Double-diffusive convection in Europa's icy shell and subsurface ocean

木卫二是木星的一颗中等大小的卫星，是一个独特的冰体，它显示出各种各样的特征，表明它的表面很年轻，活动范围很广。伽利略号宇宙飞船的磁场数据和哈勃太空望远镜拍摄的潜在喷发羽流的图像为地下海洋的存在提供了令人信服的证据。在水合矿物中，表面的彩色带和破碎的地形被强化，这可能表明地下海洋的组成。这些观察结果提出了关于物质如何通过热液羽流从海底输送到地表的各种假设，并提出了关于传热的问题。在这里，我们建议研究双扩散对流，在这个过程中，温度和成分（或盐度，如果化学成分是盐）都涉及到对流层的动力学，因此对我们旨在理解的木卫二的热演化有重大影响。化学非均质性影响对流的活力、羽流的形式、海洋中分层结构或指状结构的产生和破坏，从而影响热量和物质从内部到表面的输送。我们将首先将冰壳和地下海洋中的对流作为单独的系统进行建模，以研究它们的行为作为其性质的函数。在建立对每个系统的基本理解之后，我们将检查它们的耦合演化，以了解一个系统中的动态如何影响另一个系统。研究冰壳（无限普朗特数）和海洋（低普朗特数）的数值方法有很多（有限体积法、有限元法、谱法）。这些结果将使我们能够(1)约束冰壳的深度，(2)估计混合的时间尺度，(3)推断热通量，(4)为地表特征的形成提供线索。本研究中开发的方法将与研究蕴藏地下海洋的冰体有关，这些冰体是未来探索的潜在目的地。这项研究的预测可以通过即将到来的ESA JUICE和NASA木卫二快船任务的预期观测来验证。这将进一步加深我们对木卫二的物理过程和演化的了解，对这颗卫星的演化、表面环境和可居住性具有重要意义。

Europa, a mid-sized satellite of Jupiter, is a unique icy body which shows a remarkable variety of features, indicating a young surface and widespread activity. Magnetic field data from the Galileo spacecraft and the images of potentially erupting plumes from the Hubble Space Telescope provided compelling evidence for the presence of a subsurface ocean. Colored bands and disrupted terrains on the surface are enhanced in hydrated minerals, potentially indicative of the composition of the subsurface ocean. These observations invoke various hypotheses of how materials are being transported from the seafloor to the surface by hydrothermal plumes, and raises questions on heat transfer. Here we propose to investigate double-diffusive convection, the process in which both the temperature and composition (or salinity if the chemical constituent is salt) are involved the dynamics of the convecting layer, therefore having significant consequences on the thermal evolution of Europa which we aim to understand. Chemical heterogeneity affects the vigour of convection, the forms of plumes, the generation and destruction of stratified or finger structures in the ocean, and thus the transport of heat and materials from the interior to the surface. We will model convection in the icy shell and the subsurface ocean first as separate systems to study their behavior as a function of theirproperties. After establishing a basic understanding of each system, we will examine their coupled evolution to see how the dynamics in one system affect the other. A number of numerical codes (finite volume, finite element, spectral methods) are available to study the icy shell (infinite Prandtl number) and the ocean (low Prandtl number). The results will enable us to (1) constrain the depth of the icy shell, (2) estimate the timescale of mixing, (3) infer the heat flux, and (4) provide clues about the formation of surface features. The approach developed in this research will be pertinent to studying icy bodies harbouring subsurface oceans, which are prospective destinations for future exploration. The predictions from this study can be tested by expected observations from the upcoming ESA JUICE and NASA Europa Clipper missions. This will further our knowledge on the physical processes and the evolution of Europa, having important implications for the satellite's evolution, surface environment, and habitability.