Earth's rugged lower mantle

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Science  15 Feb 2019:
Vol. 363, Issue 6428, pp. 696-697
DOI: 10.1126/science.aaw4601

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To know a planet is to know its boundaries, where rapid changes in state and/or composition occur. The rock-atmosphere boundary is the one we surface dwellers are most familiar with, but other boundaries lie hidden deep within Earth; for example, the crust-mantle boundary is a change from more silicon-rich rock to denser, more magnesium-rich rock. The transport of heat and rock between the upper and lower mantle largely determines the evolution of our planet, but little is known about this boundary at small scales. On page 736 of this issue, Wu et al. (1) report seismic-array data that suggest the existence of 1- to 3-km ripples along the top of the lower mantle. Such a structure can only be maintained across boundaries with distinct chemistry, indicating that portions of the lower mantle may contain distinct relics from the planet's earliest history.