The best constraints on the internal structures of giant planets have historically originated from measurements of their gravity fields^1,2,3. These data are inherently mostly sensitive to a planet’s outer regions, stymieing efforts to measure the mass and compactness of the cores of Jupiter^2,4,5 and Saturn^6,7. However, studies of Saturn’s rings have detected waves driven by pulsation modes within the planet^8,9,10,11, offering independent seismic probes of Saturn’s interior^12,13,14. The observations reveal gravity-mode pulsations, which indicate that part of Saturn’s deep interior is stable against convection¹³. Here, we compare structural models with gravity and seismic measurements from Cassini to show that the data can only be explained by a diffuse, stably stratified core–envelope transition region in Saturn extending to approximately 60% of the planet’s radius and containing approximately 17 Earth masses of ice and rock. This gradual distribution of heavy elements constrains mixing processes at work in Saturn, and it may reflect the planet’s primordial structure and accretion history.

历史上，对巨行星内部结构的最佳限制源自对其重力场的测量^1,2,3。这些数据本质上对行星的外部区域很敏感，阻碍了测量木星^2,4,5和土星^6,7核心的质量和致密性的努力。然而，土星环的研究已经检测到由行星^8,9,10,11内的脉动模式驱动的波，提供了土星内部^12,13,14的独立地震探测器。观测揭示了重力模式脉动，这表明土星内部深处的一部分对对流是稳定的¹³. 在这里，我们将结构模型与卡西尼号的重力和地震测量值进行比较，以表明数据只能用土星中扩散的、稳定分层的核心-包络过渡区来解释，该过渡区延伸到地球半径的大约 60%，包含大约 17 个地球质量冰和岩石。这种重元素的逐渐分布限制了土星的混合过程，它可能反映了这颗行星的原始结构和吸积历史。