| 摘要: | 行星大氣的外球層在熱平衡狀態，氣體分子遵循馬克斯威爾速度分佈，其中動能超過重力位能者可能脫離該天體，即所謂的大氣逃逸。已知土星環系統主要由中性氣體雲組成，這些氣體一部分由來自土星的衛星，例如土衛六泰坦提供了氮氣、甲烷與氫氣。2004年10月26日至2014年12月10日止，卡西尼(Cassini)探測器共計30次飛掠泰坦，透過離子與中性物質質譜儀(INMS)與惠更斯大氣結構分析儀(HASI)的紀錄，泰坦大氣結構與成分便逐一揭曉(Waite et al., 2005; Fulchignoni et al., 2005)。
Yelle et al. (2008)、Strobel (2009)與Cui et al. (2012)以流體模型擬合INMS觀測的泰坦大氣甲烷密度分佈，並預測甲烷(CH_4)逃逸率相當於〖10〗^27 s^(-1)，這個遠大於瓊斯逃逸率的數值飽受爭議。Tucker & Johnson (2009)與Schaufelberger et al. (2012)以蒙地卡羅直接模擬法 (DSMC)計算泰坦外球層區域氣體分子之間的碰撞與位移，顯示即使採用瓊斯逃逸率也能夠重建同樣的觀測資料。
根據甲烷從泰坦大氣逃逸的可能，我們選擇瓊斯逃逸率6.3×〖10〗^17 s^(-1)作甲烷的產率，在土星-泰坦系統內以限制性三體運動模擬大氣逃逸並考慮甲烷在系統內主要受光化學分解作用而產生CH_3、CH_2、CH與C，進一步計算這些粒子的密度分佈。此外，我們參考雙重馬克斯能量分佈，由(2-5)%超高溫甲烷增加高速粒子的比例(Jiang et al., 2017)，分別揀選兩個特別高的甲烷產率1.2×〖10〗^25 CH_4 s^(-1)與2.2×〖10〗^27 CH_4 s^(-1)並建立兩組密度分佈與原本的瓊斯逃逸模型互相比較。
所有的模型均顯示甲烷家族的密度分佈集中於泰坦的軌道與土星A環之間，高產率與被拓寬的高速分佈則分別使氣體雲的密度更大且分佈範圍更寬廣，假設甲烷的逸散率計算正確，泰坦就是土星環系統碳元素的主要來源之一；但，來自卡西尼電漿光譜儀(CAPS)的觀測數據，並沒有足夠數量的含碳離子證實甲烷氣環的存在(Johnson, 2009; Arridge et al., 2011)，這些觀測反映流體擴散模型可能高估甲烷的逃逸率，或土星環內存在其它未知的機制抑制了碳離子的產量。
;Atoms and molecules in an atmosphere in thermodynamic equilibrium have velocities which follow the Maxwell-Boltzmann distribution law. Some of these particles may escape to space if its energy is kinetic greater than its gravitational binding energy. The ring system of Saturn is dominated by neutral gas clouds originating from the icy satellites. One significant source of them, Titan, continuously provides〖〖 N〗_2,CH〗_4,H_2. Thirty Cassini flybys with Titan, from 26 October 2004 to 10 December 2014, reveal the structure of its upper atmosphere by the Ion and Neutral Mass Spectrometer (INMS) and Huygens Atmospheric Structure Instrument (HASI) measurements (Waite et al., 2005; Fulchignoni et al., 2005).
With the CH_4 density profile based on the observations fitted by fluid model calculations, Yelle et al., (2008), Strobel (2009) and Cui et al. (2012) predicted a CH_4 escape rate at the level of 〖10〗^27 〖 s〗^(-1). Such a high value greater than the Jeans escape rate has been disputed by a number of authors. Describing the transition region of Titan’s atmosphere, Tucker & Johnson (2009) and Schaufelberger et al. (2012) show that the INMS data could be reproduced with a value similar with the Jeans escape rate by the Direct Simulation Monte Carlo (DSMC).
On the basis of the CH_4 emitted from Titan’s atmosphere, we simulate the escape process with the Jeans escape rate of 6.3×〖10〗^17 s^(-1) and calculate the methane-group (i.e., CH_4,CH_3, CH_2, CH and C) density distribution in the Saturn-Titan system by coupling the restricted three-body motion and the photochemical effects. In addition, we consider the double Maxwellian energy distribution contributed by a superthermal CH_4 population of (2-5)% on the high-energy tail, assuming two representative high CH_4 escape rates of 1.2×〖10〗^25 CH_4 s^(-1) and 2.2×〖10〗^27 CH_4 s^(-1) respectively (Jiang et al., 2017), and build two models in comparison with the original Jeans one.
All models suggest the methane-group densities peak between the orbit of Titan and the A ring. Gas clouds with high production rates and high-velocity population have more high density and extended distributions respectively. If the CH_4 escape rate calculation is correct, Titan is one of the major source of carbon. However, the Cassini Plasma Spectrometer (CAPS) has been no reported detection of carbon-containing ions in outer magnetosphere of Saturn commensurate with the compatible CH_4 torus (Johnson, 2009; Arridge et al., 2011). These observations reflect that fluid models may overestimate the CH_4 escape rate, or an extremely large chemical loss of carbon-containing ions due to dissociative mechanisms has not been found. |
