論文 / 著書情報
Article / Book Information
題目(和文)

土星の規則衛星のモデル表面年齢とエンケラドゥスの主要な年代層序
学事象の確立Title(English)

Establishing the model surface ages of Saturn's regular satellites andmajor chronostratigraphic events on Enceladus

著者(和文)

ウォン ワイ エミリー

Author(English)

WONG Wai Emily

出典(和文)

学位:博士(理学),学位授与機関:東京工業大学,
報告番号:甲第12652号,
授与年月日:2024年3月26日,
学位の種別:課程博士,
審査員:井田 茂,中本 泰史,奥住 聡,玄田 英典,関根 康人

Citation(English)

Degree:Doctor (Science),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第12652号,
Conferred date:2024/3/26,
Degree Type:Course doctor,
Examiner:,,,,

学位種別(和文)

博士論文

Category(English)

Doctoral Thesis

種別(和文)

審査の要旨

Type(English)

Exam Summary

Powered by T2R2 (Tokyo Institute Research Repository)

（博士課程）論文審査の要旨及び審査員
報告番号

甲第主査

号

学位申請者氏名

Wong Wai, Emily

氏 名

職 名

氏 名

職 名

井田茂

教授

奥住准教授

中本 泰史

教授

玄田 英典

教授

関根 康人

教授

聡

論文審査
審 査 員

審査員
審査員

論文審査の要旨（2000 字程度）
The thesis is entitled "Establishing the model surface age of Saturn's regular satellites and major chronostratigraphic eventson Enceladus" and contains 7 chapters and 3 appendices. This work combined research on the modelled crater chronologies ofSaturnian regular satellites and the global geological history of Enceladus. The author established the outer Solar System's crater
chronology through impact crater analysis and numerical N-body simulations, considering giant planet migration and refined
estimation of the primordial planetesimal disc. Applying various impactor and crater size-frequency distributions, the author
calculated surface ages for cratered plains on Mimas, Enceladus, Tethys, Dione, and Rhea. The study included a comprehensive
global crater count and analysis of geomorphologies to derive the Enceladus crater production function, enabling precise
calculation of its global surface age and exploration of its geological history.
In Chapter 1, “Introduction”, the author explored the historical impact rate on giant planets and their regular satellites,
drawing from previous dynamic studies. The overview included calculating surface ages for Saturnian satellites' icy crustthrough crater dating. Limitations in previous work, such as assumptions about crater chronology (cumulative projectile flux
through time) and crater size-frequency distribution (statistical relationship between crater diameters and their frequency ofoccurrence), were highlighted. The author emphasized the need for a tailored approach due to the unique dynamics andconditions of icy satellites in the outer Solar System. The breakthrough in the thesis lies in acquiring the crater chronology andsize-frequency distribution catered for icy satellites to enhance age estimation accuracy.
In Chapter 2, "Outer Solar System: Evolutions and Simulation", the author delineated the parameters of the N-bodysimulations. The initial focus was on simulating giant planet migration to replicate 4.5 billion years of outer solar systemevolution. However, due to the low expected impact probabilities of satellites, there were limitations in impact resolution. The
author proposed and ran a batch with cloned planetesimals, optimizing computational resources while preserving the model'saccuracy. This approach enhanced impact resolution, enabling the construction of a precise 4.5 Gyr crater chronology.
In Chapter 3, "Giant Planets and Comets: Impact Rate", the author ensured the simulations align with the current outer solarsystem conditions. Calibration was performed based on the constrained impact rate on Jupiter, integrating results from priorstudies. The author calculated initial planetesimal numbers, defined their size-frequency distribution, and assessed their rateof decline. Simulation results indicated that, after 4.5 Gyr, only 0.31% of scattered disc objects remained, suggesting an initialcount of 6.4 billion objects over 10 km in diameter. Impact count simulations over the last 1 Gyr estimated a current Jovian
impact rate of 1.57 ⨯ 10-3 per year for comets larger than 1 km in diameter.
In Chapter 4, "Icy Satellites: Crater Chronology and Absolute Surface Ages", the author introduced an improved approachto constructing a crater chronology, extending the chronological curves much further to recent times and enhancing accuracy.
With the improved chronology, the author calculated the highest expected crater density for each Saturnian icy satellite,
assuming a solidified surface and retention craters from 4.5 billion years ago. To visually convey how to calculate the surface
ages, the author plotted and compared the crater size-frequency measurements with age-isochrons ̶ representing distributions
for surfaces with varying ages. The author derived best-fit surface ages through a least-square fitting, addressing discrepancies
with previous studies and affirming the updated ages' accuracy.
In Chapter 5, "Enceladus: Untangling its long history of activity”, the author expanded the study on Enceladus. Calculating
ages not only for a specific cratered plain, but across an extensive, nearly global area, the author surveyed Enceladus using
publicly released Cassini images, reporting ages on over 80% of its surface. Despite the absence of outer Solar System satellite
crater production functions, this study introduced a comprehensive crater count method, establishing a polynomial function for
a new Enceladus crater production model. This model, combined with crater chronology, enabled age determination for surface
units. The author discussed how crater dating complemented previous geomorphological studies, offering insights into crater
asymmetry, impact population, satellite formation, and Enceladus' tidal history.
In Chapter 6, "Future work", the author outlines the vision for advancing and extending the current approach to the regularsatellites of Jupiter and Uranus and crater chronology modelling to the inner solar system. The concluding remarks and projectsummary are presented in Chapter 7.
The thesis synthesized knowledge from the dynamic evolution of the outer Solar System and remote sensing of satellites' ice
crusts to determine the ages of Saturn's regular satellites and diverse terrains on Enceladus. Its interdisciplinary and innovative
approach is judged to be of sufficient quality for the degree of Doctor of Science.
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