Topography of the core–mantle boundary and lateral homogeneity of the liquid core

Fluid Dynamics Experiments for Planetary Interiors

Understanding fluid flows in planetary cores and subsurface oceans, as well as their signatures in available observational data (gravity, magnetism, rotation, etc.), is a tremendous interdisciplinary challenge. In particular, it requires understanding the fundamental fluid dynamics involving turbulence and rotation at typical scales well beyond our day-to-day experience. To do so, laboratory experiments are fully complementary to numerical simulations, especially in systematically exploring extreme flow regimes for long duration. In this review article, we present some illustrative examples where experimental approaches, complemented by theoretical and numerical studies, have been key for a better understanding of planetary interior flows driven by some type of mechanical forcing. We successively address the dynamics of flows driven by precession, by libration, by differential rotation, and by boundary topography.

Terrestrial Single-Station Analog for Constraining the Martian Core and Deep Interior: Implications for InSight

We used a terrestrial single-station seismometer to quantify the uncertainty of InSight (INterior explorations using Seismic Investigations, Geodesy and Heat Transport) data for determining Martian core size. To mimic Martian seismicity, we formed a catalog using 917 terrestrial earthquakes, from which we randomly selected events. We stacked ScS amplitudes on modeled arrival times and searched for where ScS produced coherent seismic amplitudes. A core detection was defined by a coherent peak with small offset between predicted and user-selected arrival times. Iterating the detection algorithm with varying signal-to-noise (SNR) ranges and quantity of events determined the selection frequency of each model and quantified core depth uncertainty. Increasing the quantity of events reduced core depth uncertainty while increasing the recovery rate, while increasing event SNR had little effect. Including ScS2 multiples increased the recovery rate and reduced core depth uncertainty when we used low quantities of events. The most-frequent core depths varied by back azimuth, suggesting our method is sensitive to the presence of mantle heterogeneities. When we added 1° in source distance errors, core depth uncertainty increased by up to 11 km and recovery rates decreased by <5%. Altering epicentral distances by 25% added ~35 km of uncertainty and reduced recovery rates to <50% in some cases. From these experiments, we estimate that if InSight can detect five events with high location precision (<10 % epicentral distance errors), that there is at least an 88% chance of core depth recovery using ScS alone with uncertainty in core depth approaching 18 km and decreasing as more events are located.

Imaging the Galápagos mantle plume with an unconventional application of floating seismometers

We launched an array of nine freely floating submarine seismometers near the Galápagos islands, which remained operational for about two years. P and PKP waves from regional and teleseismic earthquakes were observed for a range of magnitudes. The signal-to-noise ratio is strongly influenced by the weather conditions and this determines the lowest magnitudes that can be observed. Waves from deep earthquakes are easier to pick, but the S/N ratio can be enhanced through filtering and the data cover earthquakes from all depths. We measured 580 arrival times for different raypaths. We show that even such a limited number of data gives a significant increase in resolution for the oceanic upper mantle. This is the first time an array of floating seismometers is used in seismic tomography to improve the resolution significantly where otherwise no seismic information is available. We show that the Galápagos Archipelago is underlain by a deep (about 1900 km) 200–300 km wide plume of high temperature, with a heat flux very much larger than predicted from its swell bathymetry. The decrease of the plume temperature anomaly towards the surface indicates that the Earth’s mantle has a subadiabatic temperature gradient.

Tidal tomography constrains Earth's deep-mantle buoyancy

Earth's body tide-also known as the solid Earth tide, the displacement of the solid Earth's surface caused by gravitational forces from the Moon and the Sun-is sensitive to the density of the two Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific. These massive regions extend approximately 1,000 kilometres upward from the base of the mantle and their buoyancy remains actively debated within the geophysical community. Here we use tidal tomography to constrain Earth's deep-mantle buoyancy derived from Global Positioning System (GPS)-based measurements of semi-diurnal body tide deformation. Using a probabilistic approach, we show that across the bottom two-thirds of the two LLSVPs the mean density is about 0.5 per cent higher than the average mantle density across this depth range (that is, its mean buoyancy is minus 0.5 per cent), although this anomaly may be concentrated towards the very base of the mantle. We conclude that the buoyancy of these structures is dominated by the enrichment of high-density chemical components, probably related to subducted oceanic plates or primordial material associated with Earth's formation. Because the dynamics of the mantle is driven by density variations, our result has important dynamical implications for the stability of the LLSVPs and the long-term evolution of the Earth system.

