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The recent successful flight demonstration of the Mars 2020 helicopter, Ingenuity, has opened doors for future Mars mission concepts that exploit modern technology, and promising investigations include low altitude magnetic field surveys. The martian crustal magnetic field has been studied extensively from orbit and those data sets have allowed global studies of the magnetic field and resulted in a range of models for the crustal magnetic field which however lack short wavelength information that is not resolvable from orbital altitudes. The InSight lander and the Chinese Zhurong missions have recently acquired magnetic measurements of the local field at their respective landing sites. However, to-date no measurements at scales in between those of local surface and global orbital data have been collected. Such measurements are key to understanding near-surface magnetizations, the processes by which they were acquired, and their interaction with magnetic fields generated above the planet’s surface. Here, we investigate data sets that a future helicopter-based magnetometer might be able to provide.We construct forward models that resemble a range of plausible subsurface geological structures that allow us to experiment with survey design, e.g., the value of multiple measurement tracks horizontally and/or vertically and their trade-offs with regional data coverage. We simulate vector magnetic field data collected by a helicopter for different geological scenarios and aim to recover our model via an inverse problem. Because such inverse problems are inherently non-unique, we investigate several approaches to find solutions, including different types of regularization, as well as modification of the model parameterization. As one example, we investigate recovery of a magnetization signature associated with a small (~200 m diameter) crater, from a few (e.g., 3) helicopter tracks over the crater. We show that smooth and sparse inversion solutions result in detection of the signal, with improved localization of the structure in the latter case. Parameterized solutions improve upon sparse solutions, but require some prior knowledge, or assumption, of the geometry (in this case a magnetized half sphere) of the source.Our investigation allows us to assess the capabilities of helicopter-based magnetic field studies in addressing some of the fundamental open questions in the field. These kinds of considerations will greatly aid in preparing for and designing future missions, optimizing their science return and demonstrating their scientific value.

The Emirates Mars Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission spacecraft, which observes ultraviolet emission between approximately 100 and 170 nm, has observed multiple instances of nightside aurora at Mars. Variations in the auroral brightness and morphology have been observed to change on timescales of tens of minutes. The brightest aurorae are typically seen following space weather events, i.e., coronal mass ejection and stream interaction region impacts. The InSight Fluxgate Magnetometer (IFG) on the Interior Explorations using Seismic Investigations, Geodesy and Heat Transport (InSight) lander measured the magnetic field at the surface of Mars. IFG has measured variations in the nightside surface magnetic field, presumably due to variations in ionospheric and magnetospheric currents. Periodic and aperiodic variations in the surface field have been observed, including with timescales of a few minutes to tens of minutes. The magnitude of the fluctuations is often larger following space weather events. We examine the connection between the presence of aurora as observed by EMUS and surface magnetic field fluctuations as measured by IFG. Coincident EMUS and IFG observations show enhanced surface magnetic field fluctuations during times when aurorae were present. Additionally, the timescale of fluctuations in the auroral brightness are similar to the timescale of surface magnetic field fluctuations for non-coincident observations. These results suggest that IFG measured the surface magnetic field effect of time varying ionospheric auroral currents.

InSight landed on Mars in November 20181 and carries the InSight FluxGate Magnetometer, IFG2 which has provided the first surface magnetic field measurements on Mars1,3. Previous magnetic field measurements taken at orbital altitudes have provided global coverage, with limited spatial resolution. Laboratory analyses of meteorites provide information on magnetic properties of martian rocks, but without detailed local context for their provenances4. Advances from the IFG are thus unique and complementary science, specifically characterizing the crustal ambient static and external time-varying fields at a single location on Mars. External fields provide information on the planet’s interaction with the interplanetary magnetic field and the ionosphere. Crustal magnetic fields carry information about the ancient dynamo and crustal conditions at the time at which magnetization was acquired, and on how the crust has been modified by subsequent exogenic and endogenic processes4. IFG data for sols 14-736 were collected almost continuously, with some data gaps from electronics anomalies. After sol 736, the magnetometer was operational for shorter periods due to power constraints (Fig. 1) 5. A range of studies have been enabled by IFG data, supported by results from other instruments such as the seismometer, and we summarize those.Strong crustal fields provide evidence for an ancient dynamo. The IFG measured a surface magnetic field strength of ~2000 nT, ~10x stronger than predicted from satellite data3,6 and consistent with an ancient Earth-like dynamo3. The strong surface field indicates that magnetization at wavelengths shorter than those resolvable from current satellite data (~150 km) contribute substantially to the overall magnetic field. Characterization of the crust through seismic measurements7 and geologic inferences3 of subsurface layering allow assessment of magnetization of the crust (Fig. 2). Depending on the depth at which the magnetization is carried, specifically whether it is in the seismically-determined deep layer of Noachian origin or also in the shallow Hesperian-aged crust, the minimum magnetization required to explain the surface field is ~2 A/m or ~0.4 A/m. Seismic characterization of crustal structure8 indicates a deep subsurface layer (> 20 km, Fig. 2) of no porosity, while the upper crust (8. Magnetization of these layers require an early active dynamo (>~4 Ga). Fractured, less porous material could have provided pathways for hydrothermal circulation and chemical remanent magnetization4,8,10. Magnetization of the most surficial layer of Hesperian age would be consistent with a long-lasting (up to ~3.7 Ga) dynamo9. IFG data also reveal time-varying fields at the planetary surface that include contributions with different periods and origins. External fields have been observed and characterized from orbit11–13. However, the degree to which external fields penetrate to and interact with the surface could not be studied prior to the InSight landing. Static and long-duration observations from a surface magnetometer are advantageous because, unlike satellite measurements, temporal variability in the field is not mixed with spatial variability. Here, we summarize different external magnetic field phenomena, transient and periodic that have been observed in IFG data (Fig. 3). Periodic variations include short period waves (100s-1000s3,14), diurnal variations15, the ~26 sol Carrington period associated with solar rotation16, and seasonal15,17 fluctuations. Transient events are observed in response to space weather18 and dust movement19,20.The inclusion of the magnetometer on InSight has provided unique and substantial scientific contributions to the overall mission results, as well as a starting point for future planetary surface magnetic field investigations. To overcome limitations of current data sets, we look forward to Mars sample return, as well as possible near-surface investigations. Including magnetometers on future missions at a variety of surface locations for long duration observations will be of great value in understanding a range of external field phenomena, including the influence of crustal magnetic fields on ionospheric currents and the effects of space weather during different phases of a solar cycle. We further advocate for regional investigations for example via a helicopter20 that can provide local magnetic field measurements at a spatial scale commensurate with detailed geological knowledge, to further constrain evolution of Mars’ ancient dynamo and explore the magnetic properties of the crust. Figure 1: a) Martian years 1 (blue) and 2 (red) of the magnetic field amplitude, B, versus solar longitude (ls). All data up to sol 1106 of InSight operations are included (PDS release 13). The blue vertical dashed line marks the beginning of the mission. (b) Corresponding sol numbers. Figure 2: The minimum magnetization required by B=2013 nT (within its 99% confidence intervals)21 for the crust below InSight8. Burial depth describes the depth extent of the unmagnetized layers above the top of the magnetized layer. A burial depth of 200 m (blue), corresponds to burial beneath the young (H: Hesperian, HNt: Hesperian-Noachian transition) near-surface lava flow3 and magnetizations are at least ~0.4 A/m if the entire underlying crust is magnetized. A burial depth of 10 km (blue) requires magnetizations >1 A/m, hosted by Noachian units. The velocity profiles show the seismically-determined interface depths7. Figure 3: A composite power spectral density (PSD) plot for the surface magnetic field strength at the InSight landing site. Estimates for longer periods are derived using a Lomb-Scargle algorithm (black), shorter periods (purple) show a Welch spectrum.[1] Banerdt, W. et al. Nat. Geosci. (2020).[2] Banfield, D. et al. SSR (2019). [3] Johnson, C. L. et al. Nat. Geosci.(2020). [4]Mitteholz, A. & Johnson, C. L. Frontiers (2022). [5] Joy, S. et. al. (2019). [6] Smrekar, S. et al. SSR (2018). [7] Knapmeyer-Endrun, B. et al. Science (2021). [8] Wieczorek, M. et al. JGR (2022). [9] Mittelholz, A. et al. Sci. Adv. (2020). [10] Gyalay, S. et al. GRL (2020). [11] Mittelholz, A. et al. JGR (2017). [12] Ramstad, R. et al. Nat. Astron. (2020). [13] Brain, D. et al. JGR (2003). [14] Chi, P. et al. LPSC (2019). [15] Mittelholz, A. et al. JGR (2020). [16] Luo, H. et al. JGR (2022). [17] Mittelholz, A. et al. LPSC (2021). [18] Mittelholz, A. et al. GRL (2021). [19] Thorne, S. et al. PSS (2022). [20] Bapst, J. et al. AAS (2021). [21] Parker, R. JGR (2003).

