how did the scientists come to know that the outer core is liquid

Scientists determined that the Earth's outer core is liquid primarily through the study of seismic waves generated by earthquakes. There are two main types of seismic waves: P-waves (primary waves) which can travel through both solids and liquids, and S-waves (secondary or shear waves) which can travel only through solids. When seismic waves travel through the Earth, scientists observed an S-wave shadow zone, an area where S-waves do not appear after passing through certain layers of the Earth. This disappearance happened beyond a certain depth, indicating that the layer they passed through—the outer core—was not solid, because S-waves cannot travel through liquids. P-waves, however, continue to travel through the outer core but slow down, which also supports that it is a less rigid, liquid layer. Additionally, the presence of Earth's magnetic field supported the idea of a liquid outer core because the magnetic field is generated by the movement of molten iron and nickel in the liquid outer core. These lines of evidence from seismic wave behavior and Earth's magnetic field together confirmed that the outer core is liquid.