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My current knowledge about how the core of the earth was discovered is the observation made from non returning/reflecting sonar signal when sent to deeper depths through the earth's crust/mantle. How was it inferred that the the core was molten?

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  • $\begingroup$ It is currently thought that there exists an outer core and an inner core. The outer core is molten, but the inner core is thought to be solid. The methods used to determine this are basically described in Ron Jeffries' answer. $\endgroup$ – StephenG Nov 26 '18 at 22:37
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    $\begingroup$ BTW: I (with help) just landed a seismometer on Mars a few hours ago. $\endgroup$ – JEB Nov 27 '18 at 1:52
  • $\begingroup$ When you say 'sonar signals' are 'sent', that suggests that artificial signals are sent using some kind of seismologist's apparatus. That is certainly not the case -- such signals would never make it down that far. Instead, the vibrations caused by earthquakes are monitored by different stations around the world. $\endgroup$ – TonyK Nov 27 '18 at 2:19
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There are basically two types of waves - shear (s) waves and primary (p) compression waves.

P waves can travel through solids and liquids but s waves do not travel in fluids.

That p waves, but not s waves, are detected from events occurring on the other side of the globe tells you that there is fluid in the way.

This is the basic picture; it is complicated/enhanced by refraction and dispersion of the waves.

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