Introduction: The edge of the pre-Noachian Borealis impact basin, thought to be the cause of the planetary dichotomy boundary [1-2], crosses the northern Valles Marineris troughs [1-3]. Intense deformation is exposed in the deepest parts of the Ophir and Hebes Chasmata, the northernmost troughs. Structural geology and mineralogical analyses motivate the tentative identification of brittle and brittle-ductile shear zones and hydrothermal activity in the Valles Marineris basement. Implications for the Borealis basin and the proto-Valles Marineris crust are examined. Structural analysis: Crustal right-lateral shear zones are identified in the pre-Noachian basement of Ophir and Hebes Chasmata (Figure 1). In Ophir Chasma, S-C-C' structures, indicate deformation in the brittle-ductile domain. In Hebes Chasma, megabreccia indicates brittle deformation. From scaling relationships [4-5], the shear zones are inferred to be at least hundreds of kilometers long. They do not extend to the surface nor even up into the interior layered deposits (ILD), and are therefore interpreted to affect the Valles Marineris basement only, which at this depth, is interpreted to be of pre-Noachian age. Mineralogy: A new method of non-linear spectral unmixing derived from the LinMin algorithm [6] is implemented and applied to three pre-Noachian basement exposures in a CRISM cube in Ophir Chasma. After gas absorption removal, two groups of minerals are robustly detected (Figure 2): primary minerals of mafic rocks (olivine, hypersthene, augite, anorthite, albite), and sulfates, most of them likely of hydrothermal origin (copiapite, jarosite, szomolnokite). Anhydrite (ROI3) is not diagnostic of any particular environment. Kieserite is interpreted as transported by wind from the neighboring ILDs. S-C-C' structures constrain the granulometry of the sheared rock which, under the assumption that all the primary minerals are detected, would be olivine-gabbronorite (ROI1) or troctolite (ROI2-3). Combined structural and mineralogical analyses point to hydrothermal alteration of a mafic intrusive basement, or contamination of this basement by hydrothermal activity in the ILDs. Relationships with the Borealis basin: The general trend of the shear zones follows the edge of the Borealis as inferred from gravity and topography [4], also of pre-Noachian age, suggesting that they may have initiated as basin ring faults and were reactivated as crustal shears. North of Valles Marineris, the radial component of the remanent magnetic field at the surface [7] shows elongated anomalies that follow the trend of the shear zones and more generally, the expected curved edge of the Borealis basin. The existence of a magnetic field (or dynamo) was coeval with formation of the planetary dichotomy boundary [8]. Two anomalies also correspond to Noachian or pre-Noachian crustal ridges in Ophir Planum, of igneous [9] or tectonic [10] origin. Mapping reveals that the ridges are fractured parallel to the magnetic anomalies, and that their topography guided a hydrologic system (Figure 3). Moreover, these fractures are parallel to a dyke swarm exposed in eastern Candor Chasma [11]. Therefore, the ridges have a volcanotectonic origin within an active hydrologic context. Figure 1. Ophir shear zone (OSZ) and Hebes shear zone (HSZ): (a) location map showing trace of the Borealis impact basin with ±5° uncertainty (dashed area) [5]; (b) zoom on S-C-C' structures in the OSZ, and illustration of shear orientations; (c) zoom on fault megabreccia in the HSZ. HiRISE images ESP_017754_1755 and ESP_040211_1790. Figure 2. Results of nonlinear spectral unmixing applied to basement exposures in Ophir Chasma: (a) CRISM cube location (frt00018b55_07_if165l_trr3); (b) the cube (bands R: 233, G: 78, B: 13); (c) mineral relative abundances, after aerosol contribution removal; (d) best fit plots and HiRISE images of the basement exposures: ESP_051999_1755, ESP_039525_1755, ESP_039525_1755. Figure 3. Features related to hydrothermal activity possibly resulting from the Borealis impact and suggested to explain magnetic banding north of Coprates Chasma. Dykes are located thanks to HiRISE images. The rose diagram was established from 26 representative dykes observed on the eastern Candor Chasma wall; the indicated strike refers to the mean resultant dyke orientation. Magnetic anomalies are from [7]. Topographic contours (spacing 500 m) are from HRSC (ESA/DLR/Freie Univ. Berlin). Circumferential magnetic anomalies are observed at some terrestrial impact craters (e.g.[12]) as well as the Chicxulub impact basin [13] and are due to crystallization of magnetic minerals in an impact-related hydrothermal system [14]. We suggest, therefore, that the magnetic anomalies measured above the Valles Marineris plateau similarly result from hydrothermal activity in response to the Borealis impact, and follow basin ring structures. This hydrothermal activity might be the surface counterpart of deep hydrothermal activity in the basement detected using spectral unmixing [15]. Conclusions: Analysis of northeastern Valles Marineris supports the interpretation of a pre-Noachian Borealis impact basin that would have underlain the later northern troughs of Valles Marineris in the presence of an active dynamo. Large shear zones in the Valles Marineris basement would be reactivated ring faults. Borealis basin formation may have triggered a huge hydrothermal system, identified along these structures and also producing magnetic minerals that generated the observed magnetic anomalies. Primary deposits of base and rare metals likely formed as well. Other evidence of hydrothermal activity at the edge of the Borealis basin would confirm these interpretations. References: [1] Andrews-Hanna J. et al. (2008) Nature, 453, 1212–1215. [2] Marinova M. M. et al. (2008) Nature, 453, 1216–1219. [3] Andrews-Hanna J. (2012) J. Geophys. Res., 117, E03006. [4] Schultz R. A. and Fossen H. (2002) J. Struct. Geol. 24, 1389–1411. [5] Fossen H. (2010) Structural Geology, Cambridge Univ. Press. [6] Schmidt F. et al. (2014) Icarus, 237, 61–74. [7] Langlais B. et al. (2019) J. Geophys Res., 109, E02008. [8] Mittelholz et al. (2020) Sci. Adv., eaba0513. [9] Tanaka K. L. et al. (2014) USGS Sci. Inv. Map 3292. [10] Viviano-Beck et al. (2017) Icarus, 284, 43–58. [11] Mège D. and Gurgurewicz J. (2017) 48th LPSC, Abstract #1087. [12] Hawke P. J. et al. (2006) Explor. Geophys., 37, 191–196. [13] Abramov O. and Kring D. A. Meteor. Planet. Sci., 42, 93–112. [14] Osinski G. R. et al. (2011) Meteor. Planet. Sci., 36, 731–745. [15] Gurgurewicz J. et al., submitted to Commun. Earth Environ.

