[P-PS06] 火星と火星衛星

[PPS06-05] Estimation of magnetic field intensity of ancient Mars using the magnetic anomaly data: implications for the histories of magnetic field and water escape

*佐藤 雅彦^1,2、潮田 雅司³、中田 亮一⁴、田村 裕二郎⁵、山本 伸次⁵、小澤 一仁¹、高橋 太⁶、坂田 遼弥⁷、関 華奈子¹ (1.東京大学、2.宇宙航空研究開発機構、3.四国総合研究所、4.海洋研究開発機構、5.横浜国立大学、6.九州大学、7.東北大学)

キーワード：Mars、Magnetic anomaly、Magnetic field、Water escape

Magnetic field observations of Mars revealed that there are strong magnetic anomalies arising from the crustal remanences, which is estimated to be about 10 times as strong as the Earth’s crustal magnetization. The strong crustal remanences require a particular origin such as the strong dynamo field of the ancient Mars, the high concentration of ferromagnetic mineral in the Martian crust, and so on. Plagioclase, one of the common constituents of terrestrial crustal rocks, sometimes contain fine-grained magnetite crystals exsolved from Fe-bearing magmatic plagioclase at subsolidus condition. The natural remanent magnetization carried by the exsolved magnetite in plagioclase is likely candidate of the source of Martian magnetic anomaly in terms of the remanence stability. In this study, a suite of experiments and calculations were carried out to estimate the paleo-planetary field intensity of Mars based on the crustal remanence records. (1) Magnetic hysteresis measurement, synchrotron radiation study, and microscopic observation in order to evaluate the content of exsolved magnetite, to determine valence state of iron in plagioclase, and to clarify the content of iron in plagioclase, respectively. (2) The remanence acquisition efficiency of exsolved magnetite was investigated by thermoremanent magnetization acquisition/demagnetization experiments for plagioclase crystals. (3) Thermodynamic calculation with the rhyolite-MELTS program was carried out to model fractional crystallization of a magma with composition of Martian crustal rocks. The results indicate that the Martian crustal rocks are high in concentrations of exsolved magnetite, which efficiently acquires the thermoremanent magnetization, resulting in the high remanence acquisition efficiency of the Martian crust. The paleo-planetary field intensity of Mars is estimated to be 10–20 μT using the crustal remanence and acquisition efficiency values. On the basis of the magnetic field intensity, we will discuss the histories of Martian magnetic field and water escape from Martian surface.