Constraints on Titanʹs middle atmosphere ammonia abundance from Herschel/SPIRE sub-millimetre spectra

Sub-millimetre spectra measured with Herschelʹs SPIRE Fourier Transform Spectrometer were used to search for ammonia (NH3) in Titanʹs stratosphere. Observations were taken during 2010 and 2011, just after Titanʹs northern spring equinox, which occurred in mid-2009. In our analysis we used high spectral resolution data (0.074 cm−1 apodised) from the SPIRE shortwave spectrometer array (SSW), which provided the best possible signal-to-noise ratio for detecting any NH3 emission features. These data have the most sensitivity to NH3 spectral emission of any currently available observations, although despite this we did not detect any significant emission features above the noise. However, we can place an improved 3-sigma upper limit on NH3 abundance of < 0.19 ppb for altitudes 65–110 km (75 km peak sensitivity), or alternatively a column abundance of < 1.23 × 10 15 molecules / cm 2 . These observations provide modest constraint for future photochemical models and are consistent with most current stratospheric predictions. Scaling of photochemical model profiles, in order to fit elevated abundances observed at 1100 km by Cassiniʹs INMS instrument, are for the most part also consistent with our observations.