The ZMOA-1990 Nutation Series

We present a new nutation series for the Earth (ZMOA-1990) based on (1) the rigid Earth nutation series developed by Zhu and Groten [1989], (2) the normalized response for an elastic, elliptical Earth with fluid-outer and solid-inner cores developed by Mathews et al. [1990], and (3) corrections for the effects of ocean tides and anelasticity, computed to be consistent with the Mathews et al. [1990] normalized response function. In deriving this series, only two parameters of the geophysical model for the Earth have been modified from their values computed with PREM: the dynamic ellipticities of the whole Earth, e, and of the fluid outer core, ef. The adopted values for these parameters, determined from the analysts of very long baseline interferometry (VLSI) data, are e=0.00328915 which is about 1% higher than the value obtained from PREM and 6×10−5 times larger than the IAU adopted value, and ef=0.002665 which is 4.6% higher than the PREM value. The above values were obtained from an adjustment of −0.3 ʺ/cent to the IAU-1976 luni-solar precession constant for e, and from the amplitude of the retrograde annual nutation for ef. The ZMOA-1990 nutation series agrees with estimates of the in-phase and the out-of-phase nutation amplitudes obtained from VLBI data to within 0.5 mas for the terms with 18.6 year period, and to better than 0.1 mas for terms at all other periods except for the out-of-phase terms with annual period (differences 0.39 mas, retrograde, and 0.13 mas, prograde), and for the in-phase term with prograde 13.66 day period (difference −0.25 mas).

Seismic monitoring in the oceans by autonomous floats

Our understanding of the internal dynamics of the Earth is largely based on images of seismic velocity variations in the mantle obtained with global tomography. However, our ability to image the mantle is severely hampered by a lack of seismic data collected in marine areas. Here we report observations made under different noise conditions (in the Mediterranean Sea, the Indian and Pacific Oceans) by a submarine floating seismograph, and show that such floats are able to fill the oceanic data gap. Depending on the ambient noise level, the floats can record between 35 and 63% of distant earthquakes with a moment magnitude M≥6.5. Even magnitudes <6.0 can be successfully observed under favourable noise conditions. The serendipitous recording of an earthquake swarm near the Indian Ocean triple junction enabled us to establish a threshold magnitude between 2.7 and 3.4 for local earthquakes in the noisiest of the three environments.

Normal-mode and free-Air gravity constraints on lateral variations in velocity and density of Earth's mantle

With the use of a large collection of free-oscillation data and additional constraints imposed by the free-air gravity anomaly, lateral variations in shear velocity, compressional velocity, and density within the mantle; dynamic topography on the free surface; and topography on the 660-km discontinuity and the core-mantle boundary were determined. The velocity models are consistent with existing models based on travel-time and waveform inversions. In the lowermost mantle, near the core-mantle boundary, denser than average material is found beneath regions of upwellings centered on the Pacific Ocean and Africa that are characterized by slow shear velocities. These anomalies suggest the existence of compositional heterogeneity near the core-mantle boundary.

