田畑 陽久

Abstract Enceladus may host a subsurface ocean with biologically relevant chemistry. Plumes released from this ocean preserve information on its chemical state, and previous analyses suggest weakly to strongly alkaline pH. Constraining the ocean’s pH is critical for assessing its habitability, as pH governs the chemical energy available for potential life. This requires identifying pH-sensitive minerals in plume deposits. Raman spectrometers, which have recently been incorporated into flight instruments, offer a potential approach for mineral identification on icy moons. However, their applicability to pH estimation from plume-derived minerals has not been investigated. We evaluate whether Raman measurements of plume particles deposited on the surface of Enceladus can distinguish between weakly and strongly alkaline ocean models. Fluids with pH values of 9 and 11 were frozen under vacuum conditions analogous to those on Enceladus. The resulting salt deposits were then analyzed using a Raman system designed to simulate the SuperCam instrument for Mars, featuring a pulsed laser, standoff geometry, and a spectral resolution coarser than 10 cm ⁻¹ . The obtained Raman spectra showed diagnostic peaks for pH-dependent carbonate precipitation: at pH 9, signals consistent with NaHCO ₃ (673, 1045, and 1267 cm ⁻¹ ) and Na ₂ CO ₃ (1081 cm ⁻¹ ) were detected, whereas only Na ₂ CO ₃ was observed at pH 11. These findings demonstrate that flight-like Raman spectrometers can distinguish these key mineral phases without high-resolution laboratory optics. This result validates that in situ Raman spectroscopy is a practical approach to constrain the pH of the subsurface ocean, a key parameter for assessing its chemical evolution and potential habitability.

Abstract Phobos always keeps the same side facing its host planet like Earth’s Moon, making it a key comparative target for studying the coevolution of planet–satellite systems. The heterogeneity of satellite surface evolution under a host planet’s gravity is crucial for understanding the evolution of such systems. This study examines the crater size–frequency distribution (CSFD) across four equatorial regions of Phobos—leading, near, trailing, and far sides—to investigate surface evolution heterogeneities linked to resurfacing and orbital dynamics following the formation of its largest crater, Stickney. We focus on the crater size indicating the number of craters lower than expected from the production function (PF) model. We also estimate the crater erasure scale due to the ejecta blanket and assess deviations in the size–frequency distribution (SFD) of impactors from the PF model. This study shows three main conclusions about surface heterogeneity in Phobos’ CSFD and ejecta blanket thickness. (1) The leading side experienced significant crater erasure, likely due to the thickest ejecta blanket from the Stickney impact. (2) The density of craters larger than $$\sim$$ 1.5 km reflects pre-Stickney conditions, whereas the densities of craters measuring 0.8–1.5 km and 400–800 m in diameter reflect post-Stickney surface evolution. (3) Crater number density on the near side was consistently lower than on the far side, likely due to planetary screening. This reduction in crater formation on the near side is particularly attributed to its proximity to Mars. Our results suggest that the leading side experienced the greatest deposition of the ejecta blanket on Phobos. This deposition likely occurred immediately after Stickney’s formation, followed by global resurfacing facilitated by Phobos’ post-Stickney impact spin. One possible explanation for the scarcity of craters smaller than 800 m is that later-arriving, low-energy impacts erased smaller preexisting craters while leaving larger ones comparatively well preserved, leading to a net reduction in their population.