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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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)

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

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

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