Models of mantle convection: one or several layers

Numerical model calculations are used to determine if convection in the Earth’s mantle could be organized in two or more layers with only limited mass exchange in between. The seismic discontinuity at 670 km depth and the top of the D"-layer at the bottom of the mantle are considered as candidates for internal boundaries. If the 670 km discontinuity is caused by an isochemical phase transition, it has to have a Clapeyron slope of dp/dT ⩽ — 6 MPa k -1 to prevent convection currents from crossing; this value is improbably low. If the discontinuity represents a chemical boundary, the intrinsic density difference has to exceed 3 % to prevent subducted lithospheric slabs from penetrating deeply into the lower mantle; also the condition is possibly hard to meet. The least improbable mechanism for a mid-mantle barrier for convection currents would be a combination of endothermic phase transition and chemical change. The boundary between upper and lower mantle would show considerable topography, and a limited material exchange is to be expected at any rate. The possibility of a downward segregation of former oceanic crust, transformed to dense eclogite, is studied in a further model series. It requires a region of low viscosity, as the Delayer probably is, and is faciliated by the decrease of the thermal expansion coefficient with pressure. About 20% of subducted oceanic crust could accumulate at the core-mantle boundary. The dense material would concentrate underneath rising therm al plumes, and some of it is entrained into the plumes, possibly affecting their geochemical signature.

Seismic modelling of the Earth’s large-scale three-dimensional structure

Several different kinds of seismological data, spanning more than three orders of magnitude in frequency, have been employed in the study of the Earth’s large-scale three-dimensional structure. These yield different but overlapping information, which is leading to a coherent picture of the Earth’s internal heterogeneity. In this article we describe several methods of seismic inversion and intercom pare the resulting models. Models of upper-mantle shear velocity based upon mantle waveforms (Woodhouse & Dziewonski ( J. geophys. Res . 89 , 5953-5986 (1984))) ( f ≲ 7 mHz) and long-period body waveforms ( f ≲ 20 mHz; Woodhouse & Dziewonski ( Eos, Wash . 67 , 307 (1986))) show the mid-oceanic ridges to be the major low-velocity anomalies in the uppermost mantle, together with regions in the western Pacific, characterized by back-arc volcanism. High velocities are associated with the continents, and in particular with the continental shields, extending to depths in excess of 300 km. By assuming a given ratio between density and wave velocity variations, and a given mantle viscosity structure, such models have been successful in explaining some aspects of observed plate motion in terms of thermal convection in the mantle (Forte & Peltier ( J. geophys. Res . 92 , 3645-3679 (1987))). An im portant qualitative conclusion from such analysis is that the magnitude of the observed seismic anomalies is of the order expected in a convecting system having the viscosity, tem perature derivatives and flow rates which characterize the mantle. Models of the lower mantle based upon P-wave arrival times ( f ≈ 1 Hz; Dziewonski ( J. geophys. Res . 89 , 5929-5952 (1984)); Morelli & Dziewonski ( Eos, Wash . 67 , 311 (1986))) SH waveforms ( f ≈ mHz; Woodhouse & Dziewonski (1986)) and free oscillations (Giardini et al . ( Nature, Lond . 325 , 405-411 (1987); J. geophys. Res. 93 , 13716—13742 (1988))) ( f ≈ 0.5-5 mHz) show a very long wavelength pattern, largely contained in spherical harmonics of degree 2, which is present over a large range of depths (1000-2700 km). This anomaly has been detected in both compressional and shear wave velocities, and yields a ratio of relative perturbations in v s and v P in the lower mantle in the range 2-2.5. Such values, which are much larger than has sometimes been assumed, roughly correspond to the case that perturbations in shear modulus dominate those in bulk modulus. It is this anomaly that is mainly responsible for the observed low-degree geoid undulations (Hager et al. Nature, Lond . 313 , 541-545 (1985))). In the upper part of the lower mantle this pattern consists of a high-velocity feature skirting the subduction zones of the Pacific and extending from Indonesia to the Mediterranean, with low velocities elsewhere; thus it appears to be associated with plate convergence and subduction. The pattern of wave speeds in the lowermost mantle is such that approximately 80% of hot spots are in regions of lower than average velocities in the D" region. The topography of the core-mantle boundary, determined from the arrival times of reflected and transmitted waves (Morelli & Dziewonski ( Nature, Lond . 325 , 678-683 (1987))), exhibits a pattern of depressions encircling the Pacific, having an amplitude of approximately ± 5 km, which has been shown to be consistent with the stresses induced by density anomalies inferred from tom ographic models of the lower mantle (Forte & Peltier ( Tectonphysics (In the press.) (1989))). By using both free oscillations (Woodhouse et al . ( Geophys. Res. Lett . 13 , 1549-1552 (1986))) and travel-time data (Morelli et al . ( Geophys. Res. Lett . 13 , 1545—1548 (1986))), the inner core has been found to be anisotropic, exhibiting high velocities for waves propagating parallel to the Earth ’s rotation axis and low velocities in the equatorial plane. Tomographic models represent an instantaneous, low-resolution image of a convecting system. They require for their detailed interpretation knowledge of mineral and rock properties that are, as yet, poorly known but that laboratory experiments can potentially determ ine. The fact that the present distribution of seismic anomalies must represent the current configuration of therm al and compositional heterogeneity advected by m antle flow, imposes a complex set of constraints on the possible modes of convection in the m antle of which the implications have not yet been worked out; this will require num erical modelling of convection in three dimensions, which only recently has become feasible. Thus the interpretation of the ‘geographical’ information from seismology in terms of geodynamical processes is a matter of considerable complexity, and we may expect that a number of the conclusions to be drawn from the seismological results lie in the future.

