What color is the earth? We received this question via our blog at the-earth-story.com/ and i

What color is the earth?We received this question via our blog at http://the-earth-story.com/ and it was a neat enough thought experiment that I thought it deserved a full post.“Earth’s mantle is usually depicted as the color of molten basalt lava, but in fact, as you go down, you start hitting olivine and other green rocks, right? So isn’t the mantle predominantly green, if only one could actually get any light down there?”The first part of this answer requires asking “what is color”? Visible light is part of the “electromagnetic spectrum”, waves of different energy that pass through atoms and space. The human eye can only detect a very tiny sliver of this spectrum – only light that has a wavelength between about 400 and 700 nanometers. Each color that we see corresponds to a certain set of these wavelengths – reddish light is towards the longer wavelengths, while blue and violet are towards the shorter wavelengths.The human eyes are tuned to these wavelengths of light for a reason. Our sun can be approximated as something called a “blackbody” – meaning the wavelength of light it gives off is just a function of its surface temperature. Because that temperature is about 6000K, the most intense light it gives off falls right in the range we see as visible light. Furthermore, the Earth’s atmosphere has compounds like water and CO2 that block some wavelengths of light, especially in the infrared, but no compound in our atmosphere blocks light in the visible wavelengths. The sun gives off light at those wavelengths and the light reaches all the way to the surface so we can see it.Our eyes then split the wavelengths of light into different colors that our brain processes. When an object reflects light at certain wavelengths to our eyes, we perceive the object as that color.The first way to answer this question is to consider an object as a blackbody like lava. A blackbody object that is colder than the sun’s surface will give off light at longer wavelengths than the sun – longer wavelengths than visible light are in the infrared and can’t be perceived by our eyes.Molten basaltic lava is at about 1500K. This lava looks to my eyes like it is red; that’s a function of it behaving like a blackbody. An object at room temperature, about 300K, is giving off most of its light in the far infrared and it gives off so little light in the visible range that we can’t see that light. Night vision goggles detect light in the infrared and so they can see objects giving off light in those wavelengths, even if our eyes cannot.Lava at 1500K gives off most of its light in the infrared, but it is hot enough that it is giving off some light in the visible range. Lava looks generally red because it is giving off more infrared light than red light, but we see the red light more than anything else. In other words, any object at a temperature between about 1000K and 5000K will look mostly red to our eyes.That temperature range basically covers the entire mantle; at 50 kilometers depth around the world it’s about 1000K and at the core-mantle boundary, it’s close to 5000K. If you brought a piece of the mantle up to the surface and kept it at a constant temperature, it would glow somewhere between reddish like lava and maybe yellowish, depending on exactly how our eyes interpret that signal.The dominant signal of light coming from the mantle would be this signal; the rocks would glow as a function of their temperature. Buried in the earth, one crystal glowing would be next to another crystal that could absorb the light emitted by the one next to it, so that light doesn’t carry long distances, but that behavior would be the dominant behavior.Beyond blackbody behavior, when we look at objects that are at lower temperatures, we see a different effect. Different minerals have different colors because they absorb and reflect light at different wavelengths. This picture shows a chunk of a peridotite xenolith, dominated by the mineral olivine with lesser amounts of pyroxenes.Olivine and pyroxenes contain significant amounts of iron and the electrons around atoms of iron interact with light at visible wavelengths. Certain wavelengths of light will be absorbed by the electrons of an iron atom and converted to heat that is radiated away, while other wavelengths are reflected back to our eyes.Olivine reflects light at about 500-600 nanometers, which is in the range our eyes perceive as green, and absorbs light at longer wavelengths. In other words, olivine reflects green light and absorbs red light, so we see the mineral olivine as having a “green” color.Iron is an extremely abundant element in the mantle: the 4th most abundant element after oxygen, silicon, and magnesium. Iron, therefore, dominates the color of the upper mantle. If you took a rock from anywhere in the upper mantle, it would generally have a greenish color, dominated by the way that the element iron interacts with visible light.The same chemistry is thought to dominate the rest of the earth, the lower mantle, but with increasing pressure some interesting things happen to iron. We see iron as green because of the spacing between the d-shell electrons in iron: a response to the shape of the orbitals around the atom. Changing the shape of electron orbitals by compressing an atom changes the way the electrons interact with light.Because the orbital spacing changes with pressure, as the pressure in the Earth increases, the color of the minerals will change. The energy of light absorbed first will move to higher wavelengths, first from green towards green/blue.So, as a consequence of pressure, about 1000 km down, if you ignored temperature, the rocks start looking closer to blue than green. But then some other crazy things happen.About 1500-2000 kilometers in the Earth, the electrons in iron start moving around. They move between a “high-spin” state where electrons are in all the orbitals to a “low-spin” state where the electrons move out of some orbitals. That spin transition doesn’t happen at a single pressure, it happens across a huge range, but as it happens the color is going to change. With this change happening over a large pressure range, the minerals basically go from reflecting green-blue light to probably not reflecting anything in the visible range. The minerals probably get pretty dark deeper in the mantle as a consequence of this spin transition.Finally, iron does one other crazy thing. Iron mostly exists as Iron (II) in the upper mantle, with a charge of +2. But with high enough pressure, iron will shift from 3 atoms with a +2 charge to two atoms with a +3 charge and one atom with a 0 charge. The iron splits into a mix of ferric iron and metallic iron, literally forming free, metal iron in the lower mantle. This change is thought to happen because it reduces the volume of the atoms, a normal response to increasing pressure.This process again would change the way the rock absorbs light. Metallic iron and ferric iron are both darker again, and so the mantle minerals would likely generally get darker and absorb light across the visible spectrum deeper in the mantle.The dominant behavior in the mantle is it behaving as a blackbody as a function of temperature. If you could ignore that behavior, the minerals in the upper mantle would look greenish like this xenolith and in the lower mantle, as iron changes its properties, it would probably get gradually darker and absorb light across more of the visible spectrum. A great question, a complicated answer, and a lot of fun to write. Thanks for getting this far!-JBBImage credit: James St. Johnhttps://flic.kr/p/pMNt2GReferences:http://www.theledlight.com/technical1.htmlhttp://bit.ly/1NAWGW3http://speclab.cr.usgs.gov/PAPERS.refl-mrs/refl4.htmlhttp://bit.ly/1MEpWpD -- source link
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