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In the deep interior of the giant planets Jupiter and Saturn, ordinary hydrogen and helium are transformed into a conducting metallic liquid at extremely high pressure. It is likely that the giant planets' observed magnetic field is constantly generated in the metallic fluid core by magnetohydrodynamic processes, converting mechanic energy in the form of convection into magnetic energy. The maximum strength of their magnetic fields is likely to be limited by magnetic field instabilities which convert the magnetic energy back into convection. The parameter which governs the occurrence of magnetic instabilities is the Elsasser number, = B
2/2, where B is the field strength, is the electrical conductivity, is the rotation rate and is the density. Since magnetic instability will be very active when exceeds a critical value
c
10 (the precise value depending on the magnetic field distribution), this imposes an upper bound on the effective electrical conductivity of the metallic fluid which comprises the bulk of Jupiter's interior and much of Saturn's.Stability calculations including both toroidal (model) and poloidal (observed) components of the magnetic field in a rapidly rotating spherical shell, have been performed. The most stable configuration of the field is when the poloidal component of field is strong and the toroidal field is weak; in this case we obtain an upper bound for electrical conductivity of 3 × 106 S/m; while the most unstable configuration of the field is when the toroidal and poloidal fields are comparable, giving rise to
m
3 × 105 S/m. The implications of the results for general dynamo theory are also discussed. 相似文献
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地球难以承载人类重负——《生命行星报告2004》解读 总被引：3，自引：1，他引：3
人类对地球自然资源的利用状况反映人类社会的可持续发展状况，要定量反映人类的可持续发展状况，就需要定量评价人类对地球自然资源的利用状况。世界自然基金会（WWF）等机构每2年发布一次的《生命行星报告》是世界上有影响的对人类对地球自然资源利用状况的定量评价分析研究报告。《生命行星报告2004》运用全球足迹网络（Global Footprint Network）提供的新的科学分析方法比较了世界上150个国家的生态足迹；以更加复杂的数据和分析、更加详细的时间趋势，测算了全球生物多样性的状况。报告肯定，地球的生态超载已经成为一个现实，人类目前消费的自然资源超过地球生产能力的20%，即人类目前使用着1.2个地球，导致了生物多样性的快速减少。 相似文献
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《Chemie der Erde / Geochemistry》2021,81(2):125735
The magma ocean concept was first conceived to explain the geology of the Moon, but hemispherical or global oceans of silicate melt could be a widespread “lava world” phase of rocky planet accretion, and could persist on planets on short-period orbits around other stars. The formation and crystallization of magma oceans could be a defining stage in the assembly of a core, origin of a crust, initiation of tectonics, and formation of an atmosphere. The last decade has seen significant advances in our understanding of this phenomenon through analysis of terrestrial and extraterrestrial samples, planetary missions, and astronomical observations of exoplanets. This review describes the energetic basis of magma oceans and lava worlds and the lava lake analogs available for study on Earth and Io. It provides an overview of evidence for magma oceans throughout the Solar System and considers the factors that control the rocks these magma oceans leave behind. It describes research on theoretical and observed exoplanets that could host extant magma oceans and summarizes efforts to detect and characterize them. It reviews modeling of the evolution of magma oceans as a result of crystallization and evaporation, the interaction with the underlying solid mantle, and the effects of planetary rotation. The review also considers theoretical investigations on the formation of an atmosphere in concert with the magma ocean and in response to irradiation from the host star, and possible end-states. Finally, it describes needs and gaps in our knowledge and points to future opportunities with new planetary missions and space telescopes to identify and better characterize lava worlds around nearby stars. 相似文献
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地磁场源于地球内部的地核发电机,经由近3000 km厚的地幔和地壳到达地面,穿过生物圈、大气层和电离层后延展至太空形成磁层.地磁场对生物圈有双重保护作用:阻挡了高能粒子向内入侵,也避免了氧和水等挥发性物质向外逃逸.尽管地磁场在几十亿年的时间里帮助维持了地球的宜居性,人们仍认为地磁倒转所导致的保护作用削弱会给生物圈带来深刻的负面影响,甚至是生物灭绝.本文梳理地磁倒转与生物灭绝因果关系研究的五十年发展历程,结合历史背景评介早期"一对一"假说的得与失,并着重阐述空间环境变化在最新提出的"多对一"假说中的重要作用.这些研究成果已经清晰地说明,从地核到磁层的地球各圈层是一个耦合的复杂系统,地球演化中的重大事件应当从地球系统科学的角度来看待,并借助比较行星学来研究和理解. 相似文献
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