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Magnetic field generation Some planets and many stars have magnetic fields that are generated by a convection-driven dynamo process. Stellar fields, known from their effect on the emitted light, are often 1,000 times stronger than that of Earth, so if the dynamo mechanism is similar for all these bodies, it is one that can produce field strengths varying over three orders of magnitude. Christensen et al . propose a simple law relating field strength to energy flux that applies to stars and planets alike, provided they are rotating sufficiently rapidly. Computer models of the geodynamo and stellar dynamos support the law, and its predictions agree with the observed fields of Earth, Jupiter and two groups of stars. Objects of intermediate mass, brown dwarf stars and supermassive extrasolar planets, should have strong detectable magnetic fields courtesy of this mechanism — but our Sun rotates too slowly to fit this template. 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, although the field strengths vary. The critical factor unifying field generation in such different objects, while still causing a large variation, has been unclear. This paper reports an extension of a scaling law derived from geodynamo models to rapidly rotating stars. The unifying principle is that the energy flux available for generating the magnetic field sets the field strength. 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 1 , 2 , 3 , 4 (the slowly rotating Sun generates its field through a different dynamo mechanism 5 ). The field strengths of planets and stars vary over three orders of magnitude, but the critical factor causing that variation has hitherto been unclear 5 , 6 . Here we report an extension of a scaling law derived from geodynamo models 7 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.

Recent estimates of the crustal thickness of Mars show a bimodal result of either ∼20 or ∼40 km beneath the InSight lander. We propose an approach based on random matrix theory applied to receiver functions (RFs) to further constrain the subsurface structure. Assuming a spiked covariance model for our data, we first use the phase transition properties of the singular value spectrum of random matrices to detect coherent arrivals in the waveforms. Examples from terrestrial data show how the method works in different scenarios. We identify three previously undetected converted arrivals in the InSight data, including the first multiple from a deeper third interface. We then use this information to jointly invert RFs with the absolute S‐wave velocity information in the polarization of body waves. Results show a crustal thickness of 43 ± 5 km beneath the lander with two mid‐crustal interfaces at depths of 8 ± 1 and 21 ± 3 km. Plain Language Summary Recent analysis of seismic data from InSight shows that the crustal thickness beneath the InSight lander can be either 20  or 40 km. To resolve this ambiguity, we apply results from random matrix theory to receiver function (RF) analysis. The distribution of singular values of a random matrix shows well‐behaved deterministic properties that can be used to separate them from those of an underlying coherent signal if present. We use examples from terrestrial data to show how the method works. When applied to RFs computed from InSight seismic data, we identify three new energy arrivals, including one that supports the existence of a deeper third layer. Using this information, we simultaneously inverted the RF data along with the measured incidence angle of body waves. Results show a crustal thickness of 43 ± 5 km beneath the lander with two mid‐crustal interfaces at depths of 8 ± 1 and 21 ± 3 km. Key Points We apply recent results from random matrix theory to identify crustal phases in noisy receiver functions for Mars from InSight data Once identified, we jointly invert these phases with frequency‐dependent apparent S‐wave velocity curves Results show a crustal thickness of 43 km with two inter‐crustal discontinuities at 8 and 21 km beneath the lander

Mercury: playing the field The latest numerical models of the geodynamo can account for the behaviour of Earth's magnetic field pretty well: Mercury has proved a harder nut to crack. Like Earth, it has a dipolar magnetic field, probably generated by a dynamo from convective motions in the planet's liquid iron core. But Mercury's field is a hundred times weaker than Earth's, and this poses a problem for the dynamo theory. A new explanation for the discrepancy has been proposed, accounting for both the observed field strength and the magnetic field geometry observed during the Mariner 10 flybys. The new model assumes that the dynamo operates only deep down in the core, where it generates a strong field. The outer regions of the core are stably layered, so do not convect heat, but they are electrically conducting and the dynamo-generated field is therefore strongly damped. Data from NASA's Messenger probe, en route to Mercury, and ESA's planned Bepi Colombo mission should provide a thorough test for the model. Mercury has a global magnetic field of internal origin and it is thought that a dynamo operating in the fluid part of Mercury’s large iron core is the most probable cause. However, the low intensity of Mercury’s magnetic field—about 1% the strength of the Earth’s field—cannot be reconciled with an Earth-like dynamo. With the common assumption that Coriolis and Lorentz forces balance in planetary dynamos 1 , a field thirty times stronger is expected. Here I present a numerical model of a dynamo driven by thermo-compositional convection associated with inner core solidification. The thermal gradient at the core–mantle boundary is subadiabatic 2 , 3 , and hence the outer region of the liquid core is stably stratified with the dynamo operating only at depth, where a strong field is generated. Because of the planet’s slow rotation the resulting magnetic field is dominated by small-scale components that fluctuate rapidly with time. The dynamo field diffuses through the stable conducting region, where rapidly varying parts are strongly attenuated by the skin effect, while the slowly varying dipole and quadrupole components pass to some degree. The model explains the observed structure and strength of Mercury’s surface magnetic field and makes predictions that are testable with space missions both presently flying and planned.

