吡啶-乙炔共晶体的实验表征及其对土卫六表面的启示
Experimental Characterization of the Pyridine:Acetylene Co-crystal and Implications for Titan's Surface
摘要：
土卫六是土星最大的卫星，在大气层和表面上有大量的有机化合物，它们相互作用。低温矿物，如共晶，可能会影响土卫六表面的地质过程和化学成分，这反过来又告诉我们土卫六是如何进化的，表面是如何继续变化的，以及土卫六的可居住性程度。先前的研究表明，吡啶：乙炔（1：1）共晶体在特定的温度和实验条件下形成;然而，这尚未在钛相关条件下得到证明。我们的工作表明，吡啶：乙炔共晶体是稳定的，从90 K，土卫六的平均地面温度，高达180 K的气氛下的N2。特别是，共晶形式通过液体-固体相互作用在几分钟内混合的成分在150 K，如由不同的，新的拉曼谱带和谱带位移证明。XRD结果表明，在90-150 K之间，沿三个主轴方向存在适度的各向异性热膨胀（约0.5% - 1.1%）。此外，共晶在暴露于液态乙烷后是可检测的，这意味着泰坦上残留乙烷“润湿”场景的稳定性。这些结果表明，吡啶：乙炔共晶体可以在土卫六上的特定地质背景下形成，允许液体吡啶可以持续存在的温暖环境，因此，这种低温矿物可能保留了撞击，冰火山作用或表面材料中地下运输的证据。
英文摘要：
Titan, Saturn's largest moon, has a plethora of organic compounds in the atmosphere and on the surface that interact with each other. Cryominerals such as co-crystals may influence the geologic processes and chemical composition of Titan's surface, which in turn informs our understanding of how Titan may have evolved, how the surface is continuing to change, as well as the extent of Titan's habitability. Previous work has shown that a pyridine:acetylene (1:1) co-crystal forms under specific temperatures and experimental conditions; however, this has not yet been demonstrated under Titan-relevant conditions. Our work here demonstrates that the pyridine:acetylene co-crystal is stable from 90 K, Titan's average surface temperature, up to 180 K under an atmosphere of N2. In particular, the co-crystal forms via liquid-solid interactions within minutes upon mixing of the constituents at 150 K, as evidenced by distinct, new Raman bands and band shifts. XRD results indicate moderate anisotropic thermal expansion (about 0.5% - 1.1%) along the three principal axes between 90-150 K. Additionally, the co-crystal is detectable after being exposed to liquid ethane, implying stability in a residual ethane "wetting" scenario on Titan. These results suggest that the pyridine:acetylene co-crystal could form in specific geologic contexts on Titan that allow for warm environments in which liquid pyridine could persist, and as such, this cryomineral may preserve evidence of impact, cryovolcanism, or subsurface transport in surface materials.
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