The Galilean satellites exhibit a clear trend in composition from the rocky Io, close to Jupiter, to the icy and distant Callisto. Proposed causes of this trend can be roughly divided by when the trend developed—either as a result of the material accreted, as a byproduct of the accretion process, or due to the subsequent diverging evolution of the moons. While the first two options have been heavily favored in the existing literature and were therefore studied previously, in this work, we directly address the last of these possibilities. To do so, we determine the range of plausible tidal heating experienced by these moons and how efficiently that energy could be converted into mass loss. We find that while the total tidal energy does exceed the energy required to lose an ice shell, the loss process would have to be highly energy efficient. Examining a range of loss processes, we find that only in extreme cases could enough mass be lost from Europa and no cases where enough mass could be lost from Io. We conclude that this compositional gradient must have been in place by the end of accretion.

伽利略卫星在成分上表现出明显的趋势，从靠近木星的岩石艾奥到冰冷而遥远的卡利斯托。这种趋势的拟议原因可以大致分为趋势发展的时间 - 物质吸积的结果，吸积过程的副产品，或由于卫星随后的发散演化。虽然前两个选项在现有文献中非常受欢迎，因此之前也进行了研究，但在这项工作中，我们直接解决了最后一个可能性。为此，我们确定了这些卫星经历的似是而非的潮汐加热范围，以及这些能量如何有效地转化为质量损失。我们发现，虽然总潮汐能确实超过了失去冰壳所需的能量，但损失过程必须非常节能。检查一系列损失过程，我们发现只有在极端情况下，木卫二才能损失足够的质量，而木卫一不会损失足够的质量。我们得出结论，这种成分梯度必须在吸积结束时就位。