King E.M.,University of California at Berkeley |
Stellmach S.,Institute For Geophysik |
Buffett B.,University of California at Berkeley
Journal of Fluid Mechanics | Year: 2013
Rotating Rayleigh-Bénard convection provides a simplified dynamical analogue for many planetary and stellar fluid systems. Here, we use numerical simulations of rotating Rayleigh-Bénard convection to investigate the scaling behaviour of five quantities over a range of Rayleigh (103 Ra 109), Prandtl (1≤Pr100) and Ekman (10-6E) numbers. The five quantities of interest are the viscous and thermal boundary layer thicknesses, δv and δ T , mean temperature gradients, β , characteristic horizontal length scales, ℓ , and flow speeds, Pe}. Three parameter regimes in which different scalings apply are quantified: non-rotating, weakly rotating and rotationally constrained. In the rotationally constrained regime, all five quantities are affected by rotation. In the weakly rotating regime, δT, β and Pe, roughly conform to their non-rotating behaviour, but δv and ℓ are still strongly affected by the Coriolis force. A summary of scaling results is given in table 2. © 2013 Cambridge University Press.
Tesoniero A.,Copenhagen University |
Auer L.,Institute For Geophysik |
Boschi L.,CNRS Paris Institute of Earth Sciences |
Cammarano F.,Copenhagen University |
Cammarano F.,Third University of Rome
Journal of Geophysical Research B: Solid Earth | Year: 2015
We present a new global model of shear and compressional wave speeds for the entire mantle, partly based on the data set employed for the shear velocity model savani. We invert Rayleigh and Love surface waves up to the sixth overtone in combination with major P and S body wave phases. Mineral physics data on the isotropic δlnVS/δlnVP ratio are taken into account in the form of a regularization constraint. The relationship between VP and VS that we observe in the top 300 km of the mantle has important thermochemical implications. Back-arc basins in the Western Pacific are characterized by large VP/VS and not extremely low VS at ∼150 km depth, consistently with presence of water. Most pronounced anomalies are located in the Sea of Japan, in the back-arc region of the Philippine Sea, and in the South China Sea. Our results indicate the effectiveness of slab-related processes to hydrate the mantle and suggest an important role of Pacific plate subduction also for the evolution of the South China Sea. We detect lateral variations in composition within the continental lithospheric mantle. Regions that have been subjected to rifting, collisions, and flood basalt events are underlain by relatively large VP/VS ratio compared to undeformed Precambrian regions, consistently with a lower degree of chemical depletion. Compositional variations are also observed in deep lithosphere. At ∼200 km depth, mantle beneath Australia and African cratons has comparable positive VS anomalies with other continental regions, but VP is ∼1% higher. ©2015. American Geophysical Union. All Rights Reserved.
Cheng J.S.,University of California at Los Angeles |
Stellmach S.,Institute For Geophysik |
Ribeiro A.,University of California at Los Angeles |
Grannan A.,University of California at Los Angeles |
And 2 more authors.
Geophysical Journal International | Year: 2015
We present laboratory and numerical models investigating the behavioural regimes of rapidly rotating convection in high-latitude planetary core-style settings. Our combined laboratorynumerical approach, utilizing simplified geometries, can access more extreme parameters (e.g. Rayleigh numbers Ra ≲ 1013; Nusselt numbers Nu ≲ 103; Ekman numbers E ≳ 3 × 10-8) than current global-scale dynamo simulations. Using flow visualizations and heat transfer measurements, we study the axialized flows that exist near the onset of rotating convection, as well as the 3-D flows that develop with stronger forcing. With water as the working fluid (Prandtl number Pr ≲ 7), we find a steep scaling trend for rapidly rotating convective heat transfer, Nu~(Ra/RaC)3.6, that is associated with the existence of coherent, axialized columns. This rapidly rotating trend is steeper than the trends found at moderate values of the Ekman number, and continues a trend of ever-steepening scalings as the rotation rate of the system is increased. In contrast, in more strongly forced or lower rotation rate cases, the heat transfer scaling consistently follows a shallower slope equivalent to that of non-rotating convection systems. The steep heat transfer scaling in the columnar convection regime, corroborated by our laboratory flow visualizations, imply that coherent, axial columns have a relatively narrow range of stability. Thus, we hypothesize that coherent convection columns are not stable in planetary core settings,where the Ekman number is estimated to be~10-15. As a consequence, convective motions in the core may not be related to the columnar motions found in presentday global-scale models. Instead, we hypothesize that turbulent rotating convection cascades energy upwards from 3-D motions to large-scale quasi-2-D flow structures that are capable of efficiently generating planetary-scale magnetic fields. We argue that the turbulent regimes of rapidly rotating convection are essential aspects of core dynamics and will be necessary components of robust, next-generation and multiscale dynamo models. © The Authors 2015.
