1. Local inversion of Martian magnetic anomalies : geological implications Y. Quesnel, C. Sotin, B. Langlais and S. Le Mouélic Laboratoire de Planétologie et Géodynamique de Nantes (France) RST 2004 - Strasbourg, France 23/09/2004

2. RST 2004 – Strasbourg, France Y. Quesnel, C. Sotin, B. Langlais and S. Le Mouélic – LPG Nantes (France) 23/09/2004 One of the MGS MAG/ER results : A remanent field with strong magnetic crustal anomalies in the South Hemisphere (Acuña et al., 1999 ; Connerney et al., 2001) Question : Sources of these strong anomalies ? Methods : Local MGS MAG data simulation with uniformly magnetized spheres (Blakely, 1995) and prisms (Plouff, 1976) Generalized non-linear inversion (Tarantola and Valette, 1982) (Acuña et al., 1999)

3. Local MGS MAG Observations Old crust No correction of altitude : real data ~10 times stronger than the Earth crustal anomalies for the same altitudes ! 0 200 km Bz AB data (80 < altitude < 250 km)Bz MO night data (altitude ~ 380 km) RST 2004 – Strasbourg, France Y. Quesnel, C. Sotin, B. Langlais and S. Le Mouélic – LPG Nantes (France) 23/09/2004

4. Model of uniformly magnetized sphere (Blakely, 1995) b 240°-45° 45 A/m 60 km 191°E-32°N DeclinationInclinationMRadiusDepthLongitudeLatitude m r Surface Magnetic moment m = volume V * magnetization MSphere dipole RST 2004 – Strasbourg, France Y. Quesnel, C. Sotin, B. Langlais and S. Le Mouélic – LPG Nantes (France) 23/09/2004

5. Model of uniformly magnetized prisms (Plouff, 1976) 180°-60° 40 A/m 20 km30 km35600 km 2 Declination Inclination Magnetization Top depth ThickArea The « west prism » All the prisms have a strong magnetization This model fits well the MO night data too ! DataBest model RST 2004 – Strasbourg, France Y. Quesnel, C. Sotin, B. Langlais and S. Le Mouélic – LPG Nantes (France) 23/09/2004

6. λ1λ1 λ2λ2 φ1φ1 φ2φ2 z1z1 z2z2 MID -8.5°N-11.5°N173°E176°E-5 km5 km13 A/m20° -9.5°N-10.5°N174°E175°E5 km40 km13 A/m20° λφzrMID -10°N174.5°E50 km 15 A/m-20°200° Both models are 1° southward the volcano center location. This simulation shows that Apollinaris Patera is a magnetic source. Data Sphere Prisms AB < 150 kmMO night data Models 200 km of altitude Sphere Prisms nT Is Apollinaris Patera (-8°N, 174°E) a magnetic source ? Magnetized volume : 523 600 km 3 Magnetized volume : 382 000 km 3 perturbation

7. 185°E 190 200 210°E I = -79° Z = 41 km m = 2,8.10 16 A.m 2 I = -62° Z = 47 km m = 1,5.10 16 A.m 2 I = -46° Z = 93 km m = 7,8.10 16 A.m 2 I = 31° Z = 86 km m = 8,5.10 16 A.m 2 I = -9° Z = 74 km m = 2,1.10 16 A.m 2 0 200 km -26°N -30 -35 -40°N Generalized non-linear inversion with a least-square criterion (Tarantola and Valette, 1982) Input : MGS data + a priori parameters of dipoles (location, depth Z, dipolar moment m, inclination I and declination D) Output : best parameters of dipoles I < 0 I > 0 RST 2004 – Strasbourg, France Y. Quesnel, C. Sotin, B. Langlais and S. Le Mouélic – LPG Nantes (France) 23/09/2004

8. First conclusions 1.Good simulation with strongly magnetized material 2.No coherence between dipoles after inversion 3.No correlation with craters Which material ? Iron-bearing minerals : magnetite, hematite, pyrrhotite, or pure iron Which process ? Volcanism, plutonism, hydration, or weathering RST 2004 – Strasbourg, France Y. Quesnel, C. Sotin, B. Langlais and S. Le Mouélic – LPG Nantes (France) 23/09/2004

9. Geological model : serpentinization 30 (Mg 0.8 ; Fe 0.2 ) SiO 3 + 18 H 2 O8 Mg 3 Si 2 O 5 (OH) 4 + 2 Fe 3 O 4 + 2 H 2 + 14 SiO 2 Enstatite (20 % Fe) Water Lizardite Magnetite Dihydrogen Quartz Assumptions : - hydrated crust - mantle convection - Earth-like dynamo A 40 A/m magnetization is compatible with a 10 % mass-fraction of magnetite corresponding to a 8 % hydration. RST 2004 – Strasbourg, France Y. Quesnel, C. Sotin, B. Langlais and S. Le Mouélic – LPG Nantes (France) 23/09/2004

10. Contribution of the Mars Express OMEGA imaging spectrometer data : comparison with terrestrial spectra of olivine, pyroxene, serpentine and magnetite (example of the Ronda massif : Launeau et al., 2002) Mineralogical composition of Mars surface and below (crater ejecta…) Perspectives RST 2004 – Strasbourg, France Y. Quesnel, C. Sotin, B. Langlais and S. Le Mouélic – LPG Nantes (France) 23/09/2004 (S. Le Mouélic, personal communication, 2004)

11. Mars Express Cover up until now (S. Le Mouélic, personal communication, 2004)

12. Acknowledgments This work benefited from the support of the European Community’s Improving Human Potential Programme under contract RTN2-2001-00414, MAGE.