text: This entity exists on each of the terrestrial planets, as well as at least 3 different moons.
full_question: This entity exists on each of the terrestrial planets, as well as at least 3 different moons. Most of them--including Earth's--are primarily composed of silicate rock, but Triton has one made of ice. The Kola Super Deep Borehole took 20 years to drill here. Subduction occurs when a tectonic plate of the Earth is pushed into this layer, which had upper, lower, and transition zones. For 10 points, name this layer of a planetary body that lies below the crust but above the core, and is typically molten.
first_sentence: This entity exists on each of the terrestrial planets, as well as at least 3 different moons.