International audience; The NASA InSight lander has recorded many pressure drops attributed to convective vortices during its first full year of data collection. However, although dust-carrying vortices (dust devils) are a common phenomenon on Mars, they have not been observed in InSight images. On Earth, magnetic signals associated with some dust devils have been reported. Data from the InSight Fluxgate Magnetometer (IFG) provide the first opportunity for similar investigations on Mars. Here, we evaluate whether magnetic signals are associated with daytime vortices. We incorporate observations of environmental conditions, measurements of ground tilt from seismic data, and data from the lander's solar panels, and consider the potential for dust-laden vortices to generate observable magnetic field signals. We find that 7.7% of pressure drop events greater than 1 Pa show a resolvable magnetic field signal at the time of the pressure drops. The resolvable magnetic signals, typically seen on the horizontal field components, are less than 1 nT in amplitude, and have no clear correlation with local time, duration, or pressure drop magnitude. During nine pressure drop events we found smoothly varying magnetic signals of at least 0.3 nT on any one component. To investigate the origin of these magnetic signals we evaluated three possible sources: solar array currents (SAC), ground and lander tilt, and triboelectric effects of lofted dust. We find that SAC and tilt could contribute a change in the magnetic field but cannot solely explain the observed signals. The observed changes in field strength could theoretically be produced via triboelectric effects, but only in the case of exceptionally large dust devils that pass close to the lander. The lack of imaged dust devils and the small number of observed magnetic signatures despite numerous measured pressure drops is consistent with at most a small proportion of dust laden convective vortices at InSight and associated predicted triboelectric effects.

The ESA-JAXA BepiColombo mission will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric and exospheric dynamics at Mercury as well as their interactions with the solar wind, radiation, and interplanetary dust. Many scientific instruments onboard the two spacecraft will be completely, or partially devoted to study the near-space environment of Mercury as well as the complex processes that govern it. Many issues remain unsolved even after the MESSENGER mission that ended in 2015. The specific orbits of the two spacecraft, MPO and Mio, and the comprehensive scientific payload allow a wider range of scientific questions to be addressed than those that could be achieved by the individual instruments acting alone, or by previous missions. These joint observations are of key importance because many phenomena in Mercury's environment are highly temporally and spatially variable. Examples of possible coordinated observations are described in this article, analysing the required geometrical conditions, pointing, resolutions and operation timing of different BepiColombo instruments sensors., Comment: 78 pages, 14 figures, published

International audience

Magnetic fields provide a window into a planet’s interior structure and evolution, including its atmospheric and space environments. Satellites at Mars have measured crustal magnetic fields indicating an ancient dynamo. These crustal fields interact with the solar wind to generate transient fields and electric currents in Mars’s upper atmosphere. Surface magnetic field data play a key role in understanding these effects and the dynamo. Here we report measurements of magnetic field strength and direction at the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) landing site on Mars. We find that the field is ten times stronger than predicted by satellite-based models. We infer magnetized rocks beneath the surface, within ~150 km of the landing site, consistent with a past dynamo with Earth-like strength. Geological mapping and InSight seismic data suggest that much or all of the magnetization sources are carried in basement rocks, which are at least 3.9 billion years old and are overlain by between 200 m and ~10 km of lava flows and modified ancient terrain. Daily variations in the magnetic field indicate contributions from ionospheric currents at 120 km to 180 km altitude. Higher-frequency variations are also observed; their origin is unknown, but they probably propagate from even higher altitudes to the surface. We propose that the time-varying fields can be used to investigate the electrical conductivity structure of the martian interior. The magnetic field measured by the InSight lander on Mars varies daily and is ten times stronger than expected. The field is inferred to originate from components of basement rocks magnetized by an ancient dynamo of Earth-like strength.

International audience

Open access funding provided by Università degli Studi di Roma La Sapienza within the CRUI-CARE Agreement. In preparation for the ESA/JAXA BepiColombo mission to Mercury, thematic working groups had been established for coordinating the activities within the BepiColombo Science Working Team in specific fields. Here we describe the scientific goals of the Geodesy and Geophysics Working Group (GGWG) that aims at addressing fundamental questions regarding Mercury’s internal structure and evolution. This multidisciplinary investigation will also test the gravity laws by using the planet Mercury as a proof mass. The instruments on the Mercury Planetary Orbiter (MPO), which are devoted to accomplishing the GGWG science objectives, include the BepiColombo Laser Altimeter (BELA), the Mercury orbiter radio science experiment (MORE), and the MPO magnetometer (MPO-MAG). The onboard Italian spring accelerometer (ISA) will greatly aid the orbit reconstruction needed by the gravity investigation and laser altimetry. We report the current knowledge on the geophysics, geodesy, and evolution of Mercury after the successful NASA mission MESSENGER and set the prospects for the BepiColombo science investigations based on the latest findings on Mercury’s interior. The MPO spacecraft of the BepiColombo mission will provide extremely accurate measurements of Mercury’s topography, gravity, and magnetic field, extending and improving MESSENGER data coverage, in particular in the southern hemisphere. Furthermore, the dual-spacecraft configuration of the BepiColombo mission with the Mio spacecraft at higher altitudes than the MPO spacecraft will be fundamental for decoupling the internal and external contributions of Mercury’s magnetic field. Thanks to the synergy between the geophysical instrument suite and to the complementary instruments dedicated to the investigations on Mercury’s surface, composition, and environment, the BepiColombo mission is poised to advance our understanding of the interior and evolution of the innermost planet of the solar system. We are grateful to the ESA spacecraft operations team for supporting and planning the scientific observations during BepiColombo cruise and orbital mission. A.G. and L.I. thank A. Di Ruscio (Sapienza University of Rome) for his support in the numerical simulations of the MORE investigation. A.G. and L.I. acknowledge funding from the Italian Space Agency (ASI) grant N. 2017-40-H.0. T.V.H. was financially supported by the Belgian Research Action through Interdisciplinary Networks (BRAIN.be 2.0 project STEM) and by the Belgian PRODEX program managed by the European Space Agency in collaboration with the Belgian Federal Science Policy Office. D.H. was financially supported by the German Ministerium für Wirtschaft und Energie and the German Zentrum für Luft- und Raumfahrt under contract 50 QW 1501. F.S. was financially supported by ASI through the cooperation agreement N. 2017-47-H.0. We acknowledge Gregory A. Neumann and an anonymous referee for their helpful comments to improve the quality of this paper. The data used in this study for the numerical simulations of the BepiColombo mission are available at https://www.cosmos.esa.int/web/spice/spice-for-bepicolombo. Peerreview

