Geophysics

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A water budget dichotomy of rocky protoplanets from Al-heating (1902.04026v1)

Tim Lichtenberg, Gregor J. Golabek, Remo Burn, Michael R. Meyer, Yann Alibert, Taras V. Gerya, Christoph Mordasini

2019-02-11

In contrast to the water-poor inner solar system planets, stochasticity during planetary formation and order of magnitude deviations in exoplanet volatile contents suggest that rocky worlds engulfed in thick volatile ice layers are the dominant family of terrestrial analogues among the extrasolar planet population. However, the distribution of compositionally Earth-like planets remains insufficiently constrained, and it is not clear whether the solar system is a statistical outlier or can be explained by more general planetary formation processes. Here we employ numerical models of planet formation, evolution, and interior structure, to show that a planet's bulk water fraction and radius are anti-correlated with initial Al levels in the planetesimal-based accretion framework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals prior to accretion onto larger protoplanets and yields a system-wide correlation of planet bulk abundances, which, for instance, can explain the lack of a clear orbital trend in the water budgets of the TRAPPIST-1 planets. Qualitatively, our models suggest two main scenarios of planetary systems' formation: high-Al systems, like our solar system, form small, water-depleted planets, whereas those devoid of Al predominantly form ocean worlds, where the mean planet radii between both scenarios deviate by up to about 10%.

Salt Polygons are Caused by Convection (1902.03600v1)

Jana Lasser, Joanna M. Nield, Marcel Ernst, Volker Karius, Lucas Goehring

2019-02-10

From fairy circles to patterned ground and columnar joints, natural patterns spontaneously appear in many complex geophysical settings. Here, we consider the origins of polygons in the crusts of salt playa and salt pans. These beautifully regular features, a meter or so in diameter, have a worldwide distribution and are important to the transport of salt and dust in arid regions, yet there has been no convincing mechanism known for their formation. We present the first evidence that they are the surface expression of buoyancy-driven convection in the porous soil beneath a salt crust. By combining consistent results from direct field observations, analogue experiments, linear stability theory, and numerical simulations, we further explain the conditions under which salt polygons will form, as well as how their characteristic size emerges.

Terminus Geometry as Main Control on Outlet Glacier Velocity (1902.03566v1)

Martin Peter Lüthi

2019-02-10

Ice flow velocities close to the terminus of major outlet glaciers of the Greenland Ice Sheet can vary on the time scale of years to hours. Such flow speed variations can be explained as the reaction to changes in terminus geometry with help of a 3D full-Stokes ice flow model. Starting from an initial steady state geometry, parts of an initially 7 km long floating terminus are removed. Flow velocity increases everywhere up to 4 km upstream of the grounding line, and complete removal of the floating terminus leads to a doubling of flow speed. The model results conclusively show that the observed velocity variations of outlet glaciers is dominated by the terminus geometry. Since terminus geometry is mainly controlled by calving processes and melting under the floating portion, changing ocean conditions most probably have triggered the recent geometry and velocity variations of Greenland outlet glaciers.

On the relation between parameters and discharge data for a lumped karst aquifer model (1808.07009v2)

Daniel Bittner, Mario Teixeira Parente, Steven Mattis, Barbara Wohlmuth, Gabriele Chiogna

2018-08-21

Hydrological models of karst aquifers are often semi-distributed, and physical processes such as infiltration and spring discharge generation are described in a lumped way. Several works have previously addressed the problems associated with the calibration of such models, highlighting in particular the issue of model parameter estimation and model equifinality. In this work, we investigate the problem of model calibration using the active subspace (AS) method, a novel tool for model parameter dimension reduction. We apply the method to a newly proposed hydrological model for karst aquifers, LuKARS, to investigate if the AS framework identifies catchment-specific characteristics or if the results only depend on the chosen model structure. Therefore, we consider four different case studies, three synthetic and one real case (Kerschbaum springshed in Waidhofen a.d. Ybbs, Austria), with varying hydrotope distributions and properties. We find that both the hydrotope area coverage and the catchment characteristics have major impacts on parameter sensitivities. While model parameters are similarly informed in scenarios with less varying catchment characteristics, we find significant differences in parameter sensitivities when the applied hydrotopes were different from each other. Our results show that the AS method can be used to investigate the relation between the model structure, the area of a hydrotope, the physical properties of a catchment and the discharge data. Finally, we successfully effectively reduce the parameter dimensions of the LuKARS model for the Kerschbaum case study using the AS method. The model with reduced parameter dimensions is able to reproduce the observed impacts of land use changes in the Kerschbaum springshed, highlighting the robustness of the hydrotope-based modeling approach of LuKARS and its applicability for land use change impact studies in karstic systems.

