Czaplinski Ellen C, Vu Tuan H, Cable Morgan L, Choukroun Mathieu, Malaska Michael J, Hodyss Robert
NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, United States.
ACS Earth Space Chem. 2023 Feb 28;7(3):597-608. doi: 10.1021/acsearthspacechem.2c00377. eCollection 2023 Mar 16.
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, and the extent of Titan's habitability. Previous works have 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 N. 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. X-ray diffraction (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 the evidence of impact, cryovolcanism, or subsurface transport in surface materials.
土卫六是土星最大的卫星，其大气和表面存在大量相互作用的有机化合物。诸如共晶体之类的低温矿物可能会影响土卫六表面的地质过程和化学成分，这反过来又有助于我们理解土卫六可能是如何演化的、表面如何持续变化以及土卫六的宜居程度。此前的研究表明，吡啶：乙炔（1:1）共晶体在特定温度和实验条件下形成；然而，在与土卫六相关的条件下尚未得到证实。我们在此的研究表明，在氮气氛围下，吡啶：乙炔共晶体在土卫六的平均表面温度90K至180K之间是稳定的。特别是，在150K下将各成分混合后，共晶体通过液固相互作用在几分钟内形成，这可由明显的新拉曼谱带和谱带位移证明。X射线衍射（XRD）结果表明，在90 - 150K之间，沿三个主轴的热膨胀呈中等各向异性（约0.5 - 1.1%）。此外，共晶体在暴露于液态乙烷后仍可检测到，这意味着在土卫六上残留乙烷“湿润”的情况下它具有稳定性。这些结果表明，吡啶：乙炔共晶体可能在土卫六上特定的地质环境中形成，这些环境允许有温暖的环境，使液态吡啶能够持续存在，因此，这种低温矿物可能会保留表面物质中撞击、低温火山活动或地下运输的证据。
