Planet のバックアップ(No.103)

惑星セミナー2022 †

惑星セミナーは原則として毎週金曜日の13:00から開催しています。（連絡係：伊藤祐一, 高橋実道，古家健次）
astro-phセミナーは毎週月曜日の12:00から開催しています。（連絡係：Chanoul Seo，瀧哲朗，松本侑士）

Schedule & History †

2021年度 2020年度 2019年度 2018年度 2017年度 2016年度 2015年度 2014年度

日程	発表	タイトル	Remarks	担当
前期第1回 4/14 15:00-	All members	Self-introduction		伊藤
前期第2回 4/28 15:00-	Masahiro Ikoma (NAOJ)	Five key questions answered via the analysis of 25 hot Jupiter atmospheres in eclipse		高橋
前期第3回 5/10 15:00-	Sho Shibata (University of Zurich)	Exploring formation pathways of gas giant planets using planetesimal accretion	Tuesday	古家
前期第4回 5/19 15:00-	Kenji Furuya (NAOJ)	Different degree of nitrogen and carbon depletion in protoplanetary disks		古家
前期第5回 6/2 15:00-	Tatsuya Yoshida (Tohoku University)	Hydrodynamic escape of reduced proto-atmospheres on Earth and Mars		伊藤
前期第6回 6/9 15:00-	Tadahiro Kimura (University of Tokyo)	半径谷形成モデルのレビューと博士論文構想：惑星種族合成モデルを用いたスーパーアース形成過程の制約	日本語	高橋
前期第7回 6/23 15:00-	Tomohiro Yoshida (SOKENDAI/NAOJ)	12CO/13CO Ratio in the TW Hya Disk		古家
前期第8回 7/5 15:00-	Hidenori Genda (ELSI)	Martian Moons eXploration (MMX) mission	Tuesday	高橋
前期第9回 7/14 15:00-	Yuki Kanbara (University of Tokyo)	潮汐による短周期惑星形成とAMDを用いた安定性評価	日本語	古家
前期第10回 7/21 15:00-	Daniel Koll (Peking University)	The unusual climates of habitable sub-Neptunes		伊藤
前期第11回 7/28 15:00-	Yasuhiro Hasegawa (JPL)	Solid Accretion onto Neptune-Mass Planets: Constraints from the D/H ratio of Uranus and Neptune		高橋
後期第1回 10/14 13:00-	Takuji Tsujimoto (NAOJ)	Migration of the solar system and its impact on the climate in the Earth		古家
後期第2回 10/21 13:00-	Hitoshi Miura (Nagoya City University)	What if the asteroid Ryugu is of cometary origin?		伊藤
後期第3回 11/4 13:00-	Yuki Yoshida (University of Tokyo)	TBD	学生発表	古家
後期第4回 11/10 16:00-	Billy Edwards (CEA and UCL)	TBD	Thursday	伊藤
後期第X回 11/18 13:00-	Yukari Toyoda (Kobe Univeristy)	TBD		高橋
後期第X回 12/16 13:00-	Kiyoaki Doi (SOKENDAI)	TBD	May be moved to Dec. 9th

5/10 Sho Shibata (University of Zurich), Exploring formation pathways of gas giant planets using planetesimal accretion: The composition of gas giant planets is a useful tracer of planet formation. Recent observations of gas giant planets suggest that planetesimal accretion had occurred in their formation stage. In our previous studies, we found that large amount of planetesimals can be captured by a protoplanet when the protoplanet migrates into the region which we call as sweet spot for planetesimal accretion. In this talk, we will apply the theory of sweet spot to the formation of close-in gas giant planets and Jupiter and Saturn. We will discuss the formation history of those planets using the planetesimal accretion process.

