There are two kinds of seismic waves: p waves and s waves. P waves are capable of passing through both solid and liquid, and travels faster. S waves are slower, and can't pass through liquid. The speed of these waves are variable to the substance they pass through. Scientists are able to determine the interior surfaces of the Earth by measuring the seismic waves that an earthquake emits. We know that the outer core is liquid because s waves can't penetrate them, and we also know that the inner core is solid because there are some p waves that reflect off the boundary between the inner and the outer core. By measuring the time a seismic wave travels, it gives an impression of how the Earth is composed. When there is a change in the velocity of the wave, then the wave is passing through a different substance.
<span>Much of our understanding of the basic structure and composition of Earth and the other planets in our solar system is not strenuously debated. We can infer a surprising amount of information from the size, mass and moment of inertia of the planets, all of which can be determined from routine astronomical observations. Measurements of surface chemical composition, either by direct sampling (as has been done on Earth, the moon, and Mars) or through spectroscopic observations, can be used to estimate elemental abundances and the degree of chemical differentiation that occurred as the planets condensed from the solar nebula. Remote observations of the gravitational field can be used to understand how a planet's mass is distributed, whereas the strength and shape of the magnetic field provides some constraint on the structure of a metallic core. The specifics of structure and composition, however, are much more debatable. And it is these details that tell us a much more extensive and ultimately more interesting story about the internal dynamics of the planets and their evolution. As a result, trying to determine them is frontier research in almost all fields of earth and planetary science. </span> hope that helped *smiles*
