Permalink : http://hdl.handle.net/2115/34723

太陽系の外惑星領域にある衛星は，そのほとんどがH2O主体の氷で覆われた"氷衛星"である．これらは数の多さや多様なサイズを持つが，最も注目すべき点は大規模な地質活動の跡が見られることである．地球の月とは異なるその外見は，氷衛星を覆う氷の存在とその特徴的な物性が作り出している と思われる．本稿では代表例として木星の衛星エウロパに見られる地溝帯と土星の衛星エンセラダスで発見された氷噴出現象に着目し，これらの現象の原因を探る．そこには，液体水が固化する際に体積が増加するという氷特有の物性が大きく関わっている可能性がある．
Most eminent feature of tectonic activity on the icy satellites is extensional signatures, which have been formed by tensional stress. The example is narrow ridges and bandshaped terrain on Europa, and parallel, periodically spaced fractures called by Tiger Stripes on Enceladus. These features suggest a possibility that the surface have been fractured and extended due to tensional stress. We have focused on the solidification of the liquid water and the accompanying volume change as one of the dominant sources for such stresses. To estimate the stress state in the solid ice region surrounding the water region, we first performed numerical simulations of the thermal history. Based on the resulting structural evolution, we calculated stresses in the viscoelastic ice region due to the solidification of the liquid water. Europa's liquid layer solidifies slowly and may partially survive at present, and its solidification induces sufficient tensional stress to drive extensional tectonic activity. Moreover, we investigated the possibility that solidification of subsurface water reservoir in Enceladus can attain enough excess pressure to explain the geysering. If the viscosity of the surrounding ice is viscous enough to inhibit relaxation, as high as 10^20-21Pas, sufficient excess pressure can be accumulated and the crack will be created to drive the internal water to the surface.