金星地幔对流与金星散热模式的研究

Despite of their similar sizes and compositions, Venus and the Earth show distinctly different surface tectonics and dynamic evolution. Venus is characterized by one-plate, stagnant-lid mantle convection, while the Earth is controlled by mobile-lid, plate tectonic type of convection. Unlike Earth, Venus has no measureable internal magnetic field. The statistically uniform distribution of impact craters prevents us from identifying older or younger regions and the surface has a global average age of 300-800 Ma. The entire surface of Venus was assumed to be covered by a volcanic event of global scale, even though such an event is not required by the crater distribution. In such models, the resurfacing event was followed by almost no volcanic activity to the present day. The cause for this global resurfacing event and whether it was a single event or one of a periodically recurring set of events is highly uncertain. The volcanism and the surface tectonics of the Earth are primarily associated with the loss of heat from its interior. This loss of heat drives solid state thermal convection in the mantle, and the oceanic lithospheric plates are the thermal boundary layers of mantle convection cells. Subduction of these plates is the primary means of cooling the interior of the Earth; it is responsible for 70% or more of the heat loss from the interior. The most fundamental question is how Venusian mantle loses its radiogenic heat without plate tectonics. Conduction through a stagnant lithosphere is too inefficient. If widespread melting occurs, then a magmatic mechanism is an obvious possibility. Another possibility is that Venus had some type of plate tectonics in past: either episodic overturn of the lithosphere or plate tectonics that subsequently froze. Numerical simulation on mantle convection is a useful tool to understand the dynamic mechanism of the resurfacing event and how Venus loses its internal heat. While convective structure wavelength is often prescribed in regional or 2D models, only 3D global models of mantle convection yield dynamically self-consistent convective structure. For example, stagnant-lid convection with relatively uniform mantle viscosity under the lid that is preferred by regional model, typically contains a large number of mantle plumes that may be inconsistent with the inferred nine mantle plumes for Venus. The episodic major mantle overturn or mantle ''avalanche'' associated with the endothermic phase change was suggested to cause the resurfacing of Venus, based on two-dimension models. However, new 3D models with the endothermic phase change show relatively weak time-dependence. Here we try to formulate 3D global models of mantle convection to explore the cause for the global resurfacing event and how Venus loses its internal heat, in order to enhance our knowledge about the Venus mantle dynamics and its thermal evolution.

金星在大小、质量和组成上与地球非常接近，但金星没有活动的板块构造，没有内生磁场。金星表面的平均年龄大约500±200 Ma，这显示金星发生过全球性表面快速更新。但是金星的表面更新是一次性的还是周期性的、是均匀还是突发或灾难性的过程则存众多争议。板块构造运动和岩浆活动是地球内部热量向外散失的主要方式。与地球的活动板块构造运动相联系的热量散失占地球热量散失总量的70%多。缺少活动板块构造运动的金星，内部热量是如何散失的？是以岩浆喷发的方式还是以某种板块构造活动方式实现的？开展金星地幔对流研究将有助于寻找问题的答案。最近的研究显示要认识金星的表面更新机制和热量散失模式，需要在三维全球地幔对流模型中进行深入研究。本项研究试图通过三维数值模拟的方法研究金星的地幔对流模型，进而揭示金星的表面更新机制以及金星内部热量散失模式，以提高我们对金星的地幔对流及其热演化等动力学问题的认识。

本研究采用有限元数值模拟方法，研究了金星的地幔对流模式及其热演化过程，根据金星地形、重力、表面地质特征和火山活动特征等地质和地球物理观测资料的约束，了解了金星地幔对流模式与地球地幔对流模式的差异以及原因；计算并分析了相变对金星地幔对流的影响，以及相变对金星表面更新的作用；计算了克拉通岩石圈失稳的过程和后果，探讨了岩石圈演化对金星热演化的影响；发展了金星地壳厚度计算的新方法，并计算了金星地壳厚度。提高了我们对金星动力学问题的认识。主要研究进展和成果包括：.1）通过大量数值模型计算，通过对比地质地球物理观测，发现现今金星不存在类似于地球的软流圈，在此基础上，通过大量的计算，了解到相变在金星地幔对流中具有重要影响。地球地幔对流的长波结构主要是软流圈粘性变化造成，但由于金星地幔缺水，金星不存在类似地球的软流圈，金星地幔对流的长波结构主要是吸热相变的影响；.2）早期的的二维数值模型显示，吸热相变对界面上下物质交换的阻碍作用可造成上下地幔物质雪崩似的交换，认为这是造成金星表面灾难性更新的主因。但我们的三维数值模型的结果显示：吸热相变会阻碍上下地幔物质交换，造成冷热物质分别在相变面上下堆积，这种堆积会造成一定程度的高强度物质交换，这与早前二维模型结果类似。但这种作用主要在不同局部区域存在，全球平均意义下，物质交换强度基本稳定。这意味着，与二维模型不同，三维模型不支持相变是造成金星表面灾难性更新的主因；.3）发展了计算金星地壳厚度的新方法。由于金星缺少地震观测，金星地壳厚度只能根据重力和地形资料计算，因此早前计算的地壳厚度与重力和地形都强相关。我们的数值模拟结果显示，金星重力和地形的很大一部分是与金星地幔内部动力学过程相关的，利用重力和地形计算地壳厚度需要去除这部分动力地形的影响。基于地壳均衡和我们的动力学模型，我们发展了新的计算方法，新方法计算的地壳厚度与金星表面地质更加一致；.4）在缺少观测约束，难以开展金星岩石圈演化研究的情况下，开展了地球大陆岩石圈，特别是克拉通岩石圈演化的研究。研究显示，非牛顿流体具有一定浮力的克拉通岩石圈，可以保持一定时间的稳定。当其继续冷却，底部的重力会造成岩石圈应力的扰动，进而造成整个岩石圈失稳。非牛顿流体特性使得整个地幔岩石圈发生快速拆沉，并大量释放地幔热能。从比较行星学的角度，这对进一步理解金星的散热模式和表面更新具有参考意义。