RC寂静：地幔（结合木寻云整理JJ + 版内已有JJ + 自己找的英文背景）

HHC1203

虽然不是原文，而且还缺了第二段，及第三段最后（希望有强者补齐）
不过还是希望对大家有帮助～～

2、地幔
一种现象：地幔(lower mantle)位于crust和core之间，分了很多layer。重点比较了mantle上部和下部硬度不同的原因及猜想假设 (Q1)。下地幔的构成并不是科学家直接观察得到，而是通过其它间接方式推出的，比如：科学家通过测地震波来了解地幔的机构；还通过一个xxx方法了解地幔结构。

地幔(lower mantle)位于crust和core之间，分了很多layer。重点比较了mantle上部和下部硬度不同的原因及猜想假设 (Q1)。下地幔的构成并不是科学家直接观察得到，而是通过其它间接方式推出的，比如：科学家通过测地震波来了解地幔的机构；还通过一个xxx方法了解地幔结构。
提出旧理论及其缺失：物质M，属于结晶(crystal)物质P的一种，在地幔里面随深度增加质量增加。但是这个物质本身就挺紧实的，所以不太可能里面的密度(density)比外面大。所以可能是里面这种物质的量比较多，但是又不太可能，因为里面很热，流动性强，不会一个东西很大量地堆在一起。所以推出说这个说法也不对。

提出新理论：科学家在高温高压实验条件下合成了一个post-P物质，这种物质的发现推翻了第二段关于P物质的结论。Post-P物质密度(density)比P大，晶体结构不同。而且因为post-P比P的导热性能(conduct heat)好，所以推断地核(core)以前比我们想象的更热，是从1 million year之前才冷却凝固的(solidify)，与之前结论不同。

此外，地核(inner core)可以带来磁场，磁场阻止射线对地球上的生物的伤害。因此有助于了解地球上生命的起源比预计早或晚了xx年。

Q1：第一段的作用？
答案：给读者提供背景知识

Q2：内核固化有什么影响？
答案：影响到达地球射线的情况。

Q3：(?)
有个选项选了带conducting的

*Q4：以下哪点支持(support)了post-P物质的存在？



Q5：关于物质Prexxx下列哪种说法正确？

我选了它在沉入地核(core)的过程中会逐渐变为post-Prexxx。

其他有一个选项是，Prexxx比post-Prexxx密度高(denser)。



英文背景知识：（地幔介绍）
The Earth's mantle is a 2,900 km (1,800 mi) thick shell of compressed and heated rock, beginning below the Earth's crust (lithosphere), which extends 5 km
(3.1 mi) below the ocean floor and 30 to 50 km (19- 31 mi)below the continents. The Earth's mantle makes up 70% of Earth's volume, in comparison to the Earth's crust which makes up less than 1% of the total. In fact, the crust is just a thin layer of frozen rock shielding the mantle from outer space. The crust and mantle are separated by a transition area called the Mohorovi?i? discontinuity (the "Moho") where a certain type of seismic wave quickly speeds up when transiting through.
Like the crust, the mantle is largely composed of oxide compounds such as olivine, pyroxenes, spinel, garnet, peridotite, and eclogite. The mantle differs in its chemical ratios from the crust, however. It is composed of roughly 45% oxygen, 23% magnesium, 22% silicon, 6% iron, 2% aluminum, 2% calcium, with trace amounts of sodium, potassium, and other elements. Like the crust, the mantle can broadly be regarded as silicate. Below the mantle are the outer core and inner core of the Earth, making up about 29% of the Earth's volume, and composed primarily of molten (outer core) or solid (inner core) iron and nickel.

英文背景知识：（如何探测地幔成分及构造）
The Earth has a radius of 6371 km, yet the deepest boreholes reach to a depth of only 12 km. As we cannot probe the inner Earth directly, our knowledge of its interior stems mainly from indirect evidence. Studies of the chemistry of the Solar System and the composition of meteorites can give us broad bounds on the chemistry of the Earth as a whole. Geochemical evidence from magma and rock fragments brought tectonically to the surface from depths of 200 km
together with geophysical evidence such as density and magnetic studies and, in particular, seismic studies allow us to constrain further the Earth’s composition and structure.

英文背景知识：（关于第三段的实验）
A team of scientists, made the first laboratory study of the deformation properties of a high-pressure silicate mineral named post-perovskite. McNamara, a geophysicist, modeled the stresses the mineral would typically undergo as convection（對流）

"This the first time the deformation properties of this mineral have been studied at lower mantle temperatures and pressures," says McNamara. "The goal was to observe where the weak planes are in its crystal structure and how they are oriented." The results of the combined laboratory tests and computer models, he says, show that post-perovskite doesn't fit what is known about conditions in the lowermost mantle.

Earth's mantle is a layer that extends from the bottom of the crust, about 25 miles down, to the planet's core, 1,800 miles deep. Scientists divide the mantle into two layers separated by a wide transition zone centered around a depth of about 300 miles. The lower mantle（下地幔）lies below that zone.

Most of Earth's lower mantle is made of a magnesium silicate mineral called perovskite. In 2004, earth scientists discovered that under the conditions of the lower mantle, perovskite can change into a high-pressure form, which they dubbed post-perovskite. Since its discovery, post-perovskite has been geophysicists' favorite candidate to explain the composition of a mysterious layer that forms the bottom of Earth's lower mantle.

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