Earth sustains its magnetic field by a dynamo process driven by convection in the liquid outer core. Geodynamo simulations have been successful in reproducing many observed properties of the geomagnetic field. However, although theoretical considerations suggest that flow in the core is governed by a balance between Lorentz force, rotational force, and buoyancy (called MAC balance for Magnetic, Archimedean, Coriolis) with only minute roles for viscous and inertial forces, dynamo simulations must use viscosity values that are many orders of magnitude larger than in the core, due to computational constraints. In typical geodynamo models, viscous and inertial forces are not much smaller than the Coriolis force, and the Lorentz force plays a subdominant role; this has led to conclusions that these simulations are viscously controlled and do not represent the physics of the geodynamo. Here we show, by a direct analysis of the relevant forces, that a MAC balance can be achieved when the viscosity is reduced to values close to the current practical limit. Lorentz force, buoyancy, and the uncompensated (by pressure) part of the Coriolis force are of very similar strength, whereas viscous and inertial forces are smaller by a factor of at least 20 in the bulk of the fluid volume. Compared with nonmagnetic convection at otherwise identical parameters, the dynamo flow is of larger scale and is less invariant parallel to the rotation axis (less geostrophic), and convection transports twice as much heat, all of which is expected when the Lorentz force strongly influences the convection properties.

The large-scale compositional structures of planets are primarily established during early global differentiation. Advances in analytical geochemistry, the increasing diversity of extraterrestrial samples, and new paleomagnetic data are driving major changes in our understanding of the nature and timing of these early melting processes. In particular, paleomagnetic studies of chondritic and small-body achondritic meteorites have revealed a diversity of magnetic field records. New, more sensitive and highly automated paleomagnetic instrumentation and an improved understanding of meteorite magnetic properties and the effects of shock, weathering, and other secondary processes are permitting primary and secondary magnetization components to be distinguished with increasing confidence. New constraints on the post-accretional histories of meteorite parent bodies now suggest that, contrary to early expectations, few if any meteorites have been definitively shown to retain records of early solar and protoplanetary nebula magnetic fields. However, recent studies of pristine samples coupled with new theoretical insights into the possibility of dynamo generation on small bodies indicate that some meteorites retain records of internally generated fields. These results indicate that some planetesimals formed metallic cores and early dynamos within just a few million years of solar system formation.

The Sun’s complex dynamics is controlled by buoyancy and rotation in the convection zone. Large-scale flows are dominated by vortical motions 1 and appear to be weaker than expected in the solar interior 2 . One possibility is that waves of vorticity due to the Coriolis force, known as Rossby waves 3 or r modes 4 , remove energy from convection at the largest scales 5 . However, the presence of these waves in the Sun is still debated. Here, we unambiguously discover and characterize retrograde-propagating vorticity waves in the shallow subsurface layers of the Sun at azimuthal wavenumbers below 15, with the dispersion relation of textbook sectoral Rossby waves. The waves have lifetimes of several months, well-defined mode frequencies below twice the solar rotational frequency, and eigenfunctions of vorticity that peak at the equator. Rossby waves have nearly as much vorticity as the convection at the same scales, thus they are an essential component of solar dynamics. We observe a transition from turbulence-like to wave-like dynamics around the Rhines scale 6 of angular wavenumber of approximately 20. This transition might provide an explanation for the puzzling deficit of kinetic energy at the largest spatial scales. Analysis of a six-year time series of SDO/HMI images of the solar photosphere reveals the existence of global-scale equatorial Rossby waves in the Sun, which contain a large fraction of the radial vorticity at these scales.

