A combined fluid/kinetic model is developed to calculate thermally driven escape of N2 from Pluto's atmosphere for two solar heating conditions: no heating above 1450 km and solar minimum heating conditions. In the combined model, one-dimensional fluid equations are applied for the dense part of the atmosphere, while the exobase region is described by a kinetic model and calculated by the direct simulation Monte Carlo method. Fluid and kinetic parts of the model are iteratively solved in order to maintain constant total mass and energy fluxes through the simulation region. Although the atmosphere was found to be highly extended, with an exobase altitude at ~6000 km at solar minimum, the outflow remained subsonic and the escape rate was within a factor of two of the Jeans rate for the exobase temperatures determined. This picture is drastically different from recent predictions obtained solely using a fluid model which, in itself, requires assumptions about atmospheric density, flow velocity and energy flux carried away by escaping molecules at infinity. Gas temperature, density, velocity and heat flux versus radial distance are consistent between the hydrodynamic and kinetic model up to the exobase, only when the energy flux across the lower boundary and escape rate used to solve the hydrodynamic equations is obtained from the kinetic model. This limits the applicability of fluid models to atmospheric escape problems. Finally, the recent discovery of CO at high altitudes, the effect of Charon and the conditions at the New Horizon encounter are briefly considered.
The Atmosphere of Pluto as Observed by New Horizons
在新视野号飞越期间进行的观测提供了冥王星大气层当前状态的详细快照。虽然低层大气（高度小于200公里）与地面恒星掩星一致，但高层大气比遭遇前模型显示的要冷得多，也更紧凑。分子氮（N $_2$）在大气中占主导地位（海拔<1800公里左右），而甲烷（CH PDMATHAMN），乙炔（C $_2$ H $_2$），乙烯（C $_2$ H $_4$），和乙烷（C $_2$ H $_6$）是丰富的次要种，并可能导致冥王星周围的大范围烟雾的产生。寒冷的高层大气切断了冥王星大气层向太空的预期增强的牛仔裤，流体动力学般的逃逸。目前还不清楚冥王星大气层的现状是否代表了它的平均状态--在季节或地质年表上。
Escape and evolution of Mars' CO2 atmosphere: Influence of suprathermal atoms
用蒙特卡罗模型，我们调查了火星大气中的热氧和碳的逃逸在其历史上的四个时间点对应于1，3，10和20倍，目前的太阳EUV通量。我们研究和讨论了热层中热氧和碳原子的不同来源以及它们随着EUV通量的变化而变化的重要性。由于更高的EUV通量导致的更高密度而导致的生产率的增加与热层的膨胀和相应的碰撞增加相竞争。我们发现，由于光解离的逃逸随EUV能级的增加而增加。然而，对于通过一些其他反应的逃逸，例如~ O $_2^+$的解离复合，这仅在EUV水平达到当前EUV通量的10倍之前是真实的，并且然后速率开始降低。此外，我们的研究结果表明，火星在诺亚纪末期不可能有稠密的大气层，因为这样的大气层直到今天才能够逃逸。在前诺亚时代，大部分的岩浆海洋和火山活动相关的脱气CO $_2$大气可能已经失去了热，直到诺亚时代，当非热损失过程，如超热原子逃逸成为主导。因此，早期的火星在前诺亚纪时期可能是炎热潮湿的，在行星起源后的前3亿年期间，表面CO $_2$压力大于1巴。
