Thermal Evolution and Magnetic Field Generation in Terrestrial Planets and Satellites
Thermal Evolution and Magnetic Field Generation in Terrestrial Planets and Satellites
复制标题
类地行星和卫星的热演化和磁场产生
DOI：
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
T. Spohn
中科院分区：
文献类型：
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作者：
D. Breuer;S. Labrosse;T. Spohn
在类地行星中，地球和水星有自我维持的磁场，而火星和金星没有。在木卫三上记录的磁场数据被解释为磁场自生的证据。其他冰冷的伽利略卫星在其地下海洋中有磁场，而木卫一和土星卫星土卫六显然完全没有内部起源的磁场。月球的部分地壳和火星的部分地壳一样，仍然被磁化。虽然人们普遍认为火星地壳的磁化是由早期的磁场引起的，但对月球的另一种解释将磁化与大撞击产生的等离子体联系起来。在类地行星和卫星上，发电机的必要条件是存在一个富铁的核心，这个核心正在经历剧烈的流体运动。人们普遍认为流体运动是由热浮力或化学浮力或两者共同驱动的对流引起的。化学浮力随着内核的生长而释放。后者需要在核心中有一种轻合金元素，随着固体内核的生长，这种元素在外核中富集。在大多数模型中，轻合金元素被假定为硫，但其他元素，例如氧、硅和氢也有可能。类地行星中地核的存在要么被证明是毫无疑问的（地球、火星和水星），要么像金星和月球一样，地核的存在是令人信服的。伽利略卫星木卫一和木卫三很可能有核心，这是根据伽利略对这些卫星重力场的无线电跟踪数据判断的。木卫二的情况就不那么明朗了。木卫四被广泛认为是未分化或部分分化的，因此缺乏富含铁的核心。土卫六是否有地核目前还不清楚。有磁场的类地行星要么有一个已知半径和密度的内核，如地球，要么被广泛认为有一个内核，如水星。金星、火星和月球（缺乏磁场的类地天体）没有内核，虽然被认为是可能的，但还没有得到很好的证实。火星内核的组成可能接近Fe-FeS共晶，这将阻止内核的生长，只要内核没有冷却到1500开尔文左右的温度。金星可能正处于形成内核的边缘，在这种情况下，化学发电机可能会在不久的将来开始运转。火星和月球地壳的剩余磁化是火星和月球早期演化中存在发电机的证据，并表明强大的热驱动发电机是可能的。热动力和化学动力都要求地核以足够的速率被地幔冷却。对于热驱动发电机，从地核流入地幔的热流必须大于沿地核绝热传导的热量，才能使地核对流。对于像水星、木卫三和月球这样的小行星来说，这个阈值是几mW m - 2，但对于地球和金星来说，这个阈值可能高达几十mW m - 2。两个发电机的浮力必须足够强以克服欧姆耗散。在地球上，板块构造和地幔对流有效地冷却了地核。火星和金星上停滞的盖子对流冷却地核的效率较低，但这是可能的，并且已经提出，火星在其早期演化中有板块构造，金星经历了间歇性的表面重塑和地幔翻转。两者都可能对这些行星内核的演化产生深远的影响。甚至有可能，内核在火星和金星上开始生长，但随着板块构造和表面重新形成的停止，地幔的加热使生长受到阻碍。木卫三磁场的产生存在广泛的争议。模型的范围从磁流体动力学对流（在这种情况下，磁场将不能自我维持）到化学和热驱动发电机。木卫三核心可能组成的广泛范围允许模型具有完全液态的近共晶Fe - fes组成，以及具有铁内核或铁雪内核的模型。
Of the terrestrial planets, Earth and Mercury have self-sustained fields while Mars and Venus do not. Magnetic field data recorded at Ganymede have been interpreted as evidence of a self-generated magnetic field. The other icy Galilean satellites have magnetic fields induced in their subsurface oceans while Io and the Saturnian satellite Titan apparently are lacking magnetic fields of internal origin altogether. Parts of the lunar crust are remanently magnetized as are parts of the crust of Mars. While it is widely accepted that the magnetization of the Martian crust has been caused by an early magnetic field, for the Moon alternative explanations link the magnetization to plasma generated by large impacts. The necessary conditions for a dynamo in the terrestrial planets and satellites are the existence of an iron-rich core that is undergoing intense fluid motion. It is widely accepted that the fluid motion is caused by convection driven either by thermal buoyancy or by chemical buoyancy or by both. The chemical buoyancy is released upon the growth of an inner core. The latter requires a light alloying element in the core that is enriched in the outer core as the solid inner core grows. In most models, the light alloying element is assumed to be sulfur, but other elements such as, e.g., oxygen, silicon, and hydrogen are possible. The existence of cores in the terrestrial planets is either proven beyond reasonable doubt (Earth, Mars, and Mercury) or the case for a core is compelling as for Venus and the Moon. The Galilean satellites Io and Ganymede are likely to have cores judging from Galileo radio tracking data of the gravity fields of these satellites. The case is less clear cut for Europa. Callisto is widely taken as undifferentiated or only partially differentiated, thereby lacking an iron-rich core. Whether or not Titan has a core is not known at the present time. The terrestrial planets that do have magnetic fields either have a well-established inner core with known radius and density such as Earth or are widely agreed to have an inner core such as Mercury. The absence of an inner core in Venus, Mars, and the Moon (terrestrial bodies that lack fields) is not as well established although considered likely. The composition of the Martian core may be close to the Fe–FeS eutectic which would prevent an inner core to grow as long as the core has not cooled to temperatures around 1500 Kelvin. Venus may be on the verge of growing an inner core in which case a chemical dynamo may begin to operate in the geologically near future. The remanent magnetization of the Martian and the lunar crust is evidence for a dynamo in Mars’ and possibly the Moon’s early evolution and suggests that powerful thermally driven dynamos are possible. Both the thermally and the chemically driven dynamo require that the core is cooled at a sufficient rate by the mantle. For the thermally driven dynamo, the heat flow from the core into the mantle must by larger than the heat conducted along the core adiabat to allow a convecting core. This threshold is a few mW m−2 for small planets such as Mercury, Ganymede, and the Moon but can be as large as a few tens mW m−2 for Earth and Venus. The buoyancy for both dynamos must be sufficiently strong to overcome Ohmic dissipation. On Earth, plate tectonics and mantle convection cool the core efficiently. Stagnant lid convection on Mars and Venus are less efficient to cool the core but it is possible and has been suggested that Mars had plate tectonics in its early evolution and that Venus has experienced episodic resurfacing and mantle turnover. Both may have had profound implications for the evolution of the cores of these planets. It is even possible that inner cores started to grow in Mars and Venus but that the growth was frustrated as the mantles heated following the cessation of plate tectonics and resurfacing. The generation of Ganymede’s magnetic field is widely debated. Models range from magneto-hydrodynamic convection in which case the field will not be self-sustained to chemical and thermally-driven dynamos. The wide range of possible compositions for Ganymede’s core allows models with a completely liquid near eutectic Fe–FeS composition as well as models with Fe inner cores or cores in with iron snowfall.