Tidally Heated Convection and the Occurrence of Melting in Icy Satellites: Application to Europa
Tidally Heated Convection and the Occurrence of Melting in Icy Satellites: Application to Europa
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DOI：
10.1029/2019je006248
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发表时间：
2020-03
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影响因子：
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通讯作者：
K. Vilella;G. Choblet;W. Tsao;F. Deschamps
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文献类型：
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作者：
K. Vilella;G. Choblet;W. Tsao;F. Deschamps
对冰卫星的观测揭示了广泛的低温火山活动痕迹。由于含水冷岩浆具有负浮力，因此需要两个过程来解释这些观察结果：一种机制是产生足够靠近地表的熔体，另一种机制是将该熔体输送到地表。在这里，我们使用一组 85 个数值模拟，以系统的方式研究熔化的产生，在这些模拟中，我们改变了粘度对比度、瑞利数和潮汐加热速率。应用于欧罗巴，考虑到由纯水组成的水圈，我们的模拟表明，只要冰层厚度（d*）保持适度（15≤d*≤35km），就可以在靠近表面（∼5km）处生成孤立的熔体袋。然而，随着地下海洋中防冻化合物数量的增加，融化的产生变得越来越困难。此外，随着潮汐加热速率的增加，熔化的比例急剧增加。特别是，当潮汐加热速率超过阈值时，达到渐进状态，其中表面热通量保持恒定，表明高于该阈值产生的潮汐热仅用于融化冰壳。在这种情况下，我们发现表面热通量和 d* 之间存在直接关系。最后，我们对木卫二的热状态进行了新的评估。根据现有的限制，我们建议冰壳厚度应超过15公里。然而，d*∼ 15–35 km 意味着潮汐能（> 2 TW）比预期大得多。对我们的结果表明的趋势进行推断表明，更合理的潮汐能（∼1 TW）将涉及 d*∼50-90 km。
Observations of icy satellites have revealed widespread marks of cryovolcanism. Because aqueous cryomagmas are negatively buoyant, two processes are required to explain these observations: one mechanism to generate melt close enough to the surface and another one to transport this melt to the surface. Here, we investigate the generation of melting in a systematic way, using a set of 85 numerical simulations where we vary the viscosity contrast, Rayleigh number, and tidal heating rate. Applied to Europa, considering a hydrosphere composed of pure water, our simulations suggest that isolated melt pockets can be generated close to the surface ( ∼ 5 km) as long as the ice layer thickness ( d* ) remains modest ( 15≤d*≤35 km). However, the generation of melting becomes increasingly difficult as the amount of antifreeze compounds in the subsurface ocean increases. Furthermore, the proportion of melting increases very sharply with increasing tidal heating rate. In particular, when the tidal heating rate exceeds a threshold, an asymptotic regime is reached where the surface heat flux remains constant indicating that the tidal heat generated above this threshold is only used for melting the ice shell. In that regime, we found a direct relationship between the surface heat flux and d* . Finally, we provide a new assessment of Europa's thermal state. Based on available constraints, we propose that the ice shell thickness should exceed 15 km. However, d*∼ 15–35 km implies a tidal power ( > 2 TW) much larger than expected. An extrapolation of the trends suggested by our results indicates that a more reasonable tidal power ( ∼ 1 TW) would involve d*∼ 50–90 km.