Canup Robin M, Visscher Channon, Salmon Julien, Fegley Bruce
Planetary Sciences Directorate, Southwest Research Institute, Boulder, CO, 80302.
Chemistry and Planetary Sciences, Dordt College, Sioux Center, IA, 51250.
Nat Geosci. 2015;8:918-921. doi: 10.1038/ngeo2574. Epub 2015 Nov 9.
The Moon may have formed from an Earth-orbiting disk of vapor and melt produced by a giant impact. The Moon and Earth's mantles have similar compositions. However, it is unclear why lunar samples are more depleted in volatile elements than terrestrial mantle rocks, given that an evaporative escape mechanism appears inconsistent with expected disk conditions. Dynamical models suggest that the Moon initially accreted from the outermost disk, but later acquired up to 60% of its mass from melt originating from the inner disk. Here we combine dynamical, thermal and chemical models to show that volatile depletion in the Moon can be explained by preferential accretion of volatile-rich melt in the inner disk to the Earth, rather than to the growing Moon. Melt in the inner disk is initially hot and volatile-poor, but volatiles condense as the disk cools. In our simulations, the delivery of inner disk melt to the Moon effectively ceases when gravitational interactions cause the Moon's orbit to expand away from the disk, and this termination of lunar accretion occurs prior to condensation of potassium and more volatile elements. Thus, the portion of the Moon derived from the inner disk is expected to be volatile depleted. We suggest that this mechanism may explain part or all of the Moon's volatile depletion, depending on the degree of mixing within the lunar interior.
月球可能是由一次巨大撞击产生的环绕地球的蒸汽和熔体盘形成的。月球和地球的地幔成分相似。然而，鉴于蒸发逃逸机制似乎与预期的盘状条件不一致，目前尚不清楚为什么月球样本中的挥发性元素比地球地幔岩石中的更贫乏。动力学模型表明，月球最初是从最外层的盘中吸积形成的，但后来其质量的60%来自于源自内盘的熔体。在这里，我们结合动力学、热学和化学模型表明，月球中挥发性元素的贫化可以通过内盘中富含挥发性物质的熔体优先吸积到地球而不是正在形成的月球来解释。内盘中的熔体最初温度较高且挥发性物质较少，但随着盘冷却，挥发性物质会凝结。在我们的模拟中，当引力相互作用导致月球轨道远离盘时，内盘熔体向月球的输送实际上就停止了，而月球吸积的终止发生在钾和更多挥发性元素凝结之前。因此，源自内盘的那部分月球预计会贫化挥发性元素。我们认为，这种机制可能解释了月球挥发性元素贫化的部分或全部原因，这取决于月球内部的混合程度。
