Europa's Subsurface Ice Observed With The Juno Microwave Radiometer

Jupiter’s moon Europa has long been thought to hide a potentially habitable saltwater ocean beneath a surface ice shell. Radar observations of Europa indicated significant scattering7, and observations by the Galileo spacecraft showed regions of ice disruption (chaotic terrain) leading to speculation that the radar scattering may be related to subsurface cracks, pores or bubbles which, if present, could provide a pathway for oxygen and nutrient transport1,2. Estimates of the ice shell thickness range from 3 km to over 30 km3,4. Current theories predict that Europa has a regolith with pores in the ice extending to a depth between a few meters below the surface (for an impact-generated regolith) and a few km below the surface (for a tidally-generated regolith)5,6. Estimates of the pore sizes range from 1 mm to 1 m.7 Here we use microwave sounding measurements obtained during the Juno spacecraft’s flyby of Europa in September 2022 to show that Europa’s conductive ice shell, if pure water ice, is 36 ± 10 km thick, with scatterers predominantly less than a few cm in radius, extending hundreds of meters below the surface. The thickness of the ice shell and the nature of the regolith are of fundamental importance in understanding potential habitability and communication between the internal ocean and Europa’s surface.

木星的卫星木卫二（Europa）长期以来被认为在其表面冰壳之下隐藏着一片潜在可居住的咸水海洋。针对木卫二的雷达观测显示出显著的散射现象7，而伽利略号（Galileo）航天器的观测则发现了冰体扰动区域（混乱地形），由此引发推测：雷达散射可能与地下裂缝、孔隙或气泡相关——若此类结构存在，便可为氧气与营养物质的输送提供通道1,2。冰壳厚度的估算范围为3千米至30千米以上3,4。当前理论预测，木卫二的表岩屑（regolith）中冰孔隙的分布深度，从地表下数米（撞击成因的表岩屑）至地表下数千米（潮汐成因的表岩屑）不等5,6。孔隙尺寸的估算范围为1毫米至1米7。本研究利用2022年9月朱诺号（Juno）航天器飞越木卫二期间获取的微波测深测量数据，证实：若木卫二的导电冰壳为纯水冰，则其厚度为36±10千米，且散射体的半径普遍小于数厘米，分布深度可达地表下数百米。冰壳厚度与表岩屑的性质，对于理解木卫二内部海洋的潜在可居住性，以及内部海洋与木卫二表面之间的物质连通机制，具有至关重要的科学意义。