The deepest hole ever drilled reaches barely 12 km, under 0.2% of the way to the centre, so almost everything we know about the interior comes from seismic waves. A single quake sends waves through the whole planet, and beyond 103 degrees of angular distance the S wave vanishes from the trace because shear waves cannot cross a liquid — that gap is the proof that the outer core is molten. Deeper still, below the molten outer core, sits the solid inner core, 1,220 km in radius.
The deepest hole ever drilled reaches barely 12 km, under 0.2% of the way to the centre, so almost everything we know about the interior comes from seismic waves. A single quake sends waves through the whole planet, and beyond 103 degrees of angular distance the S wave vanishes from the trace because shear waves cannot cross a liquid — that gap is the proof that the outer core is molten. Deeper still, below the molten outer core, sits the solid inner core, 1,220 km in radius.
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