Implications of geomagnetism for mantle structure

Recent studies of the magnetic field at the core-mantle boundary have revealed fixed sites of either static magnetic features or persistent secular variation. This suggests that part of the magnetic-field behaviour is controlled by the mantle. The most plausible mechanism for core-mantle interaction is thermal coupling, although topography may also be significant. The magnetic sites coincide with anomalies in lower-mantle seismic velocity, as determined from tomography, and density, as determined by flow models of mantle convection constrained by tomography and the geoid. Some magnetic features coincide with subduction zones, particularly those near Indonesia; they may be caused by bumps on the core-m antle boundary beneath trenches. Palaeomagnetic pole positions suggest the magnetic behaviour has persisted for at least 5 Ma, as would be expected if it were controlled from the mantle. These conclusions could be quantified if the frozen-flux hypothesis allowed determination of fluid flow at the core surface, but unfortunately failure of the hypothesis makes all such determinations suspect. Core motions calculated so far suggest the flow is mainly toroidal. Questions about the dynamics of the flow (whether it is steady, stratified, or geostrophic) remain unresolved.

Chemical Synthesis of a 5'-Terminal TMG-Capped Triribonucleotide m(3)(2,2,7)G(5)(')pppAmpUmpA of U1 RNA

The 5'-terminal TMG-capped triribonucleotide, m(3)(2,2,7)G(5)(')pppAmpUmpA, has been synthesized by condensation of an appropriately protected triribonucleotide derivative of ppAmpUmpA with a new TMG-capping reagent. During this total synthesis, it was found that the regioselective 2'-O-methylation of 3',5'-O-(1,1,3,3-tetraisopropyldisiloxane-1,3-diyl)-N-(4-monomethoxytrityl)adenosine was achieved by use of MeI/Ag(2)O without affecting the base moiety. A new route to 2-N,2-N-dimethylguanosine from guanosine via a three-step reaction has also been developed by reductive methylation using paraformaldehyde and sodium cyanoborohydride. These key intermediates were used as starting materials for the construction of a fully protected derivative of pAmpUmpA and a TMG-capping reagent of Im-pm(3)(2,2,7)G. The target TMG-capped tetramer, m(3)(2,2,7)G(5)(')pppAmpUmpA, was synthesized by condensation of a partially protected triribonucleotide 5'-terminal diphosphate species, ppA(MMTr)mpUmpA, with Im-pm(3)(2,2,7)G followed by treatment with 80% acetic acid. The structure of m(3)(2,2,7)G(5)(')pppAmpUmpA was characterized by (1)H and (31)P NMR spectroscopy as well as enzymatic assay using snake venom phosphodiesterase, calf intestinal phosphatase, and nuclease P1.