Instructions: Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.
Input: Context: Above the stratosphere is the mesosphere. Temperatures in the mesosphere decrease with altitude. Because there are few gas molecules in the mesosphere to absorb the Suns radiation, the heat source is the stratosphere below. The mesosphere is extremely cold, especially at its top, about -90o C (-130o F). The air in the mesosphere has extremely low density: 99.9% of the mass of the atmosphere is below the mesosphere. As a result, air pressure is very low (Figure 1.1). A person traveling through the mesosphere would experience severe burns from ultraviolet light since the ozone layer, which provides UV protection, is in the stratosphere below. There would be almost no oxygen for breathing. And, of course, your blood would boil at normal body temperature. Click image to the left or use the URL below. URL:
Question: what is the heat source for the mesosphere?
Output:
the stratosphere