Final answer:
Geologists used the behaviors of P and S seismic waves, which are differently affected by liquid, to confirm the Earth's outer core is liquid. S-waves can't pass through liquid, while P-waves can, revealing the core's state. Radioactive decay keeps the core hot, preventing it from solidifying completely.
Step-by-step explanation:
Geologists were able to confirm that the Earth's outer core is liquid by analyzing the behavior of P and S seismic waves produced by earthquakes. Shear waves or S-waves cannot travel through a liquid and are thus not transmitted through the Earth's core. In contrast, compression waves or P-waves can travel through both solid and liquid materials, including the outer core. The different behaviors of these waves upon reaching the liquid outer core cause a distinctive pattern of refraction and reflection that is observable with seismographs. This seismic data allowed geologists to determine the state of the Earth's outer core.
Additionally, the temperature of the Earth's interior has been estimated using this seismological data. Despite the long cooling period since the Earth's formation, it remains hot due to the radioactive decay of primordial elements. This decay releases heat, which contributes to maintaining the high temperatures inside the Earth and preventing the core from solidifying completely.