Can the lunar crust be magnetized by shock: Experimental groundtruth

TLDR: In this article, the authors present the first experimental acquisition of shock remanence by lunar rocks in the 0.1-2-GPa range and discuss their implications for the interpretation of the paleomagnetic record of these rocks, as well as for the distribution of magnetic anomalies revealed by orbital data.

About: This article is published in Earth and Planetary Science Letters.The article was published on 2010-10-15 and is currently open access. It has received 55 citations till now. The article focuses on the topics: Natural remanent magnetization & Remanence.

Figures

Fig. 6. Normalized PRM versus AF for different maximum pressure values, for samples 14053 and NWA 5406.

Fig. 2. Orthogonal projection plot of stepwise demagnetization data. Open and solid symbols are projections on vertical and horizontal planes, respectively. Thick gray lines are the magnetization components computed using Principal Component Analysis with indication of their Maximum Angular Deviation (Kirschvink, 1980). a–d) NRM of the four studied Apollo Mare basalt samples. e–f) laser-induced SRM for Mare basalts 70215 (laser flux 3.92 GW cm−2 in 400 μT) and 14053 (laser flux 2.87 GW cm−2 in 300 μT).The ambient magnetic field during impact was vertical downward. g–h) PRM imparted in the pressure cell for samples 15556 (1.8 GPa in 1762 μT) and NWA 5406 (1.8 GPa in 881 μT).

Fig. 4. a) SRM acquired with a constant laser flux of 1.6–1.7 GW cm−2 (dots, linear fit indicated by solid line), and PRMat 0.9 GPa (empty boxes, thin broken line) and at 1.8 GPa (solid boxes, thick broken line) of sample 14053 as function of ambient field; b) SRM (normalized by the magnetizing field) of sample 14053 as a function of laser flux.

Fig. 3.Modeled peak pressure isocontours in the Mare basalts sample upon laser shock with a flux of 1, 2, and 3 GW cm−2 focused on a spot of 4.7 mm in diameter in waterconfined regime. The flat front face of each cubic sample (10 mm edge) is at the bottom, and the laser pulse is coming from the bottom up. Associated peak pressure histograms are also plotted.

Table 2 PRM and SRM results.

Fig. 5. PRM (normalized by the magnetizing field) versus pressure for Mare basalts 14053 and 70215, and highland breccia NEA 001 and NWA 5406 (empty circles). PRM after AF demagnetization at 2 mT (3 mT for NWA 5406) is also plotted (solid circles), as it allows eliminating spurious magnetization possibly acquired during the PRM experiments. For samples 14053 and 70215, the laser-induced SRM is indicated by empty stars (solid stars after AF demagnetization at 2 mT) using the median pressure in the sample (0.2 GPa).

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Can the lunar crust be magnetized by shock: experimental groundtruth" ?

Gattacceca et al. this paper showed that the surface of the Moon can be magnetized by shock. 

Q2. What have the authors stated for future works in "Can the lunar crust be magnetized by shock: experimental groundtruth" ?

Mare basalts 14053 and 70215, that have a stable NRM and reliable SRM data, have similar NRM and SRM coercivity spectra, leaving open the possibility that theNRMwas imparted during an impact at the lunar surface. Conversely, the NRM of mare basalt 15556 is not compatible with a SRM, which may imply it is a TRM and therefore require a long standing stable magnetic field ( namely a dynamo generated field ) whose intensity can be estimated to about 55 μT using the REM′ method. Even though the comparison of the coercivity spectra of NRM and different types of magnetization ( TRM, SRM, Viscous remanent magnetization etc. ) is not unique because the natural case may be more complex with superimposition of different phenomena ( such as partial shock demagnetization of a TRM, or viscous demagnetization of a SRM ), SRM experiments should become a standard technique in lunar and extraterrestrial paleomagnetism. Regarding the lunar antipodal magnetic anomaly model, their results show that lunar soils, regolith breccia and about 40 % of lunar highland rocks ( comprising regolith and impact-melt breccia ) in the upper crust can be magnetized by low pressure shocks ( b10 GPa ) to sufficient levels to account for the observed lunar antipodal anomalies, provided that the compressed ambient field in the antipodal region reaches about 100 μT as proposed by Hood and Artemieva ( 2008 ). 

