Ice-Ocean interactions and the circulation of salty oceans on Icy Moons

在液态水、一种能源和营养物质的存在下，生命的潜力大大增加。越来越多的证据表明，太阳系(例如木卫二)的一些冰冷卫星上有液态海洋，这使它们处于寻找地外生命的前沿。但目前尚不清楚这些海洋是否含盐，也就是说，是否含有对生命有用的潜在营养物质。从动力学的角度来看，咸海的行为预计与淡水海洋截然不同。这在一定程度上是由于咸水状态方程的非线性(尤其是电容式和热压性)。对于冰下海洋，由于冰-海界面的过程，热量和淡水/盐分的传输也是紧密耦合的，其中一些过程与盐度密切相关(例如冰点)。该项目的工作假设是，通过在外层冰壳和磁场上的签名，可以观察到盐度导致的海洋环流差异，这给了我们一个探测冰冻卫星海洋的化学成分和生命条件的探测器。这个博士项目是朝着这个方向迈出的第一步，解决了以下目标：1.澄清和量化咸水和淡水海洋之间的环流差异，2.确定这些环流在冰壳上的潜在特征(例如，冰更新的速率/模式)很少有研究涉及冰卫星的三维海洋环流。没有一个明确地解释了海洋-冰的相互作用和海洋-冰界面上耦合的热/水/盐通量的真实表示。因此，盐对3D海洋状态的影响尚未得到解决。在比评估冰冻卫星宜居性所需的更短的时间内，目前的工作是在理解这些卫星的动力学和演化方面的一步改变，最终提供给诸如欧洲航天局的木星冰冻卫星探索者果汁等任务。该方法将把数值模拟与最先进的大气环流模型和理想化的海洋理论模型结合起来。

The potential for life is considerably increased in the presence of liquid water, an energy source, and nutrients. There is growing evidence that some of the Icy moons of the solar system (e.g., Europa) harbor liquid oceans, putting them at the forefront of the search for extra-terrestrial life. But it remains unclear whether these oceans are salty, that is, contain potential nutrients useful for life.From a dynamical point of view, a salty ocean is expected to behave in a very differently to a freshwater ocean. This is in part due to the non-linearities of the equation of state for salty water (notably cabelling and thermobaricity). In the case of a subglacial ocean, transports of heat and freshwater/salt are also tightly coupled because of processes at the ice-ocean interface, some of which are strongly salinity-dependent (e.g., freezing point).The working hypothesis of this project is that the differences in ocean circulation due to salinity could be observable through signatures on the outer ice shell and the magnetic field, giving us a probe into the chemical composition of oceans of Icy moons and one of the conditions for life.This PhD project is a first step in this direction by addressing the following objectives:1. Clarify and quantify the differences in circulation between salty and freshwater oceans,2. Determine potential signatures of these circulations on the ice shell (e.g., rates/pattern of ice renewal)Few studies have addressed the 3-dimensional ocean circulation of the Icy moons. None has explicitly accounted for the ocean-ice interaction and a realistic representation of the coupled heat/water/salt fluxes at the ocean-ice interface. As a result, effects of salt on the 3D ocean state have not been addressed. On a shorter term than that required to evaluate the habitability of the icy moons, the present work is a step change in understanding the dynamics and evolution of these moons, eventually feeding into missions such as the ESA's Jupiter Icy Moons Explorer JUICE. The approach will combine numerical modelling with a state-of-the-art General Circulation Model and idealized theoretical models of the ocean.