GP43A-0883
Magnetic Effects of Explosive Driven Shocks on Rocks With Various Magnetic Mineralogy
The effects of shock waves on the natural remanent magnetization (NRM) and the intrinsic magnetic properties of solar system materials remains poorly known. Still, hypervelocity impacts are phenomena are of primary importance in the evolution of many extraterrestrial bodies. Hence, the interpretation of the paleomagnetic signal of most meteorites is blurred by the effects of impacts. Similarly, the remagnetization patterns associated to impact basins on Mars, the Moon or asteroids cannot be interpreted with certainty. We present new experiments in which four different terrestrial rocks (with different magnetic mineralogy) were impacted using detonators containing 0.8 g of the high-order explosive penthrite. Maximum pressure is about 10 GPa at the detonator contact (O 6 mm). The shock wave was modeled numerically and we studied the effects on the preexisting NRM as well as on the intrinsic magnetic properties of the shocked rocks. We show that in each case, the intrinsic magnetic properties of the rock are permanently modified by the shock wave. Magnetite-, titanomagnetite- and pyrrhotite-bearing rocks show a noticeable increase of their coercivity, whereas a hematite-bearing rock displays a lower coercivity after impact. These changes are not annealed even at high temperature (580 °C). Though most of the original NRM is not affected by the shock wave, we observe, in the low-coercivity component of the (titano)magnetite bearing rocks a shock demagnetization and a remagnetization. The direction of this remagnetization is poorly defined but does not seem closely related to the ambient field at the time of impact.
GP43A-0884
Magnetic coercivity and intensity of Mars crust: Dichotomy formation via unicellular convection mechanism
Intense magnetic anomalies over the Mars surface indicate that during the early history of Mars large portions of Mars crust formed in the presence of global magnetic field. Distribution of the magnetic anomalies can be divided into three zones. Zone 1 is where the magnetic signature is negligible or of low intensity. Zone 2 is the region of intermediate crustal anomalies and Zone 3 is where there are magnetic anomalies of extreme magnetic intensity. Crater demagnetization behavior suggests that at least part of the Zone 3 located near the South Pole is generated by rocks with large magnetic coercivity. TRM analyses with terrestrial rocks suggest that compositional banding significantly enhances the TRM intensity. Both magnetic coercivity and intensity values near the South pole suggest presence of a deformation and compositional zoning of the rock in the Mars crust. This can be most simply explained by contractional deformation due to density contrasts above the down welling zone at the South Pole and formation of new crust above the upwelling zone at the North pole.
GP43A-0885
Modeling Terrestrial And Martian Regional Magnetic Anomalies
Characterizing crustal magnetic sources provides key information to better understand the evolution of core and crust. The present study investigates Mars' and Earth's crustal magnetic anomalies. Forward and inverse methods are used. On Earth, both low- and high-altitude measurements are available, whereas only high-altitude data are on Mars. The method is first applied to aeromagnetic measurements acquired over the Armorican Massif (Brittany, France). Comparison with other geophysical studies and geological information provides constraints on the geometry of the magnetic anomaly and the magnetic carriers. This is a good test to compare the signal obtained at low- and high-altitude. It allows us to discuss the validity of our study of martian magnetic anomalies above the Terra Sirenum area, where only satellite measurements are available. Implications for the processes responsible for the strong martian crustal remanent magnetization are also presented.
GP43A-0886
Magnetic Paleofield of Avanhandava H4 Chondrite's Matrix and Chondrules - Implications on Magnetic Fields in Early Solar System.
The Avanhandava (H4) fall occurred in 1952 in Brazil. A total of 9.33 kg had been preserved after the meteorite brake up during the impact [1]. The meteorite contains large (0.1 - 2.0 mm) chon-drules that have clearly delineated boundaries with matrix. This characteristic allows us to pick up oriented individual chondrules and study their magnetic properties. The chondrules of the Avanhandava meteorite show a low and randomly oriented NRM (10-2 - 10-1 mAm2/kg). In contrast the matrix is strongly (100 - 101 mAm2/kg) and uniformly magnet-ized [2]. Various methods for paleofield determination have been applied on matrix and individual chondrules in order to determine possi-ble magnetizing processes and paleofields in early solar systems.. The laboratory experiments reveal approximate paleofields for matrix similar to present geomagnetic field. The paleofield de-termined for chondrules is approximately one order of magnitude lower comparing to values obtained for matrix. That suggests that chondrules are not magnetically contaminated by geomagnetic or artificial fields and they acquired their NRM prior their aggregation to Avanhandava parent body (random NRM directions). The matrix shows remarkable traces of terres-trial weathering and is uniformly magnetized. The paleofield re-sult for matrix indicates possible remagnetization caused by ter-restrial weathering. The terrestrial weathering of ordinary chon-drites is observed even on falls stored in museums and can sig-Nificantly influence meteorite magnetic records [3, 4]. References: [1] Paar W. et al. 1976. Revista Brasileira de Geo-ciencias 6: 201-210. [2] Kohout T. and Pesonen L. J. 2005. 68th Annual Meteoritical Society Meeting: 5202. [3] Kohout T. et al. 2004. Physics and Chemistry of the Earth 29: 885-897. [4] Lee M. R. and Bland P. A. 2004. Geochimica et Cosmochimica Acta 68: 893-916.