In the Earth's fluid outer core, a dynamo process converts thermal and gravitational energy into magnetic energy. The power needed to sustain the geomagnetic field is set by the ohmic losses (dissipation due to electrical resistance) 1 . Recent estimates of ohmic losses cover a wide range, from 0.1 to 3.5 TW, or roughly 0.3–10% of the Earth's surface heat flow 1 , 2 , 3 , 4 . The energy requirement of the dynamo puts constraints on the thermal budget and evolution of the core through Earth's history 1 , 2 , 3 , 4 , 5 . Here we use a set of numerical dynamo models to derive scaling relations between the core's characteristic dissipation time and the core's magnetic and hydrodynamic Reynolds numbers—dimensionless numbers that measure the ratio of advective transport to magnetic and viscous diffusion, respectively. The ohmic dissipation of the Karlsruhe dynamo experiment 6 supports a simple dependence on the magnetic Reynolds number alone, indicating that flow turbulence in the experiment and in the Earth's core has little influence on its characteristic dissipation time. We use these results to predict moderate ohmic dissipation in the range of 0.2–0.5 TW, which removes the need for strong radioactive heating in the core 7 and allows the age of the solid inner core to exceed 2.5 billion years.

We use global data from the Lunar Orbiter Laser Altimeter (LOLA) to retrieve the lunar tidal Love number h 2 and find h 2 = 0.0387 ± 0.0025 . This result is in agreement with previous estimates from laser altimetry using crossover points of LOLA profiles. The Love numbers k 2 and h 2 are key constraints on planetary interior models. We further develop and apply a retrieval method based on a simultaneous inversion for the topography and the tidal signal benefiting from the large volume of LOLA data. By the application to the lunar tides, we also demonstrate the potential of the method for future altimetry experiments at other planetary bodies. The results of this study are very promising with respect to the determination of Mercury’s and Ganymede’s h 2 from future altimeter measurements.

Multispectral images (0.44 to 0.98 μm) of asteroid (4) Vesta obtained by the Dawn Framing Cameras reveal global color variations that uncover and help understand the north-south hemispherical dichotomy. The signature of deep lithologies excavated during the formation of the Rheasilvia basin on the south pole has been preserved on the surface. Color variations (band depth, spectral slope, and eucrite-diogenite abundance) clearly correlate with distinct compositional units. Vesta displays the greatest variation of geometric albedo (0.10 to 0.67) of any asteroid yet observed. Four distinct color units are recognized that chronicle processes—including impact excavation, mass wasting, and space weathering—that shaped the asteroid's surface. Vesta's color and photometric diversity are indicative of its status as a preserved, differentiated protoplanet.

The instrument package SEIS (Seismic Experiment for Internal Structure) with the three very broadband and three short‐period seismic sensors is installed on the surface on Mars as part of NASA's InSight Discovery mission. When compared to terrestrial installations, SEIS is deployed in a very harsh wind and temperature environment that leads to inevitable degradation of the quality of the recorded data. One ubiquitous artifact in the raw data is an abundance of transient one‐sided pulses often accompanied by high‐frequency spikes. These pulses, which we term “glitches”, can be modeled as the response of the instrument to a step in acceleration, while the spikes can be modeled as the response to a simultaneous step in displacement. We attribute the glitches primarily to SEIS‐internal stress relaxations caused by the large temperature variations to which the instrument is exposed during a Martian day. Only a small fraction of glitches correspond to a motion of the SEIS package as a whole caused by minuscule tilts of either the instrument or the ground. In this study, we focus on the analysis of the glitch+spike phenomenon and present how these signals can be automatically detected and removed from SEIS's raw data. As glitches affect many standard seismological analysis methods such as receiver functions, spectral decomposition and source inversions, we anticipate that studies of the Martian seismicity as well as studies of Mars' internal structure should benefit from deglitched seismic data. Plain Language Summary The instrument package SEIS (Seismic Experiment for Internal Structure) with two fully equipped seismometers is installed on the surface of Mars as part of NASA's InSight Discovery mission. When compared to terrestrial installations, SEIS is more exposed to wind and daily temperature changes that leads to inevitable degradation of the quality of the recorded data. One consequence is the occurrence of a specific type of transient noise that we term “glitch”. Glitches show up in the recorded data as one‐sided pulses and have strong implications for the typical seismic data analysis. Glitches can be understood as step‐like changes in the acceleration sensed by the seismometers. We attribute them primarily to SEIS‐internal stress relaxations caused by the large temperature variations to which the instrument is exposed during a Martian day. Only a small fraction of glitches correspond to a motion of the whole SEIS instrument. In this study, we focus on the detection and removal of glitches and anticipate that studies of the Martian seismicity as well as studies of Mars's internal structure should benefit from deglitched seismic data. Key Points Glitches due to steps in acceleration significantly complicate seismic records on Mars Glitches are mostly due to relaxations of thermal stresses and instrument tilt We provide a toolbox to automatically detect and remove glitches