The mesosphere refers to the mantle in the region between the asthenosphere and the outer core. The upper boundary is defined as the sharp increase in seismic wave velocities and density at a depth of 660 km. As depth increases, pressure builds and forces atoms into a denser, more rigid structure; thus the difference between mesosphere and asthenosphere is likely due to density and rigidity differences, that is, physical factors, and not to any difference in chemical composition. At a depth of 660 km, spinel decomposes into perovskite and magnesiowustite. This reaction marks the boundary between upper mantle and lower mantle. This measurement is estimated from seismic data and high pressure laboratory experiments.
The base of the mesosphere includes the D'' zone which lies just above the mantle-core boundary at approximately 2600 to 2700 km. The base of the lower mantle is at about 2900 km.