The sharp boundary between the orange and blue colored atmosphere

NASA undated handout image of the moon was photographed by the Expedition 28 crew aboard the International Space Station. The Earth transitions into the orange-colored troposphere, the lowest and most dense portion of the Earth's atmosphere. The troposphere ends abruptly at the tropopause, which appears in the image as the sharp boundary between the orange and blue colored atmosphere. The silvery-blue noctilucent clouds extend far above the Earth's troposphere.