[P-PS01] Outer Solar System Exploration Today, and Tomorrow

[PPS01-06] Constraints on the Eruption Duration of Plumes in the South Polar Region of Enceladus

*Takuya Mizunuma¹, Yuto Ohtawara¹, Shunpei Nakahara¹, Tomokatsu Morota¹ (1.The University of Tokyo)

Keywords:Enceladus, Plume

Saturn's moon Enceladus is a geologically active body, with numerous tectonic features across its surface (e.g., Spencer & Nimmo, 2013). In the south polar region, there are giant fractures known as "tiger stripes," from which plumes of ice and gas are ejected (e.g., Goldstein et al., 2018). While some plume particles are released into space, the majority are redeposited on Enceladus' surface, forming regolith (e.g., Kempf et al., 2010). Previous numerical models have reported that the spatial distribution of plume particle deposition rates generally aligns with the distribution of unaltered water ice on Enceladus' surface, as inferred from reflectance spectral data (Southworth et al., 2019), suggesting that recent plume particle deposition is likely occurring at rates consistent with the estimated deposition rates. If plume activity has persisted over geological timescales, it is expected that the continuous deposition of plume particles has selectively erased small craters. Additionally, since plume material is likely to be directly supplied from Enceladus' subsurface ocean (e.g., Spencer et al., 2018), estimating the thickness of these deposits could provide constraints on the duration of plume activity and, consequently, the longevity of the internal ocean. This study aims to estimate the duration of plume activity by evaluating the extent of small crater disappearance and quantifying deposit thickness.
This study utilizes a crater database identified from Cassini imaging data (Kirchoff, 2020). Based on numerical studies of plume particle redeposition (Southworth et al., 2019), we investigated the crater size-frequency distribution in the region around 45°S, 45°W, where deposition rates are estimated to be high. The results show that for craters smaller than 3 km in diameter, the slope of the size-frequency distribution is shallower than that for craters larger than 4 km. This suggests that small craters under 3 km in diameter have been erased due to plume particle deposition. To quantify the extent of small crater disappearance, we calculated the ratio of crater densities for craters larger than 3 km relative to those larger than 1 km. The results show a weak positive correlation between crater density ratios and estimated deposition rates in the cratered plains of the southern hemisphere. This supports the idea that small craters below 3 km in diameter are selectively erased by plume particle deposition.
Furthermore, to understand the mechanism of small crater disappearance, we attempted to explain the observed data using a mathematical model. This model assumes that crater formation occurs through impactor collisions, while crater burial progresses only due to plume particle deposition. The model also assumes a constant plume particle deposition rate and that craters disappear when the thickness of the regolith layer reaches the height of the crater rim. The model parameters include surface age and the maximum size of craters that disappear. We fit the model to the observational data in two regions: one around 30°S, 190°W (Region 1), where deposition rates are high, and another around 45°S, 50°W (Region 2). The results show that the crater size-frequency distribution curves generated by the model accurately reproduce the characteristics of the observed data, suggesting that crater disappearance is driven by the accumulation of a regolith layer approximately equal to the rim height. The maximum sizes of buried craters were estimated to be approximately 2 km in Region 1 and 3 km in Region 2. Based on these maximum buried crater sizes and the estimated deposition rates, the duration of plume particle deposition was found to be approximately 1 Gyr.