Deciphering the magnetic record of planetary rocks using spacecraft and laboratory measurements

Project Information

PETRA

Grant agreement ID: 844252

DOI

10.3030/844252

Project closed

EC signature date 29 March 2019

Start date 1 September 2019

End date 26 October 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 257 619,84

EU contribution

€ 257 619,84

Coordinated by

INSTITUT DE PHYSIQUE DU GLOBE DE PARIS
France

Periodic Reporting for period 2 - PETRA (Deciphering the magnetic record of planetary rocks using spacecraft and laboratory measurements)

Reporting period: 2021-09-01 to 2022-08-31

Summary of the context and overall objectives of the project

The project PETRA aims at improving our understanding about planetary formation and evolution by deciphering the information carried by the magnetic record of billion years old extraterrestrial rocks.

The evolution over time of planetary magnetic fields as recorded by rocks are directly related to properties of the planetary bodies’ deep interiors, to surface processes such as hydrothermal activity and meteor impacts, and to atmospheric and climate evolution. Therefore, studying the magnetic record of rocks opens a window into the deep interior and the geological past of a planetary body.

Space missions that are either led by the European Space Agency (ESA) (e.g. the mission to planet Mercury, BepiColombo) or involve collaboration with ESA (e.g. the lunar program Artemis, and the sample return phase of the Mars 2020 mission) create an unprecedented opportunity to drastically expand our understanding about planetary formation and evolution. Maximizing the outcome of these missions relies on optimizing the synergy between studies based on spacecraft magnetic field measurements and laboratory studies of magnetized samples. This project lies precisely at this intersection.

It focuses on studying the geological history of planet Mars and of the Moon by studying meteorites, lunar return samples and spacecraft magnetic field measurements.

Main conclusions:
All available paired stones of the oldest Martian meteorite have had their primary magnetic records overwritten by strong hand magnets on Earth. A study on dating the magnetic record of its 4.4 Gyr old minerals prior to arrival on Earth is in progress.
The lunar magnetic field has experienced fluctuations in its intensity, at least since 3.4 Gyr ago.
The origin of the Reiner Gamma lunar magnetic anomaly, the landing site of the upcoming Lunar Vertex space mission, is probably a dike emanating from the Marius Hills volcanic complex.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We collected samples from nine different paired stones of NWA 7034, a Martian meteorite which has minerals dated to be 4.4 billion years old. According to our measurements, all samples have been exposed to strong hand magnets on Earth, a widely used meteorite identification technique which leads to the destruction of their magnetic record. In order to raise awareness around this issue, we performed a comprehensive study of the effect of magnets on the magnetic record of rocks, through numerical modeling and a controlled remagnetization experiment.

This study has been published at the peer-reviewed Journal of Geophysical Research (JGR), highlighted by the editors of JGR in Eos (the science news magazine published by the American Geophysical Union), presented at the 28th IUGG General Assembly that took place in July 2023 in Berlin, Germany, and received widespread media attention (Science Magazine, MIT News, Sky&Telescope and others). It is also the subject of a comic book I put together in collaboration with scientific illustrator Dr. Sabrina Sanchez. It is available on my website and we will present it at the European Geosciences Union (EGU) General Assembly in Vienna, Austria, in April 2024.

Further on, I conducted two paleomagnetic studies using lunar samples of the Apollo 16 and Apollo 17 missions. These rocks span two billion years of lunar history, with ages ranging from 3.7 to 1.7 billion years old. I conducted laboratory measurements in order to obtain paleointensity estimates (i.e. estimates about the strength of the lunar magnetic field) and rock magnetic measurements in order to characterize their magnetic properties. According to these studies, the lunar magnetic field at 3.7 Gy ago was approximately 40 μT, by 3.4 Gy it has dropped to below 6 μT and by 1.7 Gy ago it was less than 2 μT and possibly zero. These results, in combination with previous studies, suggest that the lunar magnetic field intensity has been fluctuating at least since 3.4 Gy ago.

The results of the Apollo 17 mare basalts have been presented at the AGU Fall meeting 2021 in New Orleans, USA and the results of the Apollo 16 regolith breccias have been presented at the LPSC 2023 in Texas, USA. I am currently working on the respective papers to be submitted to peer-reviewed journals. I also presented the main results at a seminar at IPGP, France and I will present them at an invited talk at the EGU General Assembly in Vienna, Austria, in April 2024.

I also worked on two different inversion methodologies, which can convert spacecraft magnetic field measurements into information about the magnetization of the underlying sources.

Firstly, I developed a new such inversion methodology that allows us to infer the direction of the magnetization from a local set of magnetic field measurements. I presented this novel inversion technique at the AGU Fall meeting 2020 (virtual conference).

Secondly, in collaboration with Marie Curie fellow Dr. Joana Oliveira, we evaluated the potential of an existing inversion technique for inferring the geometry of magnetic sources. We found that this method successfully localizes and delineates the two-dimensional surface projection of subsurface three-dimensional magnetized bodies. We used it to decipher the origin of two lunar magnetic anomalies. This study has been published at the peer-reviewed Journal of Geophysical Research.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We showcased through the example of the NWA 7034 meteorite that the widespread use of hand magnets during collection and curation of meteorites leads to the destruction of precious, ancient magnetic records.

We studied the magnetic record of lunar mare basalts from the Apollo 17 mission, dated to be ~ 3.7 Gy old. We found them to carry evidence of being magnetized by a uniform magnetic field, with a strength of approximately 40 μT. Moreover, we found one of the subsamples to carry two distinct magnetic signatures, one from the time the rock formed and one from a much younger impact event. These results combined offer strong evidence that these rocks’ magnetic record is primary and offer further evidence in favor of the leading hypothesis that the Moon used to have a dynamo.

We studied the magnetic record of regolith breccias from the Apollo 16 mission, dated to be 3.4 and 1.7 Gy old. We found that the lunar magnetic field was below 6 μT 3.4 Gy ago and less than 2 μT, and possibly zero, and by 1.7 Gy ago. These results, in combination with previous studies, suggest that the lunar magnetic field might have been oscillating or operating only intermittently, at least since 3.4 Gy ago.

We have developed a new methodology that allows to infer the magnetization direction from magnetic field spacecraft measurements.

We have found an existing inversion methodology, up to now used to infer the magnetization direction from magnetic field measurements, to perform well in localizing and delineating the two-dimensional surface projection of subsurface three-dimensional magnetized bodies.

Impact of the project:
- This project raised awareness about the destructive effects of exposing meteorites to magnets, both within the planetary science community and the general public.
- It improved our understanding about the evolution of the lunar magnetic field.
- It increased European expertise i) in processing and interpreting magnetic field data acquired by space missions, and ii) in studying the magnetic record of samples provided by space sample return missions, from cm- down to μm-scales.

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