1. IAGA 2005 - Toulouse, France 22/07/2005 Inversion of Terrestrial and Martian magnetic anomalies: geological implications Y. Quesnel (1), B. Langlais (1), S. Le Mouélic (1), C. Sotin (1) and A. Galdéano (2) (1) Laboratoire de Planétologie et Géodynamique de Nantes, UMR-CNRS 6112 (2) Géomagnétisme Interne, IPG Paris, UMR-CNRS 7577

2. Crustal magnetic anomalies ↔ Crustal sources ? Correlation between magnetic anomaly and geological maps (near-surface investigation…) Modeling methods Forward models (geometry, magnetization) Inversion (depth, orientation) Crustal sources IAGA 2005 - Toulouse, France 22/07/2005

3. Outline Modeling method Application to Martian magnetic satellite data Application to Terrestrial aeromagnetic data Geological implications Future work IAGA 2005 - Toulouse, France 22/07/2005

4. Method Forward models: uniformly magnetized spheres (Blakely, 1996) and prisms (Plouff, 1976) Generalized non-linear inversion (Tarantola and Valette, 1982): Data: Bx, By, Bz Parameters: mx, my, mz, x, y, z (moment and location of one or several dipoles) Equations: three components of the magnetic field created by a sphere Iterative process: minimum chi-squared criterion Final test: data misfits, gaussian distribution IAGA 2005 - Toulouse, France 22/07/2005

5. Application to Martian satellite data IAGA 2005 - Toulouse, France 22/07/2005 A remanent field ! (Acuña et al., 1999) Type of data: three vector components AB (100-250 km) and MO (400 km) MGS MAG/ER measurements Area: ancient southern highlands of Terra Sirenum AB (Br) MO (Br)

6. Application to Martian satellite data IAGA 2005 - Toulouse, France 22/07/2005 AB (100-250 km)MO (400 km) Data Results of three different data set tests: (input dipole location parameters from forward modeling studies)

7. Application to Martian satellite data IAGA 2005 - Toulouse, France 22/07/2005 AB (100-250 km)MO (400 km) Data Model (3 dipoles) 1 2 3 1 2 3

8. Nantes Application to Terrestrial aeromagnetic data Type of data: total magnetic field airborne measurements at 120 m above topography (summer 1998 ; BRGM) Area: the Armorican Massif (France) IAGA 2005 - Toulouse, France 22/07/2005 I Geological setting Nantes

9. Application to Terrestrial aeromagnetic data IAGA 2005 - Toulouse, France 22/07/2005 1 32 Forward modeling (first results): 3 spheres model: 1 32 3 prisms model: N

10. Geological implications IAGA 2005 - Toulouse, France 22/07/2005 The Armor2 seismic profile from Bitri et al., C.R. Geosciences, 335, 2003: SN 3 A/m 11 A/m 0 5 10 15 20 25 30 35 Depth (km)

11. Geological implications Results of the Martian investigation: - no correlation between adjacent sources - no links with craters in the studied area - strongly magnetized materials → magnetite ? how ? IAGA 2005 - Toulouse, France 22/07/2005 Serpentinization model (early Mars) Alteration of forsterite (10% Fe) with water: 30Fo 90 + 41H 2 O  15Lizardite + 2Fe 3 O 4 + 9Mg(OH) 2 + 2H 2

12. Future work… Inversion of terrestrial aeromagnetic data Terrestrial satellite measurements investigation Study of other areas on Mars Correlation with surface mineralogy : OMEGA Mars Express IAGA 2005 - Toulouse, France 22/07/2005 Comparison with terrestrial spectra of olivine, pyroxene, serpentine and magnetite (example of the Ronda massif: Launeau et al., 2002)

13. Acknowledgments IAGA 2005 - Toulouse, France 22/07/2005 This work benefited from the support of the European Community’s Improving Human Potential Programme under contract RTN2- 2001-00414, MAGE.