Q3. What mechanism is invoked to explain these strong anomalies?

The mechanism invoked to explain these strong anomalies is shock magnetization of impact-processed materials located at the antipode to the large basins. 

Q4. What is the simplest explanation for the remanent magnetization of lunar rocks?

Thermoremanent magnetization requires a steady magnetic field during cooling through the blocking temperatures of lunar rocks (i.e. on a time scale of at least several days), which implies an internal origin, namely a core dynamo. 

Q5. What is the effect of the ambient field on the area affected by the magnetic field?

As this magnetic field balances the plasma pressure, the size of the area affected by these maximum fields depends on the intensity of the original ambient field. 

Q6. What is the way to explain the asymmetrical magnetic anomalies?

Regarding the lunar antipodal magnetic anomaly model, their results show that lunar soils, regolith breccia and about 40% of lunar highland rocks (comprising regolith and impact-melt breccia) in the upper crust can be magnetized by low pressure shocks (b10 GPa) to sufficient levels to account for the observed lunar antipodal anomalies, provided that the compressed ambient field in the antipodal region reaches about 100 μT as proposed by Hood and Artemieva (2008). 

Q7. What is the maximum SRM intensity of a typical lunar rock?

For sample 14053, that has a complex sub-solidus reduction history (Taylor et al., 2004) and magnetic remanence dominated by cohenite (this work), SRM is 6.7 10−4×Mrs×B (B in μT, SRM in Am2 kg−1), i.e. more than5 times stronger than in other lunar rocks. 

Q8. What is the optimum magnetic field for the antipodal anomaly model?

As far as the antipodal magnetic anomalymodel is concerned, such anomalies require magnetization of about 1 Am−1 (~3 10−4 Am2 kg−1) over several kilometers of thickness (e.g., Hood and Artemieva, 2008). 

Q9. What is the reason for the shock magnetization anomalies?

It can be attributed to a combination of the excavation process (as proposed onMars for impact basins by e.g., Langlais et al., 2010), to shock demagnetization (if the ambient field was null during the impact), or to a poorly efficient shock magnetization process that would result, even in the presence of an ambient field, in a lower post-shock magnetization compared to the pre-shock one. 

Q10. How much of the SRM variation is the mean?

The experiments are reproducible: for repeated shocks using the same laser power, the SRM variations are on average 5% of the mean SRM value. 

Q11. What mechanism has been modeled for the shock magnetization of lunar rocks?

Although the mechanism for transient antipodal magnetic field enhancement and shock magnetization has been modeled numerically (Hood and Huang, 1991; Hood and Artemieva, 2008), there is at the moment no experimental constraint on the shock magnetization of lunar rocks. 

Q12. What is the transition pressure for pure Fe?

The transition pressure decreases with increasing Ni content and is about 13 GPa for pure Fe, and about 9 GPa for Fe20Ni80 (Wasilewski, 1976). 

Q13. How was the pressure of the cell calibrated?

Pressure calibration of the cell was performed by fitting the pressure demagnetization curve of a synthetic magnetite-bearing sample that had already been pressure demagnetized with another pressure cell (Bezaeva et al., 2010) whose pressure was calibrated using a manganin sensor (Sadykov et al., 2008). 

Q14. How much of the PRM variation is the same for repeated loading?

The experiments are reproducible: for repeated loading at the same pressure, the PRM variations are on average 4% of the mean PRM value. 

Q15. What is the mechanism for weaker anomalies within impact basins?

On the other hand, weaker anomalies are present within some Nectarian-aged impact basins (Halekas et al., 2003) that have recently been interpreted asplausibly due to TRMof slowly cooling impact-melt (Hood, in press; Wieczorek and Weiss, 2010).