Observations of Titan's mesosphere

We have recorded spectra of Titan's ν₄ band of CH ₄ at a higher resolving power (R = 70,000) than prior measurements to better constrain the thermal structure of Titan's atmosphere from 100 to 600 km altitude. Radiative transfer analyses of the spectra indicate a temperature profile below 300 km that is consistent with past measurements. The high resolving power of our observations provides the first infrared measurement of Titan's thermal structure between 300 and 600 km. We detect the presence of a mesosphere, with a drop of temperature above 380_-100⁺⁵⁰ km altitude of at least 15 K, consistent with radiative cooling of the atmosphere by emission from hydrocarbons.