The potential habitability of Jupiter's moon Europa has motivated two missions: NASA's Europa Clipper and ESA's JUpiter ICy moons Explorer (JUICE). Both missions are equipped with ice-penetrating radars which will transmit radio waves into the subsurface, recording reflections from interfaces defined by contrasts in ice shell dielectric properties. Assuming an MgSO₄ ocean, we show that salt layers, formed through the freezing of subsurface liquid water reservoirs, can be detected by ice-penetrating radar instruments on Europa Clipper and JUICE. Furthermore, because these features are thermodynamically stable within the minimally attenuating portion of Europa's ice shell, referred to here as the “pellucid region,” they could produce brighter reflections than deeper liquid water interfaces. We demonstrate how ice-penetrating radar measurements of salt layer thickness could establish lower bounds on the parameter space of possible initial reservoir thickness and salinity, constrain the origin of reservoirs (ice shell melt vs. ocean injection), and—if sourced through ocean injection—the ocean salinity.

中文翻译：

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木卫二冰壳中盐层的雷达表征作为了解关键冰海交换过程的窗口


木星卫星木卫二的潜在宜居性激发了两项任务：NASA 的 Europa Clipper 和 ESA 的 JUpiter ICy 卫星探测器 （JUICE）。这两项任务都配备了穿冰雷达，该雷达将无线电波传输到地下，记录由冰壳介电特性的对比定义的界面的反射。假设存在 MgSO ₄ 海洋，我们表明，通过地下液态水储层冻结形成的盐层可以被 Europa Clipper 和 JUICE 上的穿冰雷达仪器检测到。此外，由于这些特征在木卫二冰壳的最小衰减部分（此处称为“透明区域”）内具有热力学稳定性，因此它们可以产生比更深的液态水界面更亮的反射。我们展示了盐层厚度的穿冰雷达测量如何确定可能的初始储层厚度和盐度的参数空间下限，限制储层的来源（冰壳融化与海洋注入），以及 - 如果通过海洋注入获得 - 海洋盐度。