Earth, Planets and Space, 73 (1), ISSN:1343-8832, ISSN:1880-5981

International audience; Study of induced magnetic field is a powerful way to sound Earth internal structure. This work presents a full analysis and interpretation in terms of electrical conductivity and temperature of vector magnetic field measurements from Swarm Level1b data product from 26/11/2013 to 31/12/2019. Time series of the Gauss coefficients associated with the induced and inducing magnetic field are obtained from the data after removal of the core and lithospheric fields models and data selection. A Bayesian inversion of the induced field Gauss coefficients is then performed to obtain a new estimate of Earth's 1D mantle electrical conductivity down to 2000 km depth. This profile is fully compatible with the profiles derived from satellite and ground magnetic observatories data but does not present in the lower mantle the increase predicted by laboratory-based conductivity profile associated to classical mantle composition and temperature profile. Using the most recent database to model the electrical conductivity of all mineral mantle phases, two different methods are used to interpret Swarm data in terms of temperature for a given composition and water content. The first one is based on an interpretation of the conductivity estimates in terms of temperature by classical numerical root search. The second one consists in inferring a temperature probability density function from a Bayesian inversion of the Gauss coefficients associated to the induced magnetic field. Our results show that the later provide more reliable estimates of mantle temperatures, in relation to more physically grounded prior values. This second method provides also tighter constraints on the electrical conductivity estimates of the lower mantle.

With both the Mars Atmosphere and Volatile Evolution (MAVEN) mission and the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission concurrently operating at Mars, we are able to make two point comparisons of the vector magnetic field at Mars for the first time. During MAVEN overflights of the InSight landing site, we compared deviations in the ionospheric magnetic field to variations in the surface level magnetic field. We find significant orbit to orbit variability in the magnitude and direction of the ionospheric magnetic field as well as significant day to day variability of the surface level magnetic field. We attribute this variability to time varying ionospheric currents. However, when analyzing the ensemble of 16 individual MAVEN overflights of the InSight landing location, we see no clear correlation between the magnitudes or directions of the ionospheric magnetic field and the surface magnetic field as might be expected. If the presumed ionospheric currents have a small scale size, then the ionospheric magnetic field will display increased variability as MAVEN flies through the current structure. Whereas the present analysis is restricted to mostly nightside MAVEN overflights where current are expected to be weak, future analyses should incorporate dayside overflights where current are expected to be stronger and current signatures more clear.

International audience

Observations of the geomagnetic field taken at Earth's surface and at satellite altitude were combined to construct continuous models of the geomagnetic field and its secular variation from 1957 to 2020. From these parent models, we derive candidate main field models for the epochs 2015 and 2020 to the 13th generation of the International Geomagnetic Reference Field (IGRF). The secular variation candidate model for the period 2020 - 2025 is derived from a forecast of the secular variation in 2022.5, which results from a multi-variate singular spectrum analysis of the secular variation from 1957 to 2020.

International audience; The global-scale crustal structure of Mars is shaped by impact basins, volcanic provinces, and a hemispheric dichotomy with a thin crust beneath the northern lowlands and a thick crust beneath the southern highlands. The southern highlands are commonly treated as a coherent terrain of ancient crust with a common origin and shared geologic history, plausibly originating from a giant impact(s) or a hemispheric-scale mantle upwelling. Previous studies have quantified the contribution of volcanism to this crustal structure; however, the influence of large impacts remains unclear. Here we present reconstructions of the past crustal thickness of Mars (about 4.2 Gyr ago) where the four largest impact basins (Hellas, Argyre, Isidis and Utopia) are removed, assuming mass conservation, as well as the main volcanic provinces of Tharsis and Elysium. Our reconstruction shows more subdued crustal thickness variations than at present, although the crustal dichotomy persists. However, our reconstruction reveals a region of discontinuous patches of thick crust in the southern highlands associated with magnetic and geochemical anomalies. This region, corresponding to Terra Cimmeria–Sirenum, is interpreted as a discrete crustal block. Our findings suggest that the southern highlands are composed of several crustal blocks with different geological histories. Such a complex architecture of the southern highlands is not explained by existing scenarios for crustal formation and evolution. A discrete block of thick ancient crust revealed by a crustal reconstruction suggests a complex geologic history for the southern highlands of Mars.

MAVEN magnetic field data indicate that a martian dynamo field was active 4.5 and 3.7 Ga ago., The absence of crustal magnetic fields above the martian basins Hellas, Argyre, and Isidis is often interpreted as proof of an early, before 4.1 billion years (Ga) ago, or late, after 3.9 Ga ago, dynamo. We revisit these interpretations using new MAVEN magnetic field data. Weak fields are present over the 4.5-Ga old Borealis basin, with the transition to strong fields correlated with the basin edge. Magnetic fields, confined to a near-surface layer, are also detected above the 3.7-Ga old Lucus Planum. We conclude that a dynamo was present both before and after the formation of the basins Hellas, Utopia, Argyre, and Isidis. A long-lived, Earth-like dynamo is consistent with the absence of magnetization within large basins if the impacts excavated large portions of strongly magnetic crust and exposed deeper material with lower concentrations of magnetic minerals.