6/2 Tatsuya Yoshida (Tohoku University), Hydrodynamic escape of reduced proto-atmospheres on Earth and Mars: Earth and Mars likely have obtained reduced proto-atmospheres enriched in H2 and CH4 through impact degassing from planetary building blocks and gravitational capture of the surrounding nebular gas during accretion. Such reduced proto-atmospheres are expected to have been lost by hydrodynamic escape, but their fluxes and timescale for hydrogen depletion remain highly uncertain due to the ambiguity in the radiative loss of energy and chemical processes in escaping outflows. Here we develop a one-dimensional hydrodynamic escape model which includes radiative and chemical processes for a multi-component atmosphere and applied to the reduced proto-atmospheres on Mars and Earth to estimate the atmospheric escape rates and propose possibly atmospheric evolutionary tracks that are consistent with the isotopic compositions and amounts of the surface volatiles. We find that the hydrodynamic escape is suppressed due to the energy loss by the radiative cooling both on Earth and Mars. The escape rate decreases more than one order of magnitude than that of the pure H2 atmosphere when the mixing ratio of CH4 is high. As a result, the duration of the reduced hydrogen-rich environment becomes longer, implying that the early atmospheres played important roles in producing organic matters linked to the emergence of living organisms. The suppression of the hydrodynamic escape by the radiative cooling is more significant on Earth due to the larger gravity and higher temperature in the escaping outflow. The difference in the hydrodynamic escape may have contributed to the difference in the amounts and isotopic compositions of the surface volatiles between Earth and Mars.

6/9 Tadahiro Kimura (University of Tokyo),　半径谷形成モデルのレビューと博士論文構想：惑星種族合成モデルを用いたスーパーアース形成過程の制約: これまでの系外惑星探査によって，地球よりも半径の大きな「スーパーアース」が主星近傍に多数発見され，その質量や半径の分布に関して統計的な解析がされている．特に惑星半径の分布に着目すると，約2地球半径程度の位置に惑星の存在頻度が非常に少ない「半径谷」が存在することがわかった．半径谷の成因については，大気の有無や固体惑星の組成の違い（岩石惑星か氷惑星か）などが提案されており，いずれも適切な初期条件の下で，半径分布の特徴を再現できることが知られている．しかしそれらの初期条件やパラメータは必ずしも現在の惑星形成理論とは整合的ではない．そこで我々は惑星の成長や軌道進化，大気形成などを自己整合的に解く「惑星種族合成モデル」を用い，理論と観測の統計的比較を通じて，半径谷の成因を含めたスーパーアースの形成過程の解明を目指している．本発表ではまず半径谷の形成モデルに関する先行研究をレビューし，現状と課題を整理する．そして種族合成計算の結果と実際のスーパーアース分布との統計的比較について，博士論文研究の構想を発表したい．

7/5 Hidenori Genda (ELSI), Martian Moons eXploration (MMX) mission: Mars has two small moons, Phobos and Deimos. Two leading hypotheses, "capture theory" and "giant impact theory," have been considered for their origin, but they have not been settled. JAXA plans the 3rd Japanese sample return mission called Martian Moon eXploration (MMX). MMX spacecraft explores the Martian moons and brings back regolith samples from Phobos to Earth in 2029. The sample analysis should reveal their origin, but why is the origin of tiny Martian moons so important? What grand story can we draw about the solar system from the samples of the tiny small moon? In this seminar, I will briefly introduce MMX mission, and explain why we chose tiny satellites orbiting Mars.

7/21 Daniel Koll (Peking University), The unusual climates of habitable sub-Neptunes: Sub-Neptune-sized exoplanets like K2-18b are one of the most common planets in our galaxy. These planets are highly promising targets to search nearby exoplanets for biosignatures, because some temperate sub-Neptunes could be hosting liquid H2O oceans underneath their H2 envelopes. The surface climates of H2-rich worlds remain poorly understood, however. In this talk I will first discuss the onset of the runaway greenhouse in a H2 atmosphere. Extending previous work by Nakajima, Ingersoll, and others on the runaway greenhouse, I will show how the unusual low mean-molecular-weight of H2 leads to a number of unique climate effects. Next, I will show that H2-rich atmospheres should also have unusually long sunsets, with atmospheric refraction and scattering extending the twilight zone far beyond that on Earth. Many questions still remain open about the climates of habitable sub-Neptunes, but better theoretical models combined with JWST observations should allow us to make rapid progress over the next couple of years.

