15:00 〜 15:15
[MIS18-06] 土星衛星エンセラダスの亀裂地形間隔の測定に基づく過去の熱流束推定
キーワード：エンセラダス、熱史、亀裂地形、クレータ年代学
Saturnian icy moon Enceladus exhibits intense geological and thermal activity. In particular, the south polar terrain is characterized by plumes of water vapor and ice particles, accompanied by elevated heat fluxes reaching several hundred mW/m2. Additionally, gravitational field and libration measurements provide strong evidence for the presence of a global subsurface ocean beneath the ice shell, raising the possibility of a habitable environment. To evaluate the duration of the subsurface ocean, it is crucial to reconstruct the Enceladus’ thermal history based on a detailed analysis of the geological features that retain imprints of such activities.
Several tectonic fractures on Enceladus exhibit nearly parallel and evenly spaced distribution. Observations of analogous fractures on Earth, experimental studies, and finite element modeling have demonstrated that such fractures are vertically confined by an underlying layer, with the spacing between fractures being proportional to the layer thickness, and a constant of proportionality of approximately 1. Therefore, by quantifying the fracture spacing on Enceladus, we can infer the thickness of the brittle ice layer. From brittle layer thickness, the temperature at the brittle-ductile boundary can be derived, enabling an estimation of the past heat flux at the timing of fracture formation. Additionally, by counting craters that intersect these fractures and are considered younger than tectonic fractures, we can estimate the formation age of the fractures, and thus the period during which the high heat flux occurred.
In this study, we mapped tectonic fractures distributed in the low- to mid-latitude regions (<60 degree) of Enceladus and measured the fracture spacing to estimate the past heat flux. We identified 13 sets of fracture terrains that are nearly parallel and evenly spaced, and measured their spacing. Of these, 11 sets exhibited small spacing, ranging from 1 to 2 km, suggesting that the brittle ice layer was thin, also ranging from 1 to 2 km. A thin brittle layer indicates that the thermal flux was high at the timing of fracture formation, and these fracture sets, with spacing of 1 to 2 km, are estimated to have experienced a heat flux of 100 to 400 mW/m2.
To estimate the timing of this high heat flux, we counted the craters that intersect fracture sets. The crater count for three fracture sets in the cratered plain indicates that the period of high heat flux occurred more than 1 Gyr ago. Furthermore, the crater density in the fracture sets of the cratered plain is higher than that in the south polar terrain, suggesting that the period of high heat flux predates the onset of current activity in the south polar terrain.
The high heat fluxes estimated in this study are thought to have been localized rather than global. If heat fluxes of several hundred mW/m² had been globally present, the total thermal production on Enceladus would have exceeded 80 GW in the past. However, tidal heating on Enceladus is believed to have produced a maximum thermal output of only around 30 GW, making it difficult to explain global high heat fluxes. This implies that the regions with high flux were geographically limited. The existence of localized heating events in the past suggests phenomena such as convective processes in the ice shell or the ascent of hydrothermal plumes from a subsurface ocean.
Several tectonic fractures on Enceladus exhibit nearly parallel and evenly spaced distribution. Observations of analogous fractures on Earth, experimental studies, and finite element modeling have demonstrated that such fractures are vertically confined by an underlying layer, with the spacing between fractures being proportional to the layer thickness, and a constant of proportionality of approximately 1. Therefore, by quantifying the fracture spacing on Enceladus, we can infer the thickness of the brittle ice layer. From brittle layer thickness, the temperature at the brittle-ductile boundary can be derived, enabling an estimation of the past heat flux at the timing of fracture formation. Additionally, by counting craters that intersect these fractures and are considered younger than tectonic fractures, we can estimate the formation age of the fractures, and thus the period during which the high heat flux occurred.
In this study, we mapped tectonic fractures distributed in the low- to mid-latitude regions (<60 degree) of Enceladus and measured the fracture spacing to estimate the past heat flux. We identified 13 sets of fracture terrains that are nearly parallel and evenly spaced, and measured their spacing. Of these, 11 sets exhibited small spacing, ranging from 1 to 2 km, suggesting that the brittle ice layer was thin, also ranging from 1 to 2 km. A thin brittle layer indicates that the thermal flux was high at the timing of fracture formation, and these fracture sets, with spacing of 1 to 2 km, are estimated to have experienced a heat flux of 100 to 400 mW/m2.
To estimate the timing of this high heat flux, we counted the craters that intersect fracture sets. The crater count for three fracture sets in the cratered plain indicates that the period of high heat flux occurred more than 1 Gyr ago. Furthermore, the crater density in the fracture sets of the cratered plain is higher than that in the south polar terrain, suggesting that the period of high heat flux predates the onset of current activity in the south polar terrain.
The high heat fluxes estimated in this study are thought to have been localized rather than global. If heat fluxes of several hundred mW/m² had been globally present, the total thermal production on Enceladus would have exceeded 80 GW in the past. However, tidal heating on Enceladus is believed to have produced a maximum thermal output of only around 30 GW, making it difficult to explain global high heat fluxes. This implies that the regions with high flux were geographically limited. The existence of localized heating events in the past suggests phenomena such as convective processes in the ice shell or the ascent of hydrothermal plumes from a subsurface ocean.