International audienceWe study numerically an extensive set of dynamo models in rotating spherical shells, varying all relevant control parameters by at least two orders of magnitude. Convection is driven by a fixed temperature contrast between rigid boundaries. There are two distinct classes of solutions with strong and weak dipole contributions to the magnetic field, respectively. Non-dipolar dynamos are found when inertia plays a significant role in the force balance. In the dipolar regime the critical magnetic Reynolds number for self-sustained dynamos is of order 50, independent of the magnetic Prandtl number Pm. However, dynamos at low Pm exist only at sufficiently low Ekman number E. For dynamos in the dipolar regime we attempt to establish scaling laws that fit our numerical results. Assuming that diffusive effects do not play a primary role, we introduce non-dimensional parameters that are independent of any diffusivity. These are a modified Rayleigh number based on heat (or buoyancy) flux Ra*Q, the Rossby number Ro measuring the flow velocity, the Lorentz number Lo measuring magnetic field strength, and a modified Nusselt number Nu* for the advected heat flow. To first approximation, all our dynamo results can be collapsed into simple power-law dependencies on the modified Rayleigh number, with approximate exponents of 2/5, 1/2 and 1/3 for the Rossby number, modified Nusselt number and Lorentz number, respectively. Residual dependencies on the parameters related to diffusion (E, Pm, Prandtl number Pr) are weak. Our scaling laws are in agreement with the assumption that the magnetic field strength is controlled by the available power and not necessarily by a force balance. The Elsasser number ?, which is the conventional measure for the ratio of Lorentz force to Coriolis force, is found to vary widely. We try to assess the relative importance of the various forces by studying sources and sinks of enstrophy (squared vorticity). In general Coriolis and buoyancy forces are of the same order, inertia and viscous forces make smaller and variable contributions, and the Lorentz force is highly variable. Ignoring a possible weak dependence on the Prandtl numbers or the Ekman number, a surprising prediction is that the magnetic field strength is independent both of conductivity and of rotation rate and is basically controlled by the buoyancy flux. Estimating the buoyancy flux in the Earth's core using our Rossby number scaling and a typical velocity inferred from geomagnetic secular variations, we predict a small growth rate and old age of the inner core and obtain a reasonable magnetic field strength of order 1 mT inside the core. From the observed heat flow in Jupiter, we predict an internal field of 8 mT, in agreement with Jupiter's external field being 10 times stronger than that of the Earth

T he Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission landed on Mars on 26 November 2018 in Elysium Planitia 1,2 , 38 years after the end of Viking 2 lander operations. At the time, Viking's seismometer 3 did not succeed in making any convincing Marsquake detections, due to its on-deck installation and high wind sensitivity. InSight therefore provides the first direct geophysical in situ investigations of Mars's interior structure by seismology 1,4. The Seismic Experiment for Interior Structure (SEIS) 5 monitors the ground acceleration with six axes: three Very Broad Band (VBB) oblique axes, sensitive to frequencies from tidal up to 10 Hz, and one vertical and two horizontal Short Period (SP) axes, covering frequencies from ~0.1 Hz to 50 Hz. SEIS is complemented by the APSS experiment 6 (InSight Auxiliary Payload Sensor Suite), which includes pressure and TWINS (Temperature and Winds for InSight) sensors and a magnetometer. These sensors monitor the atmospheric sources of seismic noise and signals 7. After seven sols (Martian days) of SP on-deck operation, with seismic noise comparable to that of Viking 3 , InSight's robotic arm 8 placed SEIS on the ground 22 sols after landing, at a location selected through analysis of InSight's imaging data 9. After levelling and noise assessment, the Wind and Thermal Shield was deployed on sol 66 (2 February 2019). A few days later, all six axes started continuous seismic recording, at 20 samples per second (sps) for VBBs and 100 sps for SPs. After onboard decimation, continuous records at rates from 2 to 20 sps and event records 5 at 100 sps are transmitted. Several layers of thermal protection and very low self-noise enable the SEIS VBB sensors to record the daily variation of the

Subducted oceanic crust, transformed into dense mineral assemblages at high pressure, may gravitationally segregate at the bottom of the convecting mantle, for example, the D″ layer. Here it could be stored for a long enough time to develop an “enriched” isotopic signature, before it is recycled in mantle plumes and hence control the geochemical character of hot‐spot basalts. We study both the geodynamical and geochemical aspects of this hypothesis in two‐dimensional numerical convection models, in which plate motion is imposed by a velocity boundary condition. About 250,000 tracer particles are used to identify the basalt fraction in the oceanic crustmantle system. High‐, average‐, and low‐tracer densities indicate basalt or eclogite, peridotite, and harzburgite, respectively. The tracers are negatively buoyant to account for the density differences between the rock types. At surface divergence zones, crust formation is simulated by extracting tracers and transferring them into a thin layer at the surface. The tracers carry a certain amount of the relevant nuclides of the U‐Pb and Sm‐Nd systems, which are fractionated between the basalt tracers and a second species of residue tracers during crust formation. Using reasonable parameter values, we find that of the order of 1/6 of the subducted crust accumulates in pools at the bottom, which reside underneath thermal plumes. After 3.6 Ga, the [207Pb]/[204Pb], [206Pb]/[204Pb], and [143Nd]/[ 144Nd] ratios in various parts of the model cover the observed HIMU‐MORB range. A systematic study of the influence of control parameters on the results indicates that the amount of segregation and the diversity of isotope ratios (1) increases strongly with Rρ, the ratio of chemical to thermal buoyancy; (2) decreases moderately with the Rayleigh number Ra, where Ra = 106 is the highest value that we employed; (3) increases strongly with the degree of temperature dependence of the viscosity; and (4) is not very sensitive to the partitioning between internal and bottom heating. The largest uncertainty in applying our model results to the Earth lies in the lack of accurate density data for conditions at the core‐mantle boundary. We conclude that, if our density estimates are correct, segregation and reentrainment of subducted crust is of fundamental importance for the dynamics and chemistry of mantle plumes.

The magnetic fields of Earth and Jupiter, along with those of rapidly rotating, low-mass stars, are generated by convection-driven dynamos that may operate similarly (the slowly rotating Sun generates its field through a different dynamo mechanism). The field strengths of planets and stars vary over three orders of magnitude, but the critical factor causing that variation has hitherto been unclear. Here we report an extension of a scaling law derived from geodynamo models to rapidly rotating stars that have strong density stratification. The unifying principle in the scaling law is that the energy flux available for generating the magnetic field sets the field strength. Our scaling law fits the observed field strengths of Earth, Jupiter, young contracting stars and rapidly rotating low-mass stars, despite vast differences in the physical conditions of the objects. We predict that the field strengths of rapidly rotating brown dwarfs and massive extrasolar planets are high enough to make them observable.