Constraints on Titan's middle atmosphere ammonia abundance from Herschel/SPIRE sub-millimetre spectra

Sub-millimetre spectra measured with Herschel's SPIRE Fourier Transform Spectrometer were used to search for ammonia (NH₃) in Titan's stratosphere. Observations were taken during 2010 and 2011, just after Titan's 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  for altitudes 65–110  km (75 km peak sensitivity), or alternatively a column abundance of . 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 Cassini's INMS instrument, are for the most part also consistent with our observations.