GP33B-01 INVITED
Shock Magnetization and Demagnetization of Rocks: What we Have Learnt From Experimental Studies
We will present new results of simultaneous shock magnetization and shock demagnetization experiments performed on titanomagnetite-bearing basalt samples with a pulsed laser in controlled magnetic field. These new results provide the opportunity to discuss the main properties of the these two phenomena. What is the efficiency of the acquisition of shock remanent magnetization (SRM) acquisition with respect to thermoremanent magnetization? Is shock demagnetization equivalent to shock magnetization in zero field? Do we observe scattered SRM direction in shocked samples? Can we predict the shock demagnetization/remagnetization behavior of a rock knowing its rock magnetic properties? Eventually we will discuss the implications of these results for the understanding of the paleomagnetic signal of shocked rocks (meteorites in paticular) and of the magnetic anomalies above impact basins.
GP33B-02 INVITED
Unraveling the Shock History of Magnetic Materials
Renewed interest in the effects of impact-generated shocks on magnetic materials has been fueled by satellite observations of magnetic crustal remanence on Mars, new dynamic and static pressure experiments on rocks and pure minerals, and paleomagnetic studies of terrestrial craters. Attempts have been made to infer the magnetic carriers in planets and asteroids based on the experimental response of minerals to dynamic and static pressure. For example, unmagnetized crust surrounding impact basins on Mars has been attributed to low pressure (a few GPa) shock demagnetization. Dynamic and static experiments at low pressures (<1GPa) show that remanence decreases after the application of stress. The reduction of magnetic moment is accompanied by permanent changes in the magnetic properties. Recent experiments have expanded the pressure range of both static and dynamic experiments (up to 10-20 GPa), as well as the compositions of minerals investigated. Our compilation of the total body of data reveals universal trends, which include a decrease in magnetic remanence and an increase in bulk coercivity with increasing pressure. However, at low pressures (< a few GPa), the demagnetization trends are not unique for different minerals and compositions. Furthermore, at moderate shock pressures (~10 GPa), unusual features occur in specific phases, e.g. the remanence of pyrrhotite may increase as a result of permanent changes in its magnetic properties. Residual remanence is an unreliable measure of peak shock pressure; whereas, the changes in bulk magnetic properties are better correlated with pressure and a more robust "shock barometer." Unfortunately, the complex behavior of magnetic minerals to shock prevents the straightforward interpretation of magnetic carriers on Mars based on remote measurements of crustal remanence. At present, the nature of the intensely magnetized crust on Mars remains enigmatic.
GP33B-03
Magnetic Field Anomalies Above Large Martian Impact Structures
The Mars Global Surveyor NASA mission revealed the complex nature of the lithospheric magnetic field of Mars. Intense anomalies are located above the southern cratered highlands, while the giant impact basins (Hellas, Argyre, Utopia) and the northern smoothed lowlands do not show significant anomalies. Here we study the magnetic signal above large impact craters, with diameters ranging between 100 and 2000 km. Magnetic measurements are carefully screened and selected to avoid non static features. Then the mean magnetic field is evaluated both inside each crater rim and in its immediate vicinity, within one crater radius. The ratio of these two quantities helps to determine which craters modified the magnetic properties of the pre-impact lithosphere. In addition, this technique allows the impacts located in the strongly magnetized Terra Sirenum and Terra Cimmeria to be studied. Results of this study, as well as comparison of the magnetic measurements to predicted ones for different pre-impact magnetization directions will be presented.
GP33B-04 INVITED
Magnetic field, shock, and crustal magnetization effects of lunar basin-forming impacts
Orbital mapping has shown that lunar crustal magnetic fields are weak within young large impact basins (especially Imbrium and Orientale) but are unusually strong near the antipodes of the same basins. The weak fields near the basins have been attributed to shock demagnetization of relatively soft crustal magnetization (Helekas et al., MAPS, 2003). The strong antipodal anomalies have been attributed mainly to shock remanent magnetization (SRM) associated with the impact of ejecta in a magnetic field amplified by the converging impact vapor-melt cloud (Hood & Artemieva, Icarus, 2008). Both of these interpretations allow but do not require the existence of a former lunar core dynamo at the times of the impacts. Initial 3D simulations indicate that the time of maximum amplification of an ambient magnetic field (early solar wind or core dynamo) is ~ 1 hour after the impact, coinciding roughly with the period of ejecta convergence and impact near the antipode. During this period, shock stresses are produced within the range of 5-25 GPa where stable SRM of lunar soils has been found experimentally to occur (e.g., Fuller et al., Moon, 1974). Additional shock stresses in this range are produced by converging compressional waves from the impact; however, these waves arrive at a time (400 - 700 s) well before the antipodal field amplification. Seismic surface waves are unlikely to produce significant antipodal shock stresses because of the existence of a highly fractured (and dry) near-surface zone, which produces intense scattering and destruction of coherent surface reflections. Calculated ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally. This suggests that pre-existing ejecta materials, which would also contain abundant metallic iron remanence carriers, may be important anomaly sources. The latter possibility is consistent with enhanced magnetic anomalies observed peripheral to the South Pole-Aitken (SPA) basin, which may have been produced by amplified secondary ejecta impact shock waves in the thick SPA ejecta mantle near the antipodes of the Imbrium and Serenitatis basins.
GP33B-05
Can Shock Remanent Magnetization be Distinguished from Thermal Remanent Magnetization in the Natural Remanent Magnetization (NRM) of Apollo Samples?
The ages of lunar samples, whose NRM appears to require relatively strong lunar surface fields, shows a peak between ~3.65 and ~3.9 Gyrs., e.g Cisowski et al., (1983). However, it has been pointed out that none of these samples, with the possible exception of 62235, satisfy minimal requirement criteria for classical paleointensity determinations (Lawrence et al., 2008). Without successful paleointensity experiments, it is not clear what the peak in the age distribution of samples giving strong field estimates means. The use of AF demagnetization characteristics of remanent magnetizations can be a powerful aid in the interpretation of the NRM of lunar samples. Moreover, the origin of the NRM of samples such as 62235, which are comparatively well behaved in classical paleointensity experiments is not necessarily determined by these techniques. AF demagnetization characteristics can help to show whether the NRM of Melt Rocks and Mare Basalts is predominantly primary, acquired during initial cooling, or is secondary, having been acquired in impact related shock. Examples will be given of the use of AF demagnetization characteristics to attempt to distinguish between NRM whose origin is a Shock Remanent Magnetization and a Thermal Remanent Magnetization. This type of analysis suggests that the NRM of a number of Mare Basalts may have been acquired as they initially cooled and recorded lunar surface fields, as suggested for example by Stephenson and Collinson (1974). If the NRM of these Mare basalts can be demonstrated to have been acquired during cooling, then the recorded field must have been present over a significant period of time and the case for a lunar dynamo is strengthened, If on the other hand, their magnetization has a shock origin, there is always the possibility that a transient field associated with an impact event has been recorded.