Simulation and Instability Investigation of the Flow around a Cylinder between Two Parallel Walls (1902.02460v1)

Hua-Shu Dou, An-Qing Ben

2019-02-07

The two-dimensional flows around a cylinder between two parallel walls at Re=40 and Re=100 are simulated with computational fluid dynamics (CFD). The governing equations are Navier-Stokes equations. They are discretized with finite volume method (FVM) and the solution is iterated with PISO Algorithm. Then, the calculating results are compared with the numerical results in literature, and good agreements are obtained. After that, the mechanism of the formation of Karman vortex street is investigated and the instability of the entire flow field is analyzed with the energy gradient theory. It is found that the two eddies attached at the rear of the cylinder have no effect on the flow instability for steady flow, i.e., they don't contribute to the formation of Karman vortex street. The formation of Karman vortex street originates from the combinations of the interaction of two shear layers at two lateral sides of the cylinder and the absolute instability in the cylinder wake. For the flow with Karman vortex street, the initial instability occurs at the region inner a vortex downstream of the wake and the center of a vortex firstly loses its stability inner a vortex. For pressure driven flow, it is confirmed that the inflection point on the time-averaged velocity profile leads to the instability. It is concluded that the energy gradient theory is potentially applicable to study the flow stability and to reveal the mechanism of turbulent transition.

Influence of Magnetic Force on the Flow Stability in a Rectangular duct (1902.00620v2)

Rahman Anisur, Wenqia Xu, Kunhang Li, Hua-Shu Dou, Boo Cheong Khoo, Jie Mao

2019-02-02

The stability of the flow under the magnetic force is one of the classical problems in fluid mechanics. In this paper, the flow in a rectangular duct with different Hartmann (Ha) number is simulated. The finite volume method and the SIMPLE algorithm are used to solve a system of equations and the energy gradient theory is then used to study the (associated) stability of magnetohydrodynamics (MHD). The flow stability of MHD flow for different Hartmann (Ha) number, from Ha=1 to 40, at the fixed Reynolds number, Re=190 are investigated. The simulation is validated firstly against the simulation in literature. The results show that, with the increasing Ha number, the centerline velocity of the rectangular duct with MHD flow decreases and the absolute value of the gradient of total mechanical energy along the streamwise direction increases. The maximum of K appears near the wall in both coordinate axis of the duct. According to the energy gradient theory, this position of the maximum of K would initiate flow instability (if any) than the other positions. The higher the Hartmann number is, the smaller the K value becomes, which means that the fluid becomes more stable in the presence of higher magnetic force. As the Hartmann number increases, the K value in the parallel layer decreases more significantly than in the Hartmann layer. The most dangerous position of instability tends to migrate towards wall of the duct as the Hartmann number increases. Thus, with the energy gradient theory, the stability or instability in the rectangular duct can be controlled by modulating the magnetic force.

Quantum Magnetism in Minerals (1806.10967v3)

D. S. Inosov

2018-06-28

The discovery of magnetism by the ancient Greeks was enabled by the natural occurrence of lodestone -- a magnetized version of the mineral magnetite. Nowadays, natural minerals continue to inspire the search for novel magnetic materials with quantum-critical behavior or exotic ground states such as spin liquids. The recent surge of interest in magnetic frustration and quantum magnetism was largely encouraged by crystalline structures of natural minerals realizing pyrochlore, kagome, or triangular arrangements of magnetic ions. As a result, names like azurite, jarosite, volborthite, and others, which were barely known beyond the mineralogical community a few decades ago, found their way into cutting-edge research in solid-state physics. In some cases, the structures of natural minerals are too complex to be synthesized artificially in a chemistry lab, especially in single-crystalline form, and there is a growing number of examples demonstrating the potential of natural specimens for experimental investigations in the field of quantum magnetism. On many other occasions, minerals may guide chemists in the synthesis of novel compounds with unusual magnetic properties. The present review attempts to embrace this quickly emerging interdisciplinary field that bridges mineralogy with low-temperature condensed-matter physics and quantum chemistry.

