Compositional heterogeneities in the outer ice shell of Europa – a scale-coupled computational study

研究木星和土星系统的冰冷卫星是科学界的高度期望，因为它将a)提高我们对太阳系演化的理解，b)深入了解这些卫星在地球外孕育生命的潜力。在其内部存在液态水海洋，可能与硅酸盐核心接触（例如木卫二），使这些世界成为寻找宜居环境的理想候选者。如今冰卫星的表面和内部是进化过程的结果。特别令人感兴趣的是与地下海洋直接接触的外层冰壳的演变。冰壳中的化学成分（盐杂质）由于在冰-水界面的反应界面过程而发生变化，例如在冰-海洋边界或在构造或低温火山成因的裂缝系统中。与此同时，外冰通过固态对流的物质输送影响了表面的形态和液态海洋的组成。虽然伽利略近红外测绘光谱仪（NIMS）已经在木卫三、木卫二和木卫四等行星表面观察到成分的非均质性，但对其地下的化学异常却知之甚少。这些非均质性可能导致形成液穴，如果在较长时间内保持稳定，则构成潜在的近地表宜居环境。木星的卫星木卫二是外太阳系地下宜居性的一个特别有趣的候选者。欧空局的JUICE任务和美国宇航局的木卫二快船任务计划分别于2022年和2024年发射，未来的测量将分析木卫二表面非水冰物质的组成。对最近活跃的地表区域的调查将有助于了解地表、冰壳和海洋之间的相互作用，而额外的测量将限制冰壳的厚度、非均质程度和外冰壳中液体袋的存在。因此，有必要更好地了解木卫二外冰壳中化学非均质性的起源和演变，以便对即将到来的空间任务数据提供可靠的解释。本项目的主要目标是开发一种混合、尺度耦合的数值模拟方法，将冰-海洋边界层的计算中尺度模型与固态对流的宏观尺度模型相结合，并应用它来提高我们对冰外壳中化学异常的混合效率和分布的理解。在这个项目中，我们将专注于木卫二的冰壳。然而，我们强调提出的计算策略是通用的，我们计划在未来将我们的研究扩展到其他木星和土星的卫星。

Studying the icy moons of the Jupiter and Saturn system is highly desired by the scientific community, as it will a) improve our understanding of the solar system’s evolution, and b) yield insight into these moon’s potential to harbor life outside Earth. The presence of liquid water oceans in their interior, which may be in contact with the silicate core (e.g., Europa) makes these worlds ideal candidates for the search of habitable environments. The present-day surfaces and interiors of icy satellites are a result of evolutionary processes. Of particular interest is the evolution of the outer ice shell, which is in direct contact with the subsurface ocean. Chemical composition (salt impurities) in the ice shell is subject to change due to reactive interface processes at ice-water interface, e.g. at the ice-ocean boundary or within crack systems of tectonic or cryo-volcanic origin. At the same time, the outer ice affects both the morphology of the surface, and the composition of the liquid ocean by material transport through solid-state convection. While compositional heterogeneities have been observed at the surfaces of e.g., Ganymede, Europa and Callisto by the Galileo Near-Infrared Mapping Spectrometer (NIMS), little is known about chemical anomalies in the subsurface. These heterogeneities may lead to formation of liquid pockets, which, if stable over longer time periods, constitute potential near-surface habitable environments. The Jovian moon Europa is a particularly interesting candidate for subsurface habitability in the outer Solar System. Future measurements of the ESA’s JUICE mission and NASA's Europa Clipper mission scheduled for launch in 2022 and 2024, respectively, will analyze the composition of non-water ice material at Europa's surface. Investigation of recently active surface regions will help understand the interaction between the surface, ice shell and ocean, while additional measurements will constrain the thickness of the icy shell, the degree of heterogeneity and presence of liquid pockets in the outer ice shell. A better understanding of both origin and evolution of chemical heterogeneities in the outer ice shell of Europa is therefore necessary to provide a reliable interpretation of forthcoming space mission data.Our major goal for this project is to develop a hybrid, scale-coupled numerical modeling approach that combines a computational mesoscale model of the ice-ocean boundary layer with macroscale models of solid-state convection, and to apply it in order to improve our understanding of mixing efficiency and distribution of chemical anomalies in the outer ice shell. In this project, we will focus on Europa’s icy shell. Nevertheless, we emphasize that the proposed computational strategy is generic and it is our plan to extend our studies to other Jovian and Saturnian moons in the future.