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Paleomagnetism of extraterrestrial materials: a clue to early magnetic fields in the solar system and asteroids differentiation

Final Report Summary - EXTRAMAG (Paleomagnetism of extraterrestrial materials: a clue to early magnetic fields in the solar system and asteroids differentiation)

The study of the remanent magnetization (paleomagnetism) of extraterrestrial materials gives clues as to the history of the primitive solar system and its evolution. Indeed, paleomagnetic studies of meteorites provide a unique window into understanding early solar magnetic fields generated externally from planetesimal bodies (by the young sun during its putative T Tauri phase, or within the protoplanetary nebula), as well as magnetic fields generated within the planetesimals through convection of a molten core (dynamo field).
The aim of this project is to participate to the current effort in this field. More precisely we focused on: primordial fields in the protoplanetary disk; differentiation processes in asteroids; Lunar magnetism. We addressed these questions through detailed paleomagnetic studies of selected samples, using good practices like the determination of the nature of the natural remanence (with tests for shock and viscous remanence), advanced instrumentation (like small-scale magnetic mapping), and the interpretative framework provided by the recent progresses in the field of meteoritics (in particular thermochronology).

1. Meteorite paleomagnetism
1.1 Partial differentiation of the CV parent body
The aim of this study was to test the model of partially differentiated asteroids that has been recently put forward (Carporzen et al. 2011, PNAS). We conducted a paleomagnetic study of Kaba meteorite. The results (presented at LPSC and AGU meeting in 2013) confirm that the CV parent body was likely partially differentiated, with an outer chondritic layer overlying a differentiated interior. Publication of these results is underway.

1.2 CM chondrites: the oldest paleomagnetic record
We studied seven CM chondrite falls. All of them have recorded a weak magnetic field of a few microteslas, that was present when the parent body was aqueously altered only a few million years after the formation of the solar system. This very ancient magnetic signal may have recorded a magnetic field generated within the protoplanetary disk itself, of generated within the parent body (dynamo field). These results are being published in EPSL.

1.3. Other meteorite studies
We have started paleomagnetic studies of several other meteorite grops: enstatite chondrites (Feng et al. 2013), acapulcoites and lodranites (Schnepf et al. 2014), and an ungrouped achondrite NWA 7325 that may have originated from Mercury (Weiss et al. 2013, 2014). The preliminary results have been presented at several meetings (AGU Fall meeting 2013, 2014).
All these works deal with the same question: did the parent bodies of these meteorites possess a dynamo field? The answer to this question varies from group to group. In enstatite chondrites, our study is limited by the poor magnetic recording properties of these rocks (Feng et al. 2013). In NWA 7325, no magnetic field could be detected (Weiss et al. 2013, 2014). In acapulcoites and lodranites, an internally generated magnetic field may be recorded (Schnepf et al. 2014).

2. Lunar paleomagnetism
Previous works have proven the existence of a lunar dynamo, and paved the way to a better characterization of the lunar dynamo. In this project, we worked on new Apollo samples to better understand the end of this story.

2.1. Timing of the lunar dynamo
We have performed a paleomagnetic study of two relatively young Apollo sample (dated at 3.56 Ga) and demonstrated that they were magnetized in a stable and intense dynamo magnetic field. These results (published as Suavet et al. 2013 in PNAS) require a persistent power source for the lunar dynamo like precession of the lunar mantle and/or a compositional convection dynamo. A second study was conducted paleomagnetic studies of a number of Apollo samples to estimate the timing of the decline of the lunar dynamo. We show that he lunar dynamo likely switched off around 3.2 Ga. The results have been published in Tikoo et al. (2014, Earth and Planetary Science Letters).

2.2. Paleomagnetism of large Apollo samples
With the idea of studying the magnetic properties of large Apollo samples we have designed and built a magnetometer that allows measuring the natural remanent magnetization and magnetic susceptibility of large samples up to about 5 kg. This instrument was successfully tested at Johnson Space Center (Houston) with measurements of Apollo Mare basalt samples directly in the storage vault. This success demonstrated the feasibility and the non-destructive aspect of these measurements. We then proceeded to the measurement of 105 large Apollo samples. The aim is to track the evolution of the lunar dynamo and to use this database to select key samples for a detailed paleomagnetic study. The first results will be presented at the AGU Fall meeting in December 2014.

3. Magnetic properties and thermal history of ordinary chondrites
In this project, we have assembled a large database of hysteresis properties of ordinary chondrites. We interpret these properties as a proxy to the thermal history of these meteorites. In particular, hysteresis properties allow easy identification of tetrataenite, a mineral that is very sensitive to the initial cooling rate and later thermal disturbances. This work is now published (Gattacceca et al. 2014 in Meteoritics and Planetary Science). To better characterize the sensitivity of tetrataenite to thermal disturbances, we performed thermal disordering experiments of natural tetrataenite. We provide a full picture of the time-temperature conditions under which tetrataenite is disordered (Dos Santos et al. 2014 in Journal of Magnetism and Magnetic Materials).

4. Magnetism and pressure
In the general framework of the understanding of the effects of hypervelocity impacts on the magnetization in natural samples, we studied the effect of pressure on the magnetic properties of hematite and pyrrhotite. The results on hematite are published in Physics of the Earth and Planetary Interiors (Jiang et al. 2013), the ones about pyrrhotite will be presented at the upcoming AGU meeting (Bezaeva et al. 2014).

5. Martian magnetism
We studied the magnetic properties of the latest (and only fifth) Martian meteorite fall, named Tissint, which fell in Morocco in 2011. We showed that this meteorite was remagnetized during a major impact at the surface of Mars and that it keeps the record of the surface magnetic field on Mars as we know them today (Gattacceca et al. 2013 in Meteoritics and Planetary Science).
We studied the magnetic properties of a new type of Martian meteorite, a Noachian magmatic breccia, that was discovered in 2012. We showed that this type of rock is a plausible candidate to account for the large magnetic anomalies observed over Noachian terrains on Mars. This study reconcile for the first time the study of Martian rocks and the magnetic fields measured by satellites orbiting Mars. This work is now published (Gattacceca et al. 2014 in Geophysical Research Letters).