International audience; Key Points: 9 • We analyze crustal magnetic anomalies that are likely thermoremanent and ob-10 tain the corresponding paleopole positions. 11 • All best fitting paleopoles are found in the Southern Hemisphere. 12 • Our study strongly suggests that Mercury has evolved with time. Abstract 14 Low altitude magnetic field data acquired by MESSENGER over a small portion of Mer-15 cury's surface revealed weak crustal magnetic field signatures. Here we study the crustal 16 magnetic anomalies associated with impact craters on Mercury. We assume that the sources 17 of these anomalies consist of impact melt, enriched in impactor iron. We assume that 18 the subsurfaces of Mercury's impact craters have cooled in the presence of a constant 19 global magnetic field, thus becoming thermoremanently magnetized. We invert for the 20 crustal magnetization direction within five craters using a unidirectional magnetization 21 model which assumes that the melt impact rocks recorded the constant core magnetic 22 field present when the crater was formed, and that the crater's magnetization has not 23 been altered since its formation. From the best fitting magnetization direction we then 24 obtain the corresponding north magnetic paleopole position assuming a centered core 25 dipolar field. Results show that all five magnetic paleopoles lie in the Southern Hemi-26 sphere but are not required to be located near the present-day magnetic pole, which lies 27 near the south geographic pole. Accounting for the uncertainties, we show that our re-28 sults all agree in a common small region that excludes the current magnetic pole. This 29 strongly suggests that the dynamo has evolved with time. Our results represent valu-30 able information for understanding the evolution of Mercury, and emphasize the impor-31 tance of including more anomaly analyses to complete and refine our conclusions. 32

The evolution of Martian Crustal Magnetic Field from MGS to MAVEN and Insight missionseng

While devoid of an active magnetic dynamo field today, Mars possesses a remanent magnetic field that may reach several thousand nanoteslas locally. The exact origin and the events that have shaped the crustal magnetization remain largely enigmatic. Three magnetic field data sets from two spacecraft collected over 13 cumulative years have sampled the Martian magnetic field over a range of altitudes from 90 up to 6,000 km: (a) Mars Global Surveyor (MGS) magnetometer (1997–2006), (b) MGS Electron Reflectometer (1999–2006), and (c) Mars Atmosphere and Volatile EvolutioN (MAVEN) magnetometer (2014 to today). In this paper we combine these complementary data sets for the first time to build a new model of the Martian internal magnetic field. This new model improves upon previous ones in several aspects: comprehensive data coverage, refined data selection scheme, modified modeling scheme, discrete-to-continuous transformation of the model, and increased model resolution. The new model has a spatial resolution of ∼160 km at the surface, corresponding to spherical harmonic degree 134. It shows small scales and well-defined features, which can now be associated with geological signatures.

International audience; Mercury is characterized by a very peculiar magnetic field, as it was revealed by the MESSENGER mission. Its internal component is highly axisymmetric, dominated by the dipole, and very weak. This in turns leads to a very dynamic magnetosphere. It is known that there exist relationships between the internally generated field and the external field, although their dynamics are complex. In this study we derive steady and time‐varying spherical harmonic models of Mercury's magnetic field using MESSENGER measurements and interpret these models both in terms of correlated features and of the internal structure of Mercury. The influence of the hemispheric data distribution of MESSENGER is evaluated to grant the robustness of our models. We find a quadrupole‐to‐dipole ratio of 0.27 for the steady magnetic field. The time‐varying models reveal periodic and highly correlated temporal variations of internal and external origins. This argues for externally inducing and internally induced sources. The main period is 88 days, the orbital period of Mercury around the Sun. There is no measurable time lag between variations of external and internal magnetic fields, which place an upper limit of 1 S/m for the mantle conductivity. Finally, the compared amplitudes of external and internal time‐varying field lead to an independent (from gravity studies) estimate of the conductive core radius, at 2,060 ± 22 km. These analyses will be further completed with the upcoming BepiColombo mission and its magnetic field experiment, but the presented results already lift the veil on some of the magnetic oddities at Mercury.

International audience; Abstract The Interior exploration using Seismic Investigations, Geodesy, and Heat Trans-port (InSight) Mission will focus on Mars’ interior structure and evolution. The basic structure of crust, mantle, and core form soon after accretion. Understanding the early differentiation process on Mars and how it relates to bulk composition is key to improving our understanding of this process on rocky bodies in our solar system, as well as in other solar systems. Current knowledge of differentiation derives largely from the layers observed via seismology on the Moon. However, the Moon’s much smaller diameter make it a poor analog with respect to interior pressure and phase changes. In this paper we review the current knowledge of the thickness of the crust, the diameter and state of the core, seismic attenuation, heat flow, and interior composition. InSight will conduct the first seismic and heat flow measurements of Mars, as well as more precise geodesy. These data reduce uncertainty in crustal thickness, core size and state, heat flow, seismic activity and meteorite impact rates by a factor of 3–10× relative to previous estimates. Based on modeling of seismic wave propagation, we can further constrain interior temperature, composition, and the location of phase changes. By combining heat flow and a well constrained value of crustal thickness, we can estimate the distribution of heat producing elements between the crust and mantle. All of these quantities are key inputs to models of interior convection and thermal evolution that predict the processes that control subsurface temperature, rates of volcanism, plume distribution and stability, and convective state. Collectively these factors offer strong controls on the overall evolution of the geology and habitability of Mars.

New magnetic figures of Mars

An improved description of magnetic fields of terrestrial bodies has been obtained from recent space missions, leading to a better characterization of the internal fields including those of crustal origin. One of the striking differences in their crustal magnetic field is the signature of large impact craters. A comparative analysis of the magnetic characteristics of these structures can shed light on the history of their respective planetary-scale magnetic dynamos. This has motivated us to identify impact craters and basins, first by their quasi-circular features from the most recent and detailed topographic maps and then from available global magnetic field maps. We have examined the magnetic field observed above 27 complex craters on the Earth, 34 impact basins on Mars and 37 impact basins on the Moon. For the first time, systematic trends in the amplitude and frequency of the magnetic patterns, inside and outside of these structures are observed for all three bodies. The demagnetization effects due to the impact shock wave and excavation processes have been evaluated applying the Equivalent Source Dipole forward modeling approach. The main characteristics of the selected impact craters are shown. The trends in their magnetic signatures are indicated, which are related to the presence or absence of a planetary-scale dynamo at the time of their formation and to impact processes. The low magnetic field intensity at center can be accepted as the prime characteristic of a hypervelocity impact and strongly associated with the mechanics of impact crater formation. In the presence of an active internal field, the process of demagnetization due to the shock impact is associated with post-impact remagnetization processes, generating a more complex magnetic signature.

Hermean magnetic field measurements acquired over the northern hemisphere by the MErcury Surface Space ENvironment GEochemistry, and Ranging (MESSENGER) spacecraft provide crucial information on the magnetic field of the planet. We develop a new method, the Time Dependent Equivalent Source Dipole, to model a planetary magnetic field and its secular variation over a limited spatial region. Tests with synthetic data distributed on regular grids as well as at spacecraft positions show that our modeled magnetic field can be upward or downward continued in an altitude range of −300 to 1460 km for regular grids and in a narrower range of 10 to 970 km for spacecraft positions. They also show that the method is not sensitive to a very weak secular variation along MESSENGER orbits. We then model the magnetic field of Mercury during the first four individual sidereal days as measured by MESSENGER using the modified Equivalent Source Dipoles scheme and excluding the secular variation terms. We find a dominantly zonal field with small-scale nonaxisymmetric features corotating with the Sun in the Mercury Body Fixed system and repeating under similar local time, suggestive of external origin. When modeling the field during one complete solar day, these small-scale features decrease and the field becomes more axisymmetric. The lack of any coherent nonaxisymmetric feature recovered by our method, which was designed to allow for such small-scale structures, provides strong evidence for the large-scale and close-to-axisymmetry structure of the internal magnetic field of Mercury.

