小天体の捕獲による火星衛星の形成メカニズムを解明～火星衛星探査による地球型惑星の水・有機物の起源の解明に期待～

2025-10-14 北海道大学

北海道大学大学院理学研究院の倉本圭教授・松岡亮博士研究員らは、火星衛星フォボスとディモスの形成過程を「一時捕獲天体のガス抵抗捕獲モデル」で再現することに成功した。従来の捕獲説では、これらの衛星が示す火星赤道面に沿ったほぼ円軌道を説明できなかったが、本研究では、原始太陽系星雲中に存在した火星を想定し、小天体が火星近傍で一時的に捕獲された後、周囲のガス抵抗によって軌道エネルギーを失い、安定的な赤道円軌道に移行する過程を理論解析とシミュレーションで明示した。結果は、火星衛星が地球型惑星形成に関与した原始小天体の残存物である可能性を示唆する。2026年打ち上げ予定のJAXA火星衛星探査計画（MMX）によるサンプルリターンは、水や有機物の供給起源を探る鍵となることが期待される。成果は『Monthly Notices of the Royal Astronomical Society』誌に掲載。

図 1. ⼩天体が⼀時捕獲状態から完全捕獲状態へと⾄る様⼦。

＜関連情報＞

フォボスとディモスの起源：一時捕獲天体のガス抵抗捕獲 Origin of Phobos and Deimos: gas-drag capture of temporarily captured bodies

Ryo N Matsuoka, Kiyoshi Kuramoto
Monthly Notices of the Royal Astronomical Society Published:13 September 2025
DOI:https://doi.org/10.1093/mnras/staf1534

ABSTRACT

The gas-drag capture hypothesis for the origin of the Martian moons Phobos and Deimos, fairly consistent with their reflectance spectra like carbonaceous meteorites, faces difficulties in explaining their small orbital inclinations and the dense gas required for capture. Here, we show through numerical and theoretical analysis that gas-drag capture via temporary capture of small bodies can overcome these difficulties. Temporary capture occurs for small bodies entering the Martian Hill sphere at low speeds from the L₁or L₂ Lagrange points, allowing them to orbit Mars repeatedly even without dissipation. Such a long-range orbit around Mars enables complete capture by weak gas drag under the minimum-mass solar nebula and more dilute gas densities, with minimal dependence on the specific gas flow structure. The orbital inclination with respect to the Mars orbital plane is suppressed to within a few degrees due to the quasi-conservation law of absolute angular momentum around Mars, and the periapsis immediately after complete capture ranges around several tens of Mars radii. The continuous gas drag with torque exerted by the Martian equatorial bulge can transfer captured bodies towards the primordial low-inclined moons’ orbits inferred from tidal evolution models. If the typical planetesimal masses are close to those of the Martian moons, there are numerous opportunities for complete capture through temporary capture even at late accretion stages with depleted nebula. The last population of captured bodies may have undergone the slowest gas drag-induced migration towards Mars and some of them may have survived as the present-day moons after the nebular dissipation.