Magnetic Properties of Avanhandava H4 Meteorite Chondrules
The magnetic properties of twenty individual chondrules from Avanhandava chondritic (H4) meteorite fall were studied. Magnetic hysteresis measurements and isothermal remanent magnetization (IRM) acquisition experiments reveal two populations of chondrules with different magnetic properties within the meteorite. The first group shows magnetically hard behavior with coercivities (Hc) ranging between 20 and 60 mT and IRM acquisition curves displaying two distinct coercivity components. The low coercivity component is acquired at fields < 250 mT, while the high coercivity component is acquired mostly between 400 and 1200 mT. This high coercivity component is also apparent in first order reversal curve (FORC) diagrams. The median destructive field (MDF) for samples' saturation isothermal remanent magnetization (SIRM) was ∼ 160 mT. The second group shows magnetically soft behavior, with Hc values below 5 mT and IRM acquisition curves showing only a single low coercivity component acquired at fields < 250 mT. Additionally, there are no high coercivity components observed in the FORC diagrams of these chondrules, and MDFs are generally lower than those of the first group. The natural remanent magnetization (NRM) of the chondrules is weak (∼ 10-2 - 10-1 mAm2/kg). Surprisingly, there is no correlation between the stability of the NRM and the presence of the high coercivity component. The NRM directions of individual chondrules define a random distribution. Alternating field demagnetization reveals one or two stable components. Some chondrules display erratic variations in intensity during demagnetization, which can be caused by the presence of multi-domain grains. While the low coercivity component is likely related to the presence of multi-domain kamacite, the mineralogy of the high coercivity fraction is uncertain. Experiments are underway to identify the magnetic carriers of the two coercivity fractions. This information will be essential for isolating the NRM component held by the high coercivity carriers, which may be related to processes such as original chondrule formation, subsequent coalescence into a parent meteorite body, and/or shock events.
Correlations of Strong Martian Crustal Magnetic Fields With Valley Networks and Phyllosilicate Exposures: Implications for Magnetic Sources
A broad spatial correlation between strong martian crustal magnetic fields and the valley networks, which are indicative of surface water erosion, has previously been reported. In this paper, we report initial evidence for a correlation of phyllosilicate exposures identified to date using Mars Express OMEGA data with strong crustal fields and valley networks in the Noachian southern highlands. Two separate statistical methods confirm the significance of the phyllosilicate exposure correlation. Like the valley networks and crustal fields, the phyllosilicate exposures are distributed north and east of Hellas in the southern highlands but are nearly absent within, south, and west of this basin. Similarly, they are present north and west of Argyre but are sparse within, south, and east of this basin. All three tend to occur mainly at low paleolatitudes as inferred from magnetic anomaly modeling by several groups. We interpret the correlation to imply that the strongest crustal magnetization formed primarily while liquid water was present in the martian upper crust. The source of the water could have been from above (precipitation) or from below (mantle outgassing). A likely explanation is that the production in magnetic source regions of efficient remanence carriers was enhanced in the presence of magmatic heat through hydrothermal chemical processes. For example, magnetite production could have been enhanced through serpentinization. Geologic evidence suggests that magmatic intrusions occurred commonly in the upper highland crust during the Noachian. If these intrusions occurred in the form of dikes and/or dendritic conduits over regions as large as 200 - 600 km during time periods less than the dynamo reversal time scale, then magnetic source regions with dimensions comparable to those inferred from orbital data could be accounted for.
Using Lunar Prospector Magnetometer/Electron Reflectometer Data to Investigate Lunar Impact Demagnetization
Measurements of lunar crustal magnetization hold great promise as a tool that we can use to understand the geophysical processes that have shaped the lunar surface and interior. The current lunar magnetic field distribution suggests a Moon dominated by impact processes, with strong magnetic fields antipodal to young large impact basins, and demagnetized regions in impact sites. The distribution of crustal magnetization around the lows may tell us as much about the evolution of the Moon as that of the highs. By comparing Lunar Prospector Magnetometer (MAG) and Electron Reflectometer (ER) measurements, we can begin to unfold the properties of lunar magnetization in and around the demagnetized basins. We now present simulations of magnetic field distributions produced by impact demagnetization, with the aim of placing constraints on the properties of the initial magnetization distribution. We simulate the response of both the MAG and ER technique to the specified magnetization distributions. Both measurements have strengths and weaknesses, with the MAG providing a measurement of magnetic field at altitude, and the ER providing a remote measurement of the crustal field magnitude at the surface. MAG measurements are more straightforward to interpret, but more limited in sensitivity due to the rapid falloff of signal with altitude. ER measurements are harder to interpret, but can provide us with more information about weak and incoherent crustal magnetization near the surface. By comparing the two data sets with each other and with models, we can probe the properties of lunar crustal magnetization. We find that lunar magnetization is likely more incoherent, with less spatial correlation, than the terrestrial analogue, suggesting that local processes (for instance, impacts) have dominated the creation and evolution of lunar crustal magnetism.
Laboratory Observations of Elliptical Rotational Parametric Instability Confirm a Dynamo Mechanism
Periodic forcing of planetary core fluid, through straining of otherwise circular streamlines, can give rise to large scale flows on millennial time scales. Tidal perturbations of the rotating contained fluid core couple pairs of normal modes resulting in repetitive sequential growth, collapse and decay, provided viscous and ohmic dissipations are weak. This phenomenon as been studied in a laboratory setting through a traveling elliptical perturbation of a flexible spheroidal cavity, and a spheroidal shell of fluid. Slow growth and decay, as predicted by linear stability theory, are measured in our experiments, confirming the existence of parametric fluid instability in both geometries. Our results are shown to be consistent with Earth's fluctuating paleomagnetic field. Furthermore, parametric instabilities can drive a dynamo throughout Earth's history: with and without the presence on an inner core, and thus escape the problem of the geodynamo predating a compositional convective process. Finally, elliptical instability may also provide a means to explain the ancient Martian dynamo through periodic tidal strain from a co-orbiting body.
3-D Aeromagnetic Anomaly Modelling of Chicxulub Multiring Crater - Central Structure and Magnetic Sources
Analytic signal, inversion and forward models in three-dimensions are applied to magnetic field over the Chicxulub crater to investigate magnetic anomaly sources and crater structure in the central zone. Aeromagnetic data show three strong, well-defined concentric patterns, with a central 40-km diameter zone of high amplitude anomalies. Magnetic anomalies are interpreted to be associated with the melt sheet, upper breccias and central uplift, which present 3-4 orders of magnitude contrasts with the surrounding carbonate units. Limited rock magnetic measurements, apparent wide range in the remanent intensity and susceptibility in melt, suevitic breccias and basement and hydrothermal alteration effects limit our ability to determine the characteristics and distribution of major structural elements of the Chicxulub crater. Amplitude of the analytic signal produces maxima over magnetization contrasts, independent of the direction of magnetization. Interpretation of maxima location and depth distribution is used, in a second stage, as a priori information in the construction of an input prism assemblage magnetic configuration and its properties for three-dimensional forward modeling. Results show that sources extend to a radial distance of ~45 km from the center of the structure with average depths ranging between 2 and 4 km. Magnetic anomaly sources in the central uplift zone are located in the range from 3.5 to 8 km depth, with dominant contributions from the large body forming the structural uplift.