In this chapter we describe the remote sensing measurement of nitrogen-bearing species in Titan ’ s atmosphere by the Composite Infrared Spectrometer ( CIRS ) on the Cassini spacecraft .
This instrument , which detects the thermal infrared spectrum from 10–1500Â cm ^ { -1 } Â ( 1000–7Â \mu m ) is sensitive to vibrational emissions of gases and condensates in Titan ’ s stratosphere and lower mesosphere , permitting the measurement of ambient temperature and the abundances of gases and particulates .
Three N-bearing species are firmly detected : HCN , HC _ { 3 } N and C _ { 2 } N _ { 2 } , and their vertical and latitudinal distributions have been mapped .
In addition , ices of HC _ { 3 } N and possibly C _ { 4 } N _ { 2 } are also seen in the far-infrared spectrum at high latitudes during the northern winter .
The HC ^ { 15 } N isotopologue has been measured , permitting the inference of the ^ { 14 } N/ ^ { 15 } NÂ in this species , which differs markedly ( lower ) than in the bulk nitrogen reservoir ( N _ { 2 } ) .
We also describe the search in the CIRS spectrum , and inferred upper limits , for NH _ { 3 } Â and CH _ { 3 } CN .
CIRS is now observing seasonal transition on Titan and the gas abundance distributions are changing accordingly , acting as tracers of the changing atmospheric circulation .
The prospects for further CIRS science in the remaining five years of the Cassini mission are discussed .