为什么地球大气中表层以上的粒子密度比地面和以下的小得多?相反，为什么地球不是一个简单的由旋转的气体/液体组成的球，当你靠近核心时，它的密度会逐渐增加(因为引力在靠近核心时更强)?< / p >
如果电磁力没有这样做，就只会有一个星系大小的氢和氦核团，正如我们在暗物质晕中所看到的(引力作用而不是电磁作用)是电磁相互作用形成了化学键，以及不同化学物质之间的不同类型的相互作用，解释了为什么在地壳和大气之间，或海洋和大气之间的边界，密度会发生急剧变化。类似的阶梯式变化也存在于海洋和地壳之间，以及地球内部地幔和地核之间
粗略地说，当你尝试将油和水结合在一起时，这是同样的事情，使密度阶函数:
这些成分不混合或相互溶解，所以重力使密度更大的物质-水-沉降到底部。密度作为高度的函数，从油密度上升到水密度，当你走到界面下面。以地球为例，有几个密度逐渐增大的相不能混合。密度梯度中最大的一步是当岩石相(橄榄石、尖晶石、钙钛矿和铁方石(下图未显示)让位给密度更大的核心时。在下面的插图中，这个边界是岩石地幔的底部，D"层，遇到主要是铁的核心。< / p >
One major difference between the interior of Earth and the oil-water combination in the first picture is that much of Earth's interior is solid, so it takes a lot of heat and pressure -- both generated by gravity -- to make the materials yield and flow to their equilibrium positions. The same is true of other predominantly solid celestial bodies, so only relatively large and massive ones have enough gravitational power to enable separation of different density phases. Planetary scientists call this process differentiation.
The reason for the elemental difference in the planets is partly due to the make-up and distribution within the accretion disk, the swirl of elements that existed when our solar system formed. Also, planets that are as big as Jupiter have such a strong gravitational pull that they are able to hold onto (and pull in more) of the lighter elements like hydrogen and helium, which are the most abundant in the universe. Jupiter is actually like a half-there star, sometimes referred to as a 'failed star'.
However, the Earth is still large enough and has a protective magnetic shield generated by its iron core that allows its atmosphere not to be blasted away by the solar wind. This means we have both a stable gaseous atmosphere as well as a metallic and rocky interior, meaning a step-change in densities between solids and gases.
It is also worth noting though that our gaseous atmosphere is only 60 miles thick, whereas the Earth's radius is about 4,000 miles. The planet is much more of a homogenous solid than you might think, being as we are atmosphere dwellers.
The differentiation phenomemon is also responsible for plate tectonics; formation of continents, earthquakes, life.