variations of the gravity field are introduced in the observation equations for further usage in subsequent models, but are not here solved for. Several solutions have been derived, with or without additional information coming from the previous GRIM 5 satellite model, or from other laser satellite observations over a time period encompassing the CHAMP first year mission, with regularization (based on Kaula’s rule) and with different weighting factors. The solution which is presented is complete to degree and order 120, although a lack of power in the coefficients power spectrum is obvious above degree 40 approximately, as may be expected from a satellite solution, due to the orbit decreasing sensitivity to gravity at the present altitude. The quality of the field is assessed mainly through orbit determinations of several geodetic satellites, of CHAMP itself (especially with the residuals of the laser range measurements — not included in the gravity modelling process), and through comparisons with the geoidal surface derived from a mean altimetric sea surface — corrected for the ocean circulation, at a comparable resolution. A significant improvement in the long to medium wavelength harmonics of the model can already be seen as compared to previous satellite solutions.

Comparison of Satellite-Only Gravity Field Models Constructed with All and Parts of the GOCE Gravity Gradient Dataset

ABSTRACT The impact of GOCE Satellite Gravity Gradiometer data on gravity field models was tested. All models were constructed with the same Laser Geodynamics Satellite (LAGEOS) and Gravity Recovery and Climate Experiment (GRACE) data, which were combined with one or two of the diagonal gravity gradient components for the entire GOCE mission (November 2009 to October 2013). The Stokes coefficients were estimated by solving large normal equation (NE) systems (i.e., the direct numerical approach). The models were evaluated through comparisons with the European Space Agencys (ESA) gravity field model DIR-R5, by GPS/Leveling, GOCE orbit determination, and geostrophic current evaluations. Among the single gradient models, only the model constructed with the vertical ZZ gradients gave good results that were in agreement with the formal errors. The model based only on XX gradients is the least accurate. The orbit results for all models are very close and confirm this finding. All models constructed with two diagonal gradient components are more accurate than the ZZ-only model due to doubling the amount of data and having two complementary observation directions. This translates also to a slower increase of model errors with spatial resolution. The different evaluation methods cannot unambiguously identify the most accurate two-component model. They do not always agree, emphasizing the importance of evaluating models using many different methods. The XZ gravity gradient gives a small positive contribution to model accuracy.