Final answer:
The main difference between the two layers of the mantle is their mechanical properties. The lithosphere is a brittle, rigid solid, while the asthenosphere is partially molten and can flow.
Step-by-step explanation:
The main difference between the two layers of the mantle is their mechanical properties. The lithosphere is the outermost mechanical layer and behaves as a brittle, rigid solid. It includes the crust and the uppermost mantle, which are both brittle.
On the other hand, the asthenosphere is partially molten upper mantle material and behaves plastically, allowing it to flow.