Atomistic simulations of molten carbonates: thermodynamic and transport properties of the Li2CO3-Na2CO3-K2CO3 system (1902.02225v1)

Elsa Desmaele, Nicolas Sator, Rodolphe Vuilleumier, Bertrand Guillot

2019-02-06

Although molten carbonates only represent, at most, a very minor phase in the Earth's mantle, they are thought to be implied in anomalous high-conductivity zones in its upper part (70-350 km). Besides the high electrical conductivity of these molten salts is also exploitable in fuel cells. Here we report quantitative calculations of their properties, over a large range of thermodynamic conditions and chemical compositions, that are a requisite to develop technological devices and to provide a better understanding of a number of geochemical processes. To model molten carbonates by atomistic simulations, we have developed an optimized classical force field based on experimental data of the literature and on the liquid structure issued from ab initio molecular dynamics simulations performed by ourselves. In implementing this force field into a molecular dynamics simulation code, we have evaluated the thermodynamics (equation of state and surface tension), the microscopic liquid structure and the transport properties (diffusion coefficients, electrical conductivity and viscosity) of molten alkali carbonates (Li2CO3, Na2CO3, K2CO3 and some of their binary and ternary mixtures) from the melting point up to the thermodynamic conditions prevailing in the Earth's upper mantle (~ 1100-2100 K, 0-15 GPa). Our results are in very good agreement with the data available in the literature. To our knowledge a reliable molecular model for molten alkali carbonates covering such a large domain of thermodynamic conditions, chemical compositions and physicochemical properties has never been published yet.

Precessing spherical shells: flows, dissipation, dynamo and the lunar core (1809.05330v2)

David Cébron, Raphaël Laguerre, Jerome Noir, Nathanaël Schaeffer

2018-09-14

Precession of planets or moons affects internal liquid layers by driving flows, instabilities and possibly dynamos.The energy dissipated by these phenomena can influence orbital parameters such as the planet's spin rate.However, there is no systematic study of these flows in the spherical shell geometry relevant for planets, and the lack of scaling law prevents convincing extrapolation to celestial bodies.We have run more than 900 simulations of fluid spherical shells affected by precession, to systematically study basic flows, instabilities, turbulence, and magnetic field generation.We observe no significant effects of the inner core on the onset of the instabilities.We obtain an analytical estimate of the viscous dissipation, mostly due to boundary layer friction in our simulations.We propose theoretical onsets for hydrodynamic instabilities, and document the intensity of turbulent fluctuations.We extend previous precession dynamo studies towards lower viscosities, at the limits of today's computers.In the low viscosity regime, precession dynamos rely on the presence of large-scale vortices, and the surface magnetic fields are dominated by small scales.Interestingly, intermittent and self-killing dynamos are observed.Our results suggest that large-scale planetary magnetic fields are unlikely to be produced by a precession-driven dynamo in a spherical core.But this question remains open as planetary cores are not exactly spherical, and thus the coupling between the fluid and the boundary does not vanish in the relevant limit of small viscosity.Moreover, the fully turbulent dissipation regime has not yet been reached in simulations.Our results suggest that the melted lunar core has been in a turbulent state throughout its history.Furthermore, in the view of recent experimental results, we propose updated formulas predicting the fluid mean rotation vector and the associated dissipation in both the laminar and the turbulent regimes.

Seismogenic nodes as a viable alternative to seismogenic zones and observed seismicity for the definition of seismic hazard at regional scale (1902.02108v1)

Paolo Rugarli, Franco Vaccari, Giuliano Panza

2019-02-06

It is shown that considering a fixed increment of a given magnitude at a fault is equivalent to factoring the mechanical moment at the fault as done in structural engineering with the applied loads, by the most currently used structural engineering standards (e.g. Eurocodes). A special safety factor is introduced and related to the partial factor acting on the mechanical moment representing the fault. A comparison is then made between the hazard maps obtained for Italy with the Neo Deterministic Seismic Hazard Assessment (NDSHA) technique, using two approaches for the definition of the seismic sources considered for the computation of the synthetic seismograms. The first one is based on the magnitude of the events listed in the earthquake catalogue, and located within the active seismogenic zones. This is the standard approach, used in most of the NDSHA computations performed up to now. It is adequate for countries, like Italy, where the catalogue can be reasonably considered complete for events of M=5 and above. When the catalogue completeness is barely adequate, another approach can be adopted for the definition of the earthquake sources. It uses the seismogenic nodes identified by means of pattern recognition techniques applied to morphostructural seismic zonation (MSZ), and increases the reference magnitude by a constant variation tuned thanks to the safety factor. The two approaches have been compared for Italy using the safety factor 2.0: they mostly produce comparable hazard maps. As the two sets are fully independent and the catalogue is very long, this implies a validation of the seismogenic nodes method and a tuning of the safety factor at about 2. Notable exception is seen in the Central Alps, where nodes tend to overestimate the "observed" hazard, and in southeastern Sicily, where the nodes underestimation is only apparent and can be negligible within experimental errors.

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