Waszek L.,University of Cambridge |
Thomas C.,Institute For Geophysik |
Deuss A.,University Utrecht
Geophysical Research Letters | Year: 2015
Precursors to the core phase PKP are generated by scattering of seismic energy from heterogeneities in the mantle. Here we examine a large global data set of PKP precursors in individual seismograms and array data, to better understand scattering locations. The precursor amplitudes from individual seismograms are analyzed with respect to the inner core phase PKIKP and mantle phase PP. We find and correct for a hemispherical asymmetry in the precursor/PKIKP amplitudes, resulting from inner core structure. Using ray tracing, we locate scatterers in our array data and use these to infer scattering locations in the individual data. The scattering strength displays regional variation; however, we find no relationship with long-scale core-mantle boundary velocity structure. Scattering is observed in all regions of data coverage, as are paths with no precursors. This indicates that scattering occurs from various small-scale heterogeneities, including but not limited to ultralow velocity zones or partial melt, and slabs. ©2015. American Geophysical Union. All Rights Reserved.
Leblanc F.,University Pierre and Marie Curie |
Chassefiere E.,University Paris - Sud |
Chassefiere E.,French National Center for Scientific Research |
Gillmann C.,Institute For Geophysik |
Breuer D.,German Aerospace Center
Icarus | Year: 2012
Noble gas 40Ar may be used as a tracer of the past evolution of volatiles in Mars' crust, mantle and atmosphere. 40Ar is formed by the radioactive decay of 40K in the mantle and in the crust and is released from the mantle to the atmosphere due to volcanism and from the crust by erosion such as eolian and hydrothermal erosion. Furthermore, 40Ar can escape from the atmosphere into space via atmospheric escape mechanisms. The evolution of the atmospheric abundance of 40Ar thus depends on these three processes whose efficiencies vary with time.In the present study we reconsider atmospheric escape mechanism efficiencies and describe various possible scenarios of the evolution of 40Ar with a model describing the three main reservoirs of 40Ar, the mantle, crust and atmosphere. First, we show that atmospheric escape, which is stronger in the early evolution, does not significantly influence the present abundance of the atmospheric 40Ar. In the early evolution the atmospheric concentration of 40Ar is very low as the outgassing of 40Ar from the mantle occurs relatively late in the martian evolution. Thus, the atmospheric 40Ar concentration is essentially a tracer of Mars' outgassing history and not of the escape processes. Second, using the results of the most recent published crustal formation models, the calculated present 40Ar atmospheric abundance is smaller than its observed value. This discrepancy may be explained by a significant 40Ar supply from the crust by erosion (16-30% of the 40Ar content of the upper first 10km of crust). The knowledge of the fraction of crustal 40Ar outgassed to the atmosphere is an important constraint for any future global modelling of past Mars' hydrothermal activity aiming at better characterizing the role of subsurface aqueous alteration processes in Mars climate evolution. One of the main sources of the uncertainty of these results is the present uncertainty in the measured atmospheric 40Ar value (±20%). More precise measurements of 40Ar and 36Ar in the martian atmosphere are therefore required to better constrain the model. © 2012 Elsevier Inc.