Propane on Titan

We present the first observations of propane (C 3 H 8) on Titan that unambiguously resolve propane features from other numerous stratospheric emissions. This is accomplished using a R = λ/δλ ≈ 10 5 spectrometer (TEXES) to observe propane's ν 26 rotation-vibration band near 748 cm −1. We find a best-fit fractional abundance of propane in Titan's stratosphere of (6.2 ± 1.2) × 10 −7 in the altitude range to which we are sensitive (90-250 km or 13-0.24 mbar). 1. INTRODUCTION Titan's thick atmosphere is simultaneously analogous to and extraordinarily different from that of Earth. Both atmospheres are composed mostly of nitrogen. Both atmospheres have a major component that can exist as a solid, liquid, or gas (water on Earth, methane on Titan). Both have similar vertical temperature-pressure structures , although Titan's atmosphere is ∼200 K cooler and is greatly extended relative to Earth due to much lower surface gravity. On Titan a complicated network of pho-tochemical reactions leads from methane (CH 4) to the formation of numerous heavier hydrocarbons, including propane (C 3 H 8). Measuring the abundances of Titan's hydrocarbons provides an important test of models of Ti-tan's atmospheric chemistry and, more generally, of our fundamental understanding of atmospheric chemical and physical processes.