A detailed description of the main geomagnetic field and of its temporal variations (i.e., the secular variation or SV) is crucial to understanding the geodynamo. Although the SV is known with high accuracy at ground magnetic observatory locations, the globally uneven distribution of the observatories hampers the determination of a detailed global pattern of the SV. Over the past two decades, satellites have provided global surveys of the geomagnetic field which have been used to derive global spherical harmonic (SH) models through some strict data selection schemes to minimise external field contributions. However, discrepancies remain between ground measurements and field predictions by these models; indeed the global models do not reproduce small spatial scales of the field temporal variations. To overcome this problem we propose to directly extract time series of the field and its temporal variation from satellite measurements as it is done at observatory locations. We follow a Virtual Observatory (VO) approach and define a global mesh of VOs at satellite altitude. For each VO and each given time interval we apply an Equivalent Source Dipole (ESD) technique to reduce all measurements to a unique location. Synthetic data are first used to validate the new VO-ESD approach. Then, we apply our scheme to data from the first two years of the Swarm mission. For the first time, a 2.5° resolution global mesh of VO time series is built. The VO-ESD derived time series are locally compared to ground observations as well as to satellite-based model predictions. Our approach is able to describe detailed temporal variations of the field at local scales. The VO-ESD time series are then used to derive global spherical harmonic models. For a simple SH parametrization the model describes well the secular trend of the magnetic field both at satellite altitude and at the surface. As more data will be made available, longer VO-ESD time series can be derived and consequently used to study sharp temporal variation features, such as geomagnetic jerks.

This paper presents the first regional model of the magnetic field of Mercury developed with mathematical continuous functions. The model has a horizontal spatial resolution of about 830 km at the surface of the planet, and it is derived without any a priori information about the geometry of the internal and external fields or regularization. It relies on an extensive dataset of the MESSENGER’s measurements selected over its entire orbital lifetime between 2011 and 2015. A first order separation between the internal and the external fields over the Northern hemisphere is achieved under the assumption that the magnetic field measurements are acquired in a source free region within the magnetospheric cavity. When downward continued to the core-mantle boundary, the model confirms some of the general structures observed in previous studies such as the dominance of zonal field, the location of the North magnetic pole, and the global absence of significant small scale structures. The transformation of the regional model into a global spherical harmonic one provides an estimate for the axial quadrupole to axial dipole ratio of about g 2 0 / g 1 0 = 0.27 . This is much lower than previous estimates of about 0.40. We note that it is possible to obtain a similar ratio provided that more weight is put on the location of the magnetic equator and less elsewhere.

We present an analysis of the remaining three candidate landing sites for Mars 2020, Columbia Hills (CH), Northeast Syrtis (NES) and Jezero (JE) from the perspective of understanding Mars' crustal magnetic field. We identify how the different sites can address each of six community-defined paleomagnetic science objectives for Mars return samples. These objectives include understanding the early dynamo field and its variability, identification of magnetic minerals that carry magnetization in the samples, and characterization of any thermal and chemical alteration of samples. Satellite data have provided global and regional constraints on crustal magnetization, indicating strong magnetizations at CH and weak to no magnetization at JE and NES. However, the primary paleomagnetic interest—understanding the early dynamo—requires ground truth from a landing site at which pre-Noachian and Early Noachian deposits are accessible. This requirement is most likely met by the site NES, which contains meggabreccia deposits, and it is therefore the highest priority landing site for magnetic field investigations. Importantly, a sample return mission has never been done, and so any of the three landing sites will provide critical, new data that will contribute to understanding the history of Mars' magnetic field and crustal mineralogy and, in turn, yield constraints on the planet's evolution. Keywords: Mars 2020; paleomagnetism; Mars; magnetic field

The fate of water which was present on early Mars remains enigmatic. We propose a simple model based on serpentinization, a hydrothermal alteration process which may produce magnetite and store water. Our model invokes serpentinization during about 500 to 800 Myr, while a dynamo is active, which may have continued after the formation of the crustal dichotomy. We show that the present magnetic field measured by MGS in the Southern hemisphere is consistent with a ~500 m thick Global Equivalent Layer of water trapped in serpentine. Serpentinization results in the release of H 2 . The released H atoms are lost to space through thermal escape, increasing the D/H ratio in water reservoirs exchanging with atmosphere. We show that the value of the D/H ratio in the present atmosphere (~5) is consistent with the serpentinization of a ~500 m thick water GEL. We reassess the role of non-thermal escape in removing water from the planet. By considering an updated solar wind-ionosphere interaction representation, we show that the contribution of oxygen escape to H isotopic fractionation is negligible. Our results suggest that significant amounts of water (up to a ~330-1030 m thick GEL) present at the surface during the Noachian, similar to the quantity inferred from the morphological analysis of valley networks, could be stored today in subsurface serpentine. 1. The study Like Earth, Mars has been endowed with large amounts of water during accretion, equivalent to the content of several terrestrial oceans, corresponding to a several 10 km thick Global Equivalent Layer. The present inventory of observable water on Mars, mainly within the polar caps, is quite smaller, in the range from ~20-30 m. The mega-regolith capacity is large, with up to ~500 GEL m potentially trapped in the cryosphere, and hypothetically several additional hundreds of meters (up to ~500 m) of ground water surviving at depth below the cryosphere [1]. A ~500 m thick GEL is generally assumed to be required to explain the formation of outflow channels [2], and most of this water could be trapped today as water ice, and possibly deep liquid water, in the subsurface, and also possibly under the form of hydrated minerals.

We describe a method to estimate the total electron content (TEC) of the Mars ionosphere from the output parameters of an algorithm, called the Contrast Method (Picardi, G., Sorge, S. [2000]. Proc. SPIE. Eighth International Conference on Ground Penetrating Radar, vol. 4084, pp. 624–629; Ilyushin, Ya.A., Kunitsyn, V.E. [2004]. J. Commun. Technol. Electron. 49, 154–165), which allows to correct the phase distortion of the echoes recorded by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) (Picardi, G. et al. [2005]. Science 310, 1925–1928) in its subsurface mode. Based on the TEC values evaluated during 6 years of MARSIS activity, corresponding to about 4600 orbits, in this paper we present a global map of the night side TEC variations, which correlates well with the magnetic field model derived from Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer (MAG/ER) data. In particular, we demonstrate that regions of enhanced TEC preferentially correspond to areas where crustal magnetic field lines are quasi perpendicular to the martian surface; moreover, we demonstrate that, in regions where the magnetic field is predominantly nearly vertical, enhanced TEC values correlate with higher field intensities, while in regions where the magnetic field is predominantly nearly horizontal, such correlation is not observed. As already suggested in the past by other authors, we suggest that increased TEC values may be related to the precipitation of electrons from the martian magnetospheric tail along vertical crustal magnetic field lines.

