[P-PS23] 月の科学と探査

[PPS23-P12] SELENE-2/月電磁探査装置 (LEMS): インバージョンのテスト (2)

*松島 政貴¹、清水 久芳²、藤 浩明³、吉村 令慧⁴、高橋 太⁵、綱川 秀夫¹、渋谷 秀敏⁶、松岡 彩子⁷、小田 啓邦⁸、小川 和律⁹、田中 智⁷ (1.東京工業大学、2.東京大学地震研究所、3.京都大学、4.京都大学防災研究所、5.九州大学、6.熊本大学、7.宇宙航空研究開発機構宇宙研究所、8.産業技術総合研究所、9.東京大学)

キーワード：電磁探査, 月内部構造, SELENE-2

The so-called giant impact hypothesis is likely to explain the origin of the Moon in view of physical and chemical evidence such as angular momentum, materials possibly through magma ocean processes, and compositional similarity of the Earth and the Moon. Numerical simulations of such a giant impact indicate that most of the Moon-forming material around the proto-Earth originates from the projectile. This means that such a standard giant impact is difficult to form the Moon whose isotopic composition is essentially identical to the Earth's as found from the lunar samples in the Apollo mission. This would be a reason why new giant-impact models are devised. It should be noted that the lunar samples were obtained only from the lunar surface, and that information on bulk composition and interior structure of the Moon is still insufficient. Therefore it is of significance to obtain information regarding the whole lunar composition and interior structure, which can advance our understanding of lunar origin and evolution.

In the SELENE-2 mission, we propose a lunar electromagnetic sounder (LEMS) to estimate the electrical conductivity structure of the Moon. The electrical conductivity varies with temperature even for the same composition, and therefore it can be used to deduce the present thermal structure of the Moon.

Temporal variations in the magnetic field of lunar external origin, which can be observed by magnetometers onboard a lunar orbiter and a lunar lander, induce eddy currents in the lunar interior depending on the electrical conductivity distribution and frequencies of the temporal variations. The eddy currents, in turn, generate temporal variations in the magnetic field of lunar internal origin, which can be observed by a magnetometer onboard a lunar lander. Thus electromagnetic response of the Moon is obtained by magnetic field measurements. Then the electromagnetic response function is used to estimate the electrical conductivity structure by solving an inverse problem. We show results for some tests of inversion, assuming a one-dimensional interior structure for electrical conductivity distribution.