Modeling ammonia–ammonium aqueous chemistries in the Solar System’s icy bodies
Modeling ammonia–ammonium aqueous chemistries in the Solar System’s icy bodies
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模拟太阳系冰体中的氨-铵水化学反应
DOI：
10.1016/j.icarus.2012.06.016
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发表时间：
2012
期刊：
影响因子：
3.2
通讯作者：
J. Lunine
中科院分区：
文献类型：
--
作者：
G. Marion;J. Kargel;D. Catling;J. Lunine
氨和铵化合物在寒冷的地下盐水中的性质对于理解外行星冰冷卫星的行为非常重要。水化学的FREZCHEM模型主要设计用于低温和高压，但不包含氨和铵化合物。我们在FREZCHEM中添加了氨和铵化合物，并探索了这些化学物质在土卫二和土卫六上的作用，注意到它们的天体生物学意义。在新的FREZCHEM版本中，为氨和铵化合物开发了Na-K-NH 4-Mg-Ca-Fe（II）-Fe（III）-Al-H-Cl-ClO 4-Br-SO 4-NO3-OH-HCO 3-CO 3-CO2-O2-CH 4-NH3-Si-H2O体系的Pitzer参数、体积参数和平衡常数，其温度范围为173- 298 K，压力范围为1- 1000 bar。氨溶解度扩展到173 K，此时NH3·2 H2O和NH3·H2O沉淀，这是现有FREZCHEM版本中的最低温度。土卫二表面的“海洋”在253 K下进行了模拟，气压为1- 10 bar，Na+、Cl-、HCO 3-、CO2（g）、CH 4（g）和NH3（aq）导致冰、NaHCO 3和气体水合物（CO2·6 H2O和CH 4·6 H2O）沉淀。土卫二上的pH值范围为5.74至6.76，水活度aw范围为0.80至0.82，这对生命相对有利。土卫六上的地下“海洋”在173- 273 K的温度范围内用NH 4+、Cl-、SO 42-、CH 4（g）和NH3（aq）模拟，导致（NH 4）2SO 4、冰、CH 4·6 H2O和NH 4Cl的沉淀。CH 4笼形物应该漂浮在盐水之上，并且相对于H2O冰具有浮力，因此有可能成为CH 4的来源，以补充土卫六大气中随着时间的推移而被光化学破坏的物质。泰坦模拟中的pH值（图11）范围为11.24至18.03（后者可能不准确），并从0.28到0.72，这是相对不利的生活，因为我们知道它。泰坦模拟，总压力为10，250和1000巴，导致类似的沉积，除了在1000巴压力下无法形成的冰。在过去，人们一直在争论为什么泰坦，考虑到与地球相似的早期环境，可能是我们太阳系中非常适合生命的天体。但是如果土卫六的海洋是强碱性的，pH值很高，而土卫二的海洋是中等pH值，那么后者似乎是我们所知道的更适合生命的环境。但是请记住，在量化氨/铵的情况时必须谨慎，因为这些化学物质在FREZCHEM模型中的复杂性和局限性。
The properties of ammonia and ammonium compounds in cold, subsurface brines are important for understanding the behavior of outer planet icy moons. The FREZCHEM model of aqueous chemistry was primarily designed for cold temperatures and high pressures, but does not contain ammonia and ammonium compounds. We added ammonia and ammonium compounds to FREZCHEM, and explored the role of these chemistries on Enceladus and Titan, mindful of their astrobiological implications. For the new FREZCHEM version, Pitzer parameters, volumetric parameters, and equilibrium constants for the Na–K–NH4–Mg–Ca–Fe(II)–Fe(III)–Al–H–Cl–ClO4–Br–SO4–NO3–OH–HCO3–CO3–CO2–O2–CH4–NH3–Si–H2O system were developed for ammonia and ammonium compounds that cover the temperature range of 173–298K and the pressure range of 1–1000bars. Ammonia solubility was extended to 173K, where NH3·2H2O and NH3·H2O precipitate, which is the lowest temperature in existing FREZCHEM versions. A subsurface “ocean” on Enceladus was simulated at 253K with gas pressures, 1–10bars, and with Na+,Cl-,HCO3-,CO2(g),CH4(g), and NH3(aq) that led to precipitation of ice, NaHCO3, and gas hydrates (CO2·6H2O and CH4·6H2O). The pH on Enceladus (Fig. 10) ranged from 5.74 to 6.76, and water activity, aw, ranged from 0.80 to 0.82, which are relatively favorable for life. A subsurface “ocean” on Titan was simulated with NH4+,Cl-,SO42-,CH4(g), and NH3(aq) over the temperature range of 173–273K that led to precipitation of (NH4)2SO4, ice, CH4·6H2O, and NH4Cl. The CH4clathrate should float above the brine and is buoyant with respect to H2O ice, so has the potential to be a source of CH4to replenish what has been photochemically destroyed in Titan’s atmosphere over time. The pH in the Titan simulation (Fig. 11) ranged from 11.24 to 18.03 (latter may not be accurate), and awranged from 0.28 to 0.72, which are relatively unfavorable for life as we know it. The Titan simulations, with total pressures of 10, 250, and 1000bars, led to similar depositions, except for ice that failed to form under 1000bars of pressure. In the past, there have been arguments for why Titan, given an early environment similar to Earth, could be a highly favorable body in our Solar System for life. But if Titan oceans are strongly alkaline with high pH whereas Enceladus’ oceans have moderate pH, as simulated, the latter would seem a better environment for life as we know it. But bear in mind, caution must be exercised in quantifying the ammonia/ammonium cases because of the complexities and limitations of these chemistries in the FREZCHEM model.