International audience; We submit three candidate models following the call for IGRF-11. We apply a simple modeling approach in spherical harmonics based on a quadratic Taylor expansion for the internal field time variations. We use the Dst magnetic index as a proxy for the external field variations. In order to compensate for the limitations incurred by such a conventional approach, we focus on the optimal selection of satellite data in space and time. We also show that some a priori knowledge about the core field state helps us to avoid the pitfall encountered in the case of rapid changes of core field accelerations. Indeed, various acceleration events of relevance for the IGRF 11th occurred between 2003 and 2010, one of them being a geomagnetic jerk. They could entail disagreements between IGRF candidate models for the secular variation (SV) if data prior to 2008 are used. Our SV and main field (MF) candidate models have a root mean square uncertainty less than 6 nT/yr and 8 nT, respectively, with respect to the modeled magnetic field contributions. These values correspond to the intrinsic error associated with truncating SV and MF models to spherical harmonic degree 8 and 13, respectively, as requested for IGRF models.

The lithospheric contribution to the Earth’s magnetic field is concealed in magnetic field data that have now been measured over several decades from ground to satellite altitudes. The lithospheric field results from the superposition of induced and remanent magnetisations. It therefore brings an essential constraint on the magnetic properties of rocks of the Earth’s sub-surface that would otherwise be difficult to characterize. Measuring, extracting, interpreting and even defining the magnetic field of the Earth’s lithosphere is however challenging. In this paper, we review the difficulties encountered. We briefly summarize the various contributions to the Earth’s magnetic field that hamper the correct identification of the lithospheric component. Such difficulties could be partially alleviated with the joint analysis of multi-level magnetic field observations, even though one cannot avoid making compromises in building models and maps of the magnetic field of the Earth’s lithosphere at various altitudes. Keeping in mind these compromises is crucial when lithospheric field models are interpreted and correlated with other geophysical information. We illustrate this discussion with recent advances and results that were exploited to infer statistical properties of the Earth’s lithosphere. The lessons learned in measuring and processing Earth’s magnetic field data may prove fruitful in planetary exploration, where magnetism is one of the few remotely accessible internal properties.

The nature of strong martian crustal field sources is investigated by mapping and modeling of Mars Global Surveyor magnetometer data near Apollinaris Patera, a previously proposed volcanic source, supplemented by large-scale correlative studies. Regional mapping yields evidence for positive correlations of orbital anomalies with both Apollinaris Patera and Lucus Planum, a nearby probable extrusive pyroclastic flow deposit that is mapped as part of the Medusae Fossae Formation. Iterative forward modeling of the Apollinaris Patera magnetic anomaly assuming a source model consisting of one or more uniformly magnetized near-surface disks indicates that the source is centered approximately on the construct with a scale size several times larger and comparable to that of the Apollinaris Patera free-air gravity anomaly. A significantly lower rms deviation is obtained using a two-disk model that favors a concentration of magnetization near the construct itself. Estimates for the dipole moment per unit area of the Lucus Planum source together with maximum thicknesses of ∼3 km based on topographic and radar sounding data lead to an estimated minimum magnetization intensity of ∼50 A/m within the pyroclastic deposits. Intensities of this magnitude are similar to those obtained experimentally for Fe-rich Mars analog basalts that cooled in an oxidizing (high fO2) environment in the presence of a strong (⩾10 μT) surface field. Further evidence for the need for an oxidizing environment is provided by a broad spatial correlation of the locations of phyllosilicate exposures identified to date using Mars Express OMEGA data with areas containing strong crustal magnetic fields and valley networks in the Noachian-aged southern highlands. This indicates that the presence of liquid water, which is a major crustal oxidant, was an important factor in the formation of strong magnetic sources. The evidence discussed here for magnetic sources associated with relatively young volcanic units suggests that a martian dynamo existed during the late Noachian/early Hesperian, after the last major basin-forming impacts and the formation of the northern lowlands.

We present a method—named as MILMA for modelling and inversion of local magnetic anomalies—that combines forward and inverse modelling of aeromagnetic data to characterize both magnetization properties and location of unconstrained local sources. Parameters of simple-shape magnetized bodies (cylinder, prism or sphere) are first adjusted by trial and error to predict the signal. Their parameters provide a priori information for inversion of the measurements. Here, a generalized nonlinear approach with a least-squares criterion is adopted to seek the best parameters of the sphere (dipole). This inversion step allows the model to be more objectively adjusted to fit the magnetic signal. The validity of the MILMA method is demonstrated through synthetic and real cases using aeromagnetic measurements. Tests with synthetic data reveal accurate results in terms of depth source, whatever be the number of sources. The MILMA method is then used with real measurements to constrain the properties of the magnetized units of the Champtoceaux complex (France). The resulting parameters correlate with the crustal structure and properties revealed by other geological and geophysical surveys in the same area. The MILMA method can therefore be used to investigate the properties of poorly constrained lithospheric magnetized sources.

International audience; CH4 has been observed on Mars both by remote sensing and in situ during the past 15 yr. It could have been produced by early Mars serpentinization processes that could also explain the observed Martian remanent magnetic field. Assuming a cold early Mars, a cryosphere could trap such CH4 as clathrates in stable form at depth. The maximum storage capacity of such a clathrate cryosphere has been recently estimated to be 2 x 10(19) to 2 x 10(20) moles of methane. We estimate how large amounts of serpentinization-derived CH4 stored in the cryosphere have been released into the atmosphere during the Noachian and the early Hesperian. Due to rapid clathrate dissociation and photochemical conversion of CH4 to H-2, these episodes of massive CH4 release may have resulted in transient H-2-rich atmospheres, at typical levels of 10-20% in a background 1-2 bar CO2 atmosphere. The collision-induced heating effect of H-2 present in such an atmosphere has been shown to raise the surface temperature above the water freezing point. We show how local and rapid destabilization of the cryosphere can be induced by large events (such as the Hellas Basin or Tharsis bulge formation) and lead to such releases. Our results show that the early Mars cryosphere had a sufficient CH4 storage capacity to have maintained H-2-rich transient atmospheres during a total time period up to several million years or tens of million years, having potentially contributed to the formation of valley networks during the Noachian/early Hesperian.

Mars is thought to have hosted large amounts of water and carbon dioxide at primitive epochs. The morphological analysis of the surface of Mars shows that large bodies of water were probably present in the North hemisphere at late Noachian (3.7–4 Gyr ago). Was this water solid or liquid? For maintaining liquid water at this time, when the Sun was (likely) less bright than now, a CO2 atmosphere of typically 2 bars is required. Can sputtering, still presently acting at the top of the Martian atmosphere, have removed such a dense atmosphere over the last 3.5–4 Gyr? What was the fate of the 100–200 m global equivalent layer of water present at late Noachian? When did Martian magnetic dynamo vanish, initiating a long period of intense escape by sputtering? Because sputtering efficiency is highly non-linear with solar EUV flux, with a logarithmic slope of ≈7:Φsput≈ΦEUV7, resulting in enhanced levels of escape at primitive epochs, when the sun was several times more luminous than now in the EUV, there is a large uncertainty on the cumulated amount of volatiles removed to space. This amount depends primarily on two factors: (i) the exact value of the non-linearity exponent (≈7 from existing models, but this value is rather uncertain), (ii) the exact time when the dynamo collapsed, activating sputtering at epochs when intense EUV flux and solar wind activity prevailed in the solar system. Both parameters are only crudely known at the present time, due the lack of direct observation of sputtering from Martian orbit, and to the incomplete and insufficiently spatially resolved map of the crustal magnetic field. Precise timing of the past Martian dynamo can be investigated through the demagnetisation signature associated with impact craters. A designated mission to Mars would help in answering this crucial question: was water liquid at the surface of Mars at late Noachian? Such a mission would consist of a low periapsis (≈100 km) orbiter, equipped with a boom-mounted magnetometer, for mapping the magnetic field, as well as adequate in situ mass and energy spectrometers, for a full characterization of escape and of its response to solar activity variations. Surface based observations of atmospheric noble gas isotopic ratios, which keep the signatures of past escape processes, including sputtering for the lightest of them (Ne, Ar), would bring a key constraint for escape models extrapolated back to the past.

