日本地球惑星科学連合2015年大会

講演情報

口頭発表

セッション記号 P (宇宙惑星科学) » P-CG 宇宙惑星科学複合領域・一般

[P-CG32] 惑星大気圏・電磁圏

2015年5月26日(火) 11:00 〜 11:45 A03 (アパホテル&リゾート 東京ベイ幕張)

コンビーナ:*今村 剛(宇宙航空研究開発機構 宇宙科学研究本部)、関 華奈子(名古屋大学太陽地球環境研究所)、高橋 幸弘(北海道大学・大学院理学院・宇宙理学専攻)、高橋 芳幸(神戸大学大学院理学研究科)、深沢 圭一郎(京都大学学術情報メディアセンター)、中川 広務(東北大学 大学院理学研究科 地球物理学専攻太陽惑星空間物理学講座 惑星大気物理学分野)、座長:今村 剛(宇宙航空研究開発機構 宇宙科学研究本部)

11:00 〜 11:03

[PCG32-P01] 木星デカメータ電波スペクトルに基づく氷衛星地下海探査の実現可能性

ポスター講演3分口頭発表枠

*熊本 篤志1笠羽 康正1三澤 浩昭1土屋 史紀1 (1.東北大学大学院理学研究科)

キーワード:木星の氷衛星, 地下海, 木星デカメータ電波, 干渉

Subsurface liquid ocean of the Jupiter's icy moons, which is suggested by several studies, is one of the most important targets in the Jovian exploration missions. We propose a new method for determination of the depth of the boundary between the icy crust and liquid ocean below the icy crust by using interference patterns found in the spectrogram of the Jovian decametric radio emissions (DAM). If we can operate an wave receiver onboard the icy moon orbiter, we can obtain spectrograms of the DAM propagated from Jupiter. Because the emissions directly from Jupiter can be interfered with the emissions reflected at the icy moon's surface and subsurface boundaries, we will find interference patterns in the measured spectrograms. In case of the Moon, the lunar orbiter SELENE detected the interference patters in the spectrograms of auroral kilometric radiation (AKR) [Ono et al., 2010]. Because the interference occurs between AKR directly from the earth and AKR reflected at the lunar surface, the amplitude of the interference patterns are almost constant. In case of Jupiter's icy moons, DAM directly from Jupiter, DAM reflected at the icy crust surface, and DAM reflected at the boundary between icy crust and liquid ocean are interfered with each other. Due to slight phase difference between DAM emissions reflected at the surface and subsurface boundaries, the amplitude of the interference patterns will be modulated. The depth of the liquid ocean can be determined the frequency width of the modulation. Assuming that the frequency of DAM is ~25 MHz, the permittivity of the icy crust is 3, permittivity of the liquid ocean is 87, loss rate in the icy crust is 1 dB/km, and the depth of the ocean is 5 or 10 km, spacecraft and receiver's specifications needed for measurement of the interference patterns in the spectrogram are as follows: (1) Spacecraft height below 200 km, (2) Receiver bandwidth of <1 kHz, and (3) Receiver level resolution of <5 dB. In addition, the following two issues have to be considered in actual application of this method: (a) DAM itself has band structures in the spectrogram due to anisotropy of the emission at the source. (b) The roughness of the surface and subsurface boundaries have to be within the wavelength (~10 m) in order that the interference occurs.