Gas Drag and the Orbital Evolution of a Captured Triton
Gas Drag and the Orbital Evolution of a Captured Triton
复制标题
捕获的海卫一的气体阻力和轨道演化
DOI：
10.1006/icar.1995.1199
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发表时间：
1995
期刊：
影响因子：
3.2
通讯作者：
A. Leith
中科院分区：
文献类型：
--
作者：
W. McKinnon;A. Leith
摘要气体拖曳是捕获海卫一的一种可能机制。我们研究的气体阻力捕获的能量，比较气体阻力在随后的轨道演化与潮汐的作用，并评估是否捕获的气体阻力海卫一可以避免螺旋进入海王星。气体阻力本身会导致轨道的倾角变得更大，海卫一的逆行状态可能会对由于气体阻力而损失的轨道能量设置上限，从而对潮汐演变和伴随的加热量设置下限。一个假定的海王星星云是仿照最小质量的天王星星云，并假定是等温和冷却。海卫一的初始远心位于海王星希尔球的半径，而其初始近心位于星云的径向范围内。尽管强烈的演变的两个半长轴和偏心率，海卫一的倾角只有几个度的变化，由于气体阻力，当解耦的倾角变化，由于太阳扭矩引起的岁差。结果是不敏感的径向表面密度分布或温度的变化，固定的星云质量。因此，海卫一可能仅通过气体阻力演化到目前的逆行圆形轨道，而大规模的潮汐加热并不是捕获的必然结果。然而，由于海卫一的后气体拖曳偏心率，当海卫一达到今天的角动量时，剩余的角动量很可能超过了0.2。尽管海卫一的质量，单程捕获被证明是积极的有利于我们的名义星云。对于（i）质量小一个数量级的星云，如果海卫一的日心轨道在能量上接近暂时的引力捕获，以及（ii）在形成的早期阶段流入海王星吸积球的太阳气体，捕获在能量上也是可能的。在最后一种情况下，海卫一不太可能抵抗气体阻力而存活下来，但低质量的情况可能适用于在各种吸积场景中形成于天王星和海王星等行星周围的致密H和He耗尽星云。一旦被捕获，在没有太阳扰动的情况下，演化是迅速的，对于名义星云来说，轨道角动量在103年内减少到海卫一的当前值。然而，太阳潮汐导致海卫一的轨道在原卫星星云内外振荡，并可以将气体拖曳时间尺度延长到104-105年。因此，海卫一可能在一个短暂的（约103年），炽热，动荡的星云中幸存下来，这个星云可能是由与海王星的巨大撞击形成的。它也可能在一个温度较低、湍流较小但寿命较长（约106年）的低质量星云中幸存下来：海卫一和低质量（氦耗尽）星云的质量和角动量相当，因此这种捕获的轨道演化可能会“自我终止”。“特别是，海卫一可能会在多次穿过星云后清除星云中的环形区域，并使星云质量的径向重新分布足够缓慢，完全停止气体阻力的演变。
Abstract Gas drag is a possible mechanism for capturing Triton. We examine the energetics of gas drag capture, compare the role of gas drag in subsequent orbital evolution with that of tides, and evaluate whether a Triton captured by gas drag could have avoided spiraling into Neptune. On its own, gas drag causes the inclination of an orbit to become more prograde, and Triton's retrograde status potentially places an upper limit on the orbital energy lost due to gas drag and hence a lower limit on the amount of tidal evolution and concomitant heating. A putative Neptune nebula is modeled after a minimum mass Uranus nebula and is assumed to be isothermal and cool. Triton's initial apocenter is set at the radius of Neptune's Hill sphere and its initial pericenter within the radial range of the nebula. Despite strong evolution of both semimajor axis and eccentricity, Triton's inclination only changes by a few degrees due to gas drag, when uncoupled to inclination variations due to solar-torque-induced precession. Results are insensitive to variations in radial surface density distribution or temperature, for fixed nebular mass. Thus, Triton could have evolved to its present retrograde circular orbit by gas drag alone, and massive tidal heating is not an inevitable consequence of capture. Nontrivial tidal heating is still likely, though, because Triton's post-gas-drag eccentricity, i.e., that remaining when Triton's present-day angular momentum was reached, is likely to have exceeded ∼0.2. Despite Triton's mass, single-pass capture is shown to be energetically favorable for our nominal nebula. Capture is also energetically possible for (i) nebulae that are less massive by an order of magnitude, if Triton's heliocentric orbit was energetically close to a temporary gravitational capture, and (ii) the solar gas flowing into Neptune's accretion sphere at an earlier stage of formation. Survival of Triton against gas drag in the last case is unlikely, but the low-mass case may apply to the compact H- and He-depleted nebulae that form around planets such as Uranus and Neptune in various accretion scenarios. Once captured, evolution is rapid in the absence of solar perturbations, with the orbital angular momentum reduced to Triton's present value in under 103 years for the case of the nominal nebula. Solar tides, however, cause Triton's orbit to oscillate in and out of the protasatellite nebula and can extend the gas drag time scale, for certain initial conditions, to ∼104-105 years. Therefore Triton could have survived a short-lived (∼103 years), hot, turbulent nebula that may have formed by a giant impact with Neptune. It might have also survived a cooler, less turbulent, but longer-lived (∼106 years) low-mass nebula: the mass and angular momentum of Triton and that of a low-mass (Hand He-depleted) nebula are comparable, so orbital evolution for such a capture may "self-terminate." In particular, Triton may clear an annular region in the nebula after multiple passes through it, and for sufficiently slow radial redistribution of nebular mass, cease evolving by gas drag entirely.