A Martian paleomagnetic pole is calculated from a magnetic anomaly associated with the late Noachian age (and older) volcano Apollinaris Patera. This isolated volcano, located near the crustal dichotomy boundary at the Martian equator, has a correlative gravity anomaly, and was likely active for more than 10 7 years. It is one of the only volcanoes on Mars known to have a substantial magnetic anomaly associated with it, and one of the only examples of correlative magnetic and gravity sources. Magnetic directions calculated using either low- or high-altitude data, and single or multiple equivalent source dipoles, are nearly horizontal and southward directed. Assuming a single dipolar source magnetization, the preferred paleopole is at 65 ∘ S , 59 ∘ E . Assuming a larger magnetized area leads to a cluster of paleopoles near 88 ∘ S , 99 ∘ E . This paleopole is very close to the current rotation pole, and very different from previously calculated paleopoles. Our preferred interpretation is that the Apollinaris Patera magnetization was acquired near the end of the life of the Martian dynamo, and that subsequent polar wander was minimal.

International audience; The 12th generation of the International Geomagnetic Reference Field (IGRF) was adopted in December 2014 by the Working Group V-MOD appointed by the International Association of Geomagnetism and Aeronomy (IAGA). It updates the previous IGRF generation with a definitive main field model for epoch 2010.0, a main field model for epoch 2015.0, and a linear annual predictive secular variation model for 2015.0-2020.0. Here, we present the equations defining the IGRF model, provide the spherical harmonic coefficients, and provide maps of the magnetic declination, inclination, and total intensity for epoch 2015.0 and their predicted rates of change for 2015.0-2020.0. We also update the magnetic pole positions and discuss briefly the latest changes and possible future trends of the Earth's magnetic field.

International audience; The martian surface exhibits a strong dichotomy in elevation, crustal thickness and magnetization between the southern and northern hemispheres. A giant impact has been proposed as an explanation for the formation of the Northern Lowlands on Mars. Such an impact probably led to strong and deep mantle heating which may have had implications on the magnetic evolution of the planet. We model the effects of such an impact on the martian magnetic field by imposing an impact induced thermal heterogeneity, and the subsequent heat flux heterogeneity, on the martian core-mantle boundary (CMB). The CMB heat flux lateral variations as well as the reduction in the mean CMB heat flux are determined by the size and geographic location of the impactor. A polar impactor leads to a north–south hemispheric magnetic dichotomy that is stronger than an east–west dichotomy created by an equatorial impactor. The amplitude of the hemispheric magnetic dichotomy is mostly controlled by the horizontal Rayleigh number RahRah which represents the vigor of the convection driven by the lateral variations of the CMB heat flux. We show that, for a given RahRah, an impact induced CMB heat flux heterogeneity is more efficient than a synthetic degree-1 CMB heat flux heterogeneity in generating strong hemispheric magnetic dichotomies. Large RahRah values are needed to get a dichotomy as strong as the observed one, favoring a reversing paleo-dynamo for Mars. Our results imply that an impactor radius of ∼1000 km could have recorded the magnetic dichotomy observed in the martian crustal field only if very rapid post-impact magma cooling took place.

Background The 12th revision of the International Geomagnetic Reference Field (IGRF) was issued in December 2014 by the International Association of Geomagnetism and Aeronomy (IAGA) Division V Working Group V-MOD (http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html). This revision comprises new spherical harmonic main field models for epochs 2010.0 (DGRF-2010) and 2015.0 (IGRF-2015) and predictive linear secular variation for the interval 2015.0-2020.0 (SV-2010-2015). Findings The models were derived from weighted averages of candidate models submitted by ten international teams. Teams were led by the British Geological Survey (UK), DTU Space (Denmark), ISTerre (France), IZMIRAN (Russia), NOAA/NGDC (USA), GFZ Potsdam (Germany), NASA/GSFC (USA), IPGP (France), LPG Nantes (France), and ETH Zurich (Switzerland). Each candidate model was carefully evaluated and compared to all other models and a mean model using well-defined statistical criteria in the spectral domain and maps in the physical space. These analyses were made to pinpoint both troublesome coefficients and the geographical regions where the candidate models most significantly differ. Some models showed clear deviation from other candidate models. However, a majority of the task force members appointed by IAGA thought that the differences were not sufficient to exclude models that were well documented and based on different techniques. Conclusions The task force thus voted for and applied an iterative robust estimation scheme in space. In this paper, we report on the evaluations of the candidate models and provide details of the algorithm that was used to derive the IGRF-12 product., Earth, Planets and Space, 67 (1), ISSN:1343-8832, ISSN:1880-5981

We propose two new analytical expressions to fit the Mauersberger–Lowes geomagnetic field spectrum at the core–mantle boundary. These can be used to estimate the radius of the outer liquid core where the geodynamo operates, or more generally the radius of the planetary dynamo regions. We show that two sub-families of the geomagnetic field are independent of spherical harmonics degree n at the core–mantle boundary and exhibit flat spectra. The first is the non-zonal field, i.e., for spherical harmonics order m different from zero. The second is the quadrupole family, i.e., n + m even. The flatness of their spectra is motivated by the nearly axisymmetric time-average paleomagnetic field (for the non-zonal field) and the dominance of rotational effects in core dynamics (for the quadrupole family). We test our two expressions with two approaches using the reference case of the Earth. First we estimate at the seismic core radius the agreement between the actual spectrum and the theoretical one. Second we estimate the magnetic core radius, where the spectrum flattens. We show that both sub-families offer a better agreement with the actual spectrum compared with previously proposed analytical expressions, and predict a magnetic core radius within less than 10 km of the Earth's seismic core radius. These new expressions supersede previous ones to infer the core radius from geomagnetic field information because the low degree terms are not ignored. Our formalism is then applied to infer the radius of the dynamo regions on Jupiter, Saturn, Uranus and Neptune. The axisymmetric nature of the magnetic field of Saturn prevents the use of the non-zonal expression. For the three other planets both expressions converge and offer independent constraints on the internal structure of these planets. These non-zonal and quadrupole family expressions may be implemented to extrapolate the geomagnetic field spectrum beyond observable degrees, or to further regularize magnetic field models constructed from modern or historical observations.