GP43A-0887
Magnetic Field Measurements As A Tool For Planetary Exploration
In the absence of surface observations, magnetic measurements on-board orbiting satellites provide a unique tool for investigating planetary properties, such as interaction with the solar wind, internal structure, or nature of the magnetic sources. Modelling and interpreting the magnetic fields and and their sources are essential to determine and understand the dynamical properties of planets, as illustrated by the example of the Earth. The core and lithospheric sources of the geomagnetic field can be quite easily separated, considering the knee of the magnetic spectra around degree 13. Assuming that the magnetic sources lie below the core-mantle boundary, a rough estimate of the radius of the outer, liquid core can be computed. Using IGRF-10 model, we find a core radius within 1% of the commonly adopted seismological value. This method is applied to Ganymede and to Mercury. Ganymede's magnetic environment was explored by the Galileo spacecraft. The Jovian satellite was found to possess an internal magnetic field, which origin is still controversial. The origin of the Hermean magnetic field is still not fully confirmed. The Messenger (launch: 2004) and the BepiColombo (launch: 2012) probe measurements are thus eagerly awaited for. The first measurements by these satellites will undoubtedly reveal the nature of the magnetic field. If the internal origin is confirmed, direct conclusion will be the presence of a liquid, conductive, convecting layer inside Mercury. Additional measurements will allow the structure and the temporal variations of the Hermean magnetic field to be modelled. In practice, the measured field by the spacecraft is the sum of the planetary field (of internal and external sources) and on-board generated magnetic fields. Here, we first synthesize different on-board generated magnetic fields as a function of the distance to the satellite body. We then predict what would be the Hermean magnetic field, assuming a fixed value for the liquid core radius. We then add the planetary and satellite contributions, considering different lengths for the magnetometer boom. We finally compute magnetic models, and compare the output to the initial hypothesis.
GP43A-0888
A Saturian Dynamo Simulation
A 3D global computer simulation of thermal convection and magnetic field generation in Saturn is presented. Convection takes the form of many isolated small-scale vortices with their axes roughly aligned with the planetary rotation axis. Vorticity is generated and differential rotation is maintained by fluid flowing through the density stratification. The differential rotation manifests itself at the surface as a fairly time independent banded zonal wind profile, similar to Saturn's. This generates multiple, migrating, toroidal magnetic fields directed eastward and westward in both hemispheres. The field above the surface is dominantly an axial dipole.
GP43A-0889
Sulfur's impact on core evolution and magnetic field generation on Ganymede
Analysis of the melting relationships of potential core forming materials in Ganymede indicate that convective motions capable of generating the satellite's magnetic field may be driven, in-part, either by iron "snow" forming below the core-mantle boundary or solid iron sulfide floating upward from the deep core. Eutectic melting temperatures in the binary Fe-FeS system decrease with increasing pressure over the interval of core pressures on Ganymede (<14 GPa). Comparison of melting temperatures to adiabatic temperature gradients in the core suggest that solid iron is thermodynamically stable at shallow levels for bulk core compositions more iron-rich than eutectic (i.e., <21 wt % S). Calculations based on high-pressure solid-liquid phase relationships in the Fe-FeS system indicate that Fe snow or floatation of solid FeS, depending on whether the core composition is more or less Fe-rich than eutectic, is an inevitable consequence of cooling Ganymede's core. Our results demonstrate that these conclusions are robust over a wide-range of plausible three-layer internal structures and thermal evolution scenarios. Using scaling arguments based on recent experimental work we estimate core Rossby and magnetic Reynolds numbers plausibly consistent with a dynamo being generated in Ganymede's core via Fe-snow. Depending on core composition, either shallow formation of Fe snow or deep precipitation and subsequent floatation of FeS is an important mechanism for driving the moon's strong internally-generated magnetic field.
