Question: How do you know what it is inside of our planet, if noone has ever been there? Suppositions?
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Hi and thank you for the answer! I really appreciate!
About those waves, i dont think they travel through all the layers, do they?
And if they don’t, how is it possible to know what’s in there?-
Hi Danut, You are very welcome. Surface waves would only travel through the surface layers, while body waves travel throughout. 🙂
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Hi donut!
you are very welcome, especially because this is a crucial point in geoscience.
Interestingly, there are many different types of seismic waves, that can be classified according to the kind of motion they induce to the material they propagate in. Some of them, for example, can propagate only in fluids and not in rocks, or the other way around, but some can actually travel all kind of materials, fluid, rocky, sandy, so we can actually measure these waves literally on the other side of the Earth, with respect to where they started by the earthquake. From this kind of measurements we can understand which materials have been in between the source and the seismograph we are looking at.
I found this sketch on the internet (at the very end of the page), I think it may help understanding how waves propagate in the different parts of the Earth interior:
http://www.cyberphysics.co.uk/topics/earth/geophysics/Seismic%20Waves%20Reading.htm -
Hi Danut:
You are right. The P waves (the primary waves and fastest) travel through all the layers. However, the S waves (secondary and slowest) do not travel through liquid. That is one of the reasons to suggest that the outer core is liquid. Because S waves are not recorded coming out of that region. P waves that travel through that region are slow. In addition, modelling of temperature, pressure and minerals at that depth help to suggest what is down there.
danut
Hi danut, that is a really great question! There are a few ways that we know about the structure and composition of our planet. First, we do have rocks on the surface that have been collected from deep within the planet. Things such as kimberlite pipes, which are associated with diamond formation, bring up rocks from deep within the earth. Other things like ophiolite complexes do the same thing. Also, when earthquakes occur they send seismic waves throughout the layers of the Earth and these waves can be monitored by geologists at the surface. How fast these waves travel can tell us about the composition of the layers of the Earth. Waves travel differently through solids, liquids, sediments, and different densities, and we can tell what they traveled through by the time it takes for a wave to go from one part of the world to another.
Another way is by ocean drilling vessels that drill cores deep into the Earth and bring back up rock and sediments for us to analyze. I probably missed something, but I hope that at least helps to get you started!
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Hi danut:
That is a good question that I also ask myself. You are right. No one has travelled to the interior of the Earth. In fact, no one has drilled to the mantle, the layer of rock just underneath the crust. Science works in the way that we use the evidence that we have to make up the best answer. So, in the case of the interior of the Earth, we have to rely on what we know about the properties of seismic waves and how they travel through the interior of the Earth. For example, we can determine was is the velocity of seismic waves as they travel through different materials. If the waves travel slower, then we can guess that the material is hotter, if the waves travel faster, then we can guess that the material is colder. If some of the waves do not propagate at all through the material, then the material is a liquid and not a solid. So, seismologists study how seismic waves (generated by large earthquakes) travel through the interior of the Earth and are recorded on seismometers. When they analyze the velocity and the types of waves that reach the seismometers (usually the use instruments that are located all over the Earth), they can make a map of the interior of the Earth.
In addition, geoscientists look at xenoliths (rocks that formed very deep in the mantle that were brought to the surface by hot magma) to study the minerals that formed at depth. Some geoscientists make experiments in the lab and expose minerals to high temperatures and pressures that would be similar to what the conditions that we should expect in the interior of the Earth. Other scientists use theoretical models based on physics and chemistry to guess what is at the core. So, it is the combination of all these data that we use to give the best answer about what is inside the Earth.
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Hi donut, interesting question indeed 🙂 Some evidence comes of course from drilling on various continents and oceans, we can see and analyse what kind of rocks we are drilling. However, this way we can just scratch the skin of the Earth. Imagine that the deepest hole ever drilled is Russia, and is 12km deep. That seems like a lot but the earth radius is about 6370 km! To know what’s deeper inside the Earth we need to model how seismic waves propagate deep inside, as my colleagues already explained. The idea is simple: imagine music is playing from the stereo in the living room. If you go around in the house, music will sound different in different rooms, depending on the thickness of the walls, the distance from the stereo, the shape of the rooms… We do the same with the earth: we listen to the earthquakes (not necessarily the big ones, even “small” earthquakes, with no consequences for people, can provide a lot of information on the Earth interior, especially, for example, if they are generated very deep) from around the earth surface, and from the “sound” that we hear we can reconstruct how the inside of the earth is done… at least in theory, since this is what is called an “inverse problem”, a class of problems that is found in many fields of science, and a difficult one! While we are quite sure about the general composition and structure of the earth interior, the details are often a lot less well understood.
There is still a lot we do not know about our own planet! That is also the reason why we also try to look at other planet’s structures, to better understand ours.
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Hi Danut,
That is really interesting and when I was younger, I had asked it to a fellow scientist 🙂
Most of the information comes from research through drilling and rocks which have surfaced on the Earth.
The other scientists managed to answer the question before me, so I would not really have much to add! 🙂
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