7/28 Yasuhiro Hasegawa (JPL), Solid Accretion onto Neptune-Mass Planets: Constraints from the D/H ratio of Uranus and Neptune: The currently available, detailed properties (e.g., isotopic ratios) of solar system planets may provide guides for constructing better approaches of exoplanet characterization. With this motivation, we explore how the measured values of the deuterium-to-hydrogen (D/H) ratio of Uranus and Neptune can constrain their formation mechanisms. Under the assumption of in-situ formation, we investigate three solid accretion modes; a dominant accretion mode switches from pebble accretion to drag-enhanced three-body accretion and to canonical planetesimal accretion, as the solid radius increases. We consider a wide radius range of solids that are accreted onto (proto)Neptune-mass planets and compute the resulting accretion rates as a function of both the solid size and the solid surface density. We find that for small-sized solids, the rate becomes high enough to halt concurrent gas accretion, if all the solids have the same size. For large-sized solids, the solid surface density needs to be enhanced to accrete enough amounts of solids within the gas disk lifetime. We apply these accretion modes to the formation of Uranus and Neptune and show that if the minimum-mass solar nebula model is adopted, solids with radius of ~ 1 m to ~ 10 km should have contributed mainly to their deuterium enrichment; a tighter constraint can be derived if the full solid size distribution is determined. This work therefore demonstrates that the D/H ratio can be used as a tracer of solid accretion onto Neptune-mass planets. Similar efforts can be made for other atomic elements that serve as metallicity indicators.

10/14 Takuji Tsujimoto (NAOJ), Migration of the solar system and its impact on the climate in the Earth: In this talk, I report the birthplace and the trajectory history of the solar system based on the comparison of an elemental abundance pattern of the Sun to those of solar twins together with the results of numerical simulations of the dynamical evolution of disk stars in a Milky Way–like galaxy. These arguments are followed by the discussion on the potential influence on the climate in the Earth as the outcome of a long journey on the disk, which makes the solar system undergo several major encounters with spiral arms.

10/21 Hitoshi Miura (Nagoya City University), What if the asteroid Ryugu is of cometary origin?: Asteroid Ryugu is a C-type near-Earth asteroid. The Japanese Hayabusa2 mission has revealed that Ryugu has a spinning top-shaped rubble-pile structure and potentially high organic content. How did asteroids with these characteristics form? We focused on the hypothesis that Ryugu is an extinct comet and examined whether the features of Ryugu can be explained by theoretically modeling the sublimation process of volatile ice from the heated cometary nucleus. First, we assumed that the cometary nucleus is a porous spherical object (parent comet) composed mainly of water ice particles. The parent comet is assumed to contain cm-sized rocks uniformly. The initial water/rock mass ratio is assumed to be 3:1 according to the solar abundance. The initial radius of the parent comet is assumed to be 1.2 km and the initial porosity is 0.8. Assuming that the parent comet was uniformly heated to 200 K, we calculated the sublimation of the internal ice, the outflow of water vapor from the surface of the parent comet, and the accumulation of the remaining rocks (contraction of the parent comet) and the associated change in spin rate. The results indicate that the ice is almost completely lost in about tens of thousands of years. After the sublimation of the ice, the remaining rocks accumulate at the center of the parent comet gravitationally, and eventually become a rocky object with a radius of about 440 m (a rubble pile asteroid). Calculations of the changes in angular momentum and moment of inertia of the parent comet during its contraction process show that the spin rate is amplified to about four times the initial rate. If the initial rotation period of the parent comet was about the same as that of a typical cometary nucleus (about 12 hr), the rotation period of the remaining asteroid after ice loss would be reduced to about 3 hr. This is shorter than the rotation period (about 3.5 hr) required for Ryugu to become spinning top-shape due to deformation caused by high-speed rotation. This result suggests that the spinning top-shape can be formed spontaneously during the sublimation of ice from the parent comet. Our results indicate that the cometary origin of Ryugu can naturally explain the physical features observed in Ryugu. In the near future, the origin of Ryugu will be revealed based on the results of the detailed analysis of Ryugu samples.