GP43A-0890
Dipole Moment Scaling for Convection-Driven Planetary Dynamos
Dipole moments from numerical dynamo models driven by various combinations of thermal and chemical convection are used to derive scaling relationships applicable to dipolar planetary magnetic fields. We compare published values of time-averaged dipole moments from dynamo models covering wide ranges of Ekman, Rayleigh, and Prandtl numbers. Three distinct moment regimes are found. In the first regime, just beyond the onset of dynamo action, the dipole moment increases linearly with Rayleigh number and the non-dipole field is typically weak. Another regime at very large Rayleigh numbers is characterized by small, highly time-variable dipole moments and strong non-dipole fields. Between these is an intermediate Rayleigh number regime in which the dipole moment tends toward a constant value that depends primarily on the core radius, rotation rate, density, and electrical conductivity, and secondarily on the Prandtl numbers of the fluid. Dynamos in this regime with homogeneous boundary conditions have the dipole moments that are relatively insensitive to the vigor of convection, the style of convection (thermal or chemical), and the concentration of internal heat sources. Planetary dynamos in this regime are expected to evolve with relatively minor changes in time-average dipole moment. The geodynamo may have been in this regime over much of Earth's history. Heterogeneous boundary conditions tend to reduce the dipole moment and large amounts of boundary heterogeneity can extinguish dynamo action entirely. The early demise of the Martian dynamo may have occurred in this regime.
GP43A-0891
Scaling properties of planetary dynamos
We have performed a parameter space survey for numerical MHD dynamos in rotating spherical shells, varying all relevant control parameters.Simple scaling laws have been derived for the typical amplitude of the convection flow, of the zonal flow and of the magnetic field. The key parameter in these scaling relationships is the buoyancy flux, which measures the quantity of energy made available to the dynamo, irrespectively of the nature (chemical or thermal) of convection. The scaling for the magnetic field is strikingly independant on rotation rate of the planet. This fact is discussed in connection with the role of the Coriolis force in the dynamo system and dimensional analysis. The scalings are then applied to evaluate the buoyancy flux in the Earth's core. The result is compatible with an aged (more than three billion years old) inner core. On Jupiter the surface magnetic field, surface zonal flow and excess heat flow are also found to be compatible with the scalings. Further modelling is presented to treat the more complex cases of Uranus and Neptune.
GP43A-0892
Non-Uniqueness of the Modeled Magnetization Vectors Used in Determining Paleopoles on Mars
This study investigates the non-Uniqueness of current magnetization models derived from magnetic anomalies on Mars observed by the Mars Global Surveyor spacecraft and modeling of magnetic anomalies on Earth. In place of elliptical and circular prismatic models of Arkani-Hamed (2001, GRL, v.28, no. 17, p. 3409-3412) and Hood and Zakharian (2001, JGR, v. 106, no. E7, p. 14601-14619), alternative, smaller, multiple source configurations developed in this study explain the anomalies designated as M10 and M3 on Mars equally well and suggest that coalescence of anomalies at high altitude might be an important effect ignored in the previous studies. The coalescence effect might have affected magnetization directions derived from other isolated anomalies in other studies. The scatter in paleopole locations computed from the alternative configurations of sources M3 and M10 covers nearly 40% of the surface of Mars and, hence, the utility of these paleopole locations in deciphering the ancient tectonics is questioned. On Earth, a 2D model of the near-surface anomaly over northeast America and neighboring Atlantic ocean near Newfoundland and the 3D models of the satellite altitude anomaly over the same region demonstrated that magnetic models based on the Amplitude of Analytic Signal (AAS) field allowed more realistic complexity similar to the near-surface magnetic anomaly patterns, which closely reflect the geology, whereas, the model deciphered from the Z-component magnetic field alone was unrealistic. This suggests on Mars the magnetic model based on the AAS field is likely to yield better source characteristics than modeling of Z-component magnetic field.
GP43A-0893
Single-Domain/Pseudo-Single-Domain Magnetic Particles in the Lower Crust of Mars: Source of the Strong Magnetic Anomalies
It is shown in this paper that single-domain or pseudo-single-domain magnetite grains can exist in the upper 30-45 km of the Martian crust. The upper ~50 km of the Martian crust has likely formed by volcanic eruptions on a stagnant lithosphere. A given lava flow cools very rapidly at the surface, but then is gradually heated up through burial heating as it is overlain by other successive flows. The thermal evolution models of a lava flow show that its temperature remains below the magnetic blocking temperatures of magnetite, 480-580C, until it reaches ~30-45 km depth. The highly magnetic single-domain or pseudo-single-domain magnetite grains can easily explain the strong magnetic anomalies of Mars, less than one percent by volume of magnetite particles is sufficient.