Why do scientists believe the outer core of Earth is liquid?

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By studying how earthquake waves travel through Earth's interior, scientists have been able to accurately deduce that the outer core of Earth is in the liquid state.  

When an earthquake occurs, it causes a variety of energy waves to begin traveling away from the focus of the earthquake. The different types of waves travel at different speeds. Additionally, certain types of waves are capable of traveling through liquids within the earth while other wave types cannot travel through liquid. Primary waves (p-waves) travel the fastest and can travel through solids, liquids, and gases. Secondary waves (s-waves) travel a little bit slower, but they are not able to travel through liquids or gases. The result is that certain seismographs will register the arrival of both p-waves and s-waves while other seismographs will register only p-waves. Additionally, as the p-waves travel from solid to liquid and back to solid, their wave path gets bent. This means that some seismographs will not register the arrival of either p-waves or s-waves. These areas are called shadow zones. By looking at all of the above information from thousands of earthquakes, scientists are quite certain that Earth's outer core is liquid.  

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