Sesquinary Catastrophe on Deimos: raw data, integrator, and analysis codes

<p>The origins of the Martian moons Phobos and Deimos are highly debated, and scenarios for their origin include formation from an impact-generated circum-Martian disk or formation from the capture of asteroids. With the impact scenario, Deimos (or its precursors) had to have formed or have been pushed out beyond the synchronous orbit of Mars. Any moons that form interior to the synchronous orbit, including Phobos (or its precursors), would evolve and resonances between these moons could potentially excite Deimos' orbit, making it eccentric and/or inclined. This contradicts the present-day orbit of Deimos, which has low eccentricity (0.00027) and moderate inclination (1.8°) relative to the Laplace plane). Tidal dissipation within Deimos is too inefficient for eccentricity damping, and therefore in the absence of any alternative mechanism for damping the orbit of Deimos, its present day orbit of Deimos would place strong constraints on the evolution of any inner moons. We propose that a type of runaway collisional cascade called the "sesquinary catastrophe'' acts as a natural barrier that prevents Deimos from having an orbit much more excited than its present one.  Using N-body simulations with collisional fragmentation, we show that if the orbit of Deimos was more excited, it would undergo a sesquinary catastrophe that would break it apart into a Roche-exterior debris disk. Collisions within the debris disk would damp it, and Deimos would then re-accrete on a more circular and less inclined orbit. Using a measure of orbital excitation called q that was previously used in the context of the sesquinary catastrophe, our simulations suggest that breakup occurs for excitation measures of q >= 8 on timescales of ~10^{3-4} years. If Deimos had ever been destroyed in a sesquinary catastrophe and re-accreted, it should be a porous sand-pile moon, consistent with its smooth surface. The sesquinary catastrophe can be applied to other planetary moons at q >= 8 assuming they are gravitational aggregates similar to Deimos.</p>

火星的两颗卫星火卫一（Phobos）与火卫二（Deimos）的起源问题至今仍存在广泛争议，其起源假说主要包括两种：一是由撞击产生的环火星盘（circum-Martian disk）形成，二是由小行星捕获而来。按照撞击起源假说，火卫二（或其前身天体）必须形成于火星同步轨道之外，或是被推移至该轨道以外。凡是形成于火星同步轨道内侧的卫星（包括火卫一及其前身天体）都会随时间演化，且这些卫星间的轨道共振可能会扰动火卫二的轨道，使其轨道偏心率和/或轨道倾角升高。这与火卫二当前的轨道特征相悖：相对于拉普拉斯平面（Laplace plane），其轨道偏心率仅为0.00027，轨道倾角仅为1.8°，均处于较低水平。火卫二内部的潮汐耗散效率过低，无法为轨道偏心率提供阻尼；因此在缺乏其他可阻尼火卫二轨道的机制的情况下，其当前轨道状态将对所有内侧卫星的演化形成极强约束。本研究提出，一种被称为“半双星灾变（sesquinary catastrophe）”的失控碰撞级联过程可作为天然屏障，阻止火卫二轨道受到远超当前水平的扰动。本研究采用包含碰撞碎裂过程的N体模拟（N-body simulations），结果表明：若火卫二轨道受到更强扰动，其将经历半双星灾变过程，被拆解为洛希极限（Roche limit）外侧的碎屑盘。碎屑盘内的碰撞过程将对轨道起到阻尼作用，随后火卫二将在更接近圆形、轨道倾角更低的轨道上重新吸积形成。本研究采用此前用于半双星灾变研究的轨道扰动程度量化指标q，模拟结果显示：当q≥8时，火卫二将在约10^(3-4)年的时间尺度内发生解体。若火卫二曾在半双星灾变中解体并重新吸积形成，其应呈现为多孔沙堆状卫星，这与其表面光滑的观测特征相符。若其他行星卫星与火卫二一样属于引力团聚体，则该半双星灾变机制同样适用于q≥8的这类天体。