Origin of Phobos and Deimos: gas-drag capture of temporarily captured bodies

KURAMOTO Kiyoshi

Department of Earth and Planetary Sciences

Oct. 14, 2025

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。各時刻（0, 100, 316, 1000年）から5年 間の⼩天体の軌道（⻘線）を、⽕星中⼼座標系（原点：⽕星、x軸：反太陽⽅向、y軸：⽕星の公転⽅ 向）で⽰す。陰影は、エネルギー的に⼩天体が到達できない領域（禁制領域）を⽰す。ガス抵抗によ る⼩天体の⼒学的エネルギー減少に伴い禁制領域が拡⼤し、⼩天体は⽕星の重⼒圏内に閉じ込められ て衛星となる（a, b）。その後もガス抵抗の作⽤により、より⽕星に近い軌道へと遷移する（b, c, d）。

Press release (in Japanese)

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 L₁ or 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 ….

Read the original article on Monthly Notices of the Royal Astronomical Society

Article inforamation
Ryo N Matsuoka, Kiyoshi Kuramoto, Origin of Phobos and Deimos: gas-drag capture of temporarily captured bodies, Monthly Notices of the Royal Astronomical Society, 543, 3, 2025, 2613–2632

DOI:10.1093/mnras/staf1534