Cassini Composite Infrared Spectrometer (CIRS) Observations of Titan 2004–2017

Abstract

From 2004 to 2017, the Cassini spacecraft orbited Saturn, completing 127 close flybys of its largest moon, Titan. Cassini's Composite Infrared Spectrometer (CIRS), one of 12 instruments carried on board, profiled Titan in the thermal infrared (7-1000 µm) throughout the entire 13-year mission. CIRS observed on both targeted encounters (flybys) and more distant opportunities, collecting 8.4 million spectra from 837 individual Titan observations over 3633 hours. Observations of multiple types were made throughout the mission, building up a vast mosaic picture of Titan's atmospheric state across spatial and temporal domains. This paper provides a guide to these observations, describing each type and chronicling its occurrences and global-seasonal coverage. The purpose is to provide a resource for future users of the CIRS data set, as well as those seeking to put existing CIRS publications into the overall context of the mission, and to facilitate future inter-comparison of CIRS results with those of other Cassini instruments, and ground-based observations.

Temperature and chemical species distributions in the middle atmosphere observed during Titan’s late northern spring to early summer

“…CIRS was a Fourier transform spectrometer acquiring spectra in the farinfrared (from 10 to 600 cm −1 ) with a 3.9-mrad FWHM circular detector on Focal Plane 1 (FP1) and in the mid-infrared through two linear arrays each composed of ten 0.273-mrad field-of-view detectors on Focal Plane 3 (FP3, from 570 to 1125 cm −1 ) and Focal Plane 4 (FP4, from 1050 to 1495 cm −1 ). A detailed description of the CIRS instrument and an overview of the different observing modes are given in Kunde et al (1996), Flasar et al (2004) and Nixon et al (2019). FP3 and FP4 were specially designed to probe the vertical structure of Titan's atmosphere, while these linear detector arrays were positioned parallel to a Titan radius when Cassini was at a distance from Titan's surface of 100 000-200 000 km, so that each detector had a vertical resolution of about one scale height (∼40 km).…”

“…Such observations had an apodized spectral resolution of 15.5 cm −1 . More details on this observation type, called midinfrared limb maps (MIRLMBMAP), are given in Nixon et al (2019).…”

Seasonal changes in the middle atmosphere of Titan from Cassini/CIRS observations: Temperature and trace species abundance profiles from 2004 to 2017

“…Each pixel has a 0.27×0.27 mrad 2 field-of-view corresponding to a vertical resolution varying from 10 to 40 km (comparable to a pressure scale height), depending on the distance of the spacecraft to Titan during the flyby. More details on the CIRS instrument are given in Kunde et al (1996), Flasar et al (2004) and Jennings et al (2017), and more details on the different types of observations are given in Nixon et al (2019).…”

Mapping the zonal structure of Titan's northern polar vortex

“…To minimise the effect of limb brightening around our polar data, we reject observations with a sub-spacecraft-latitude less than 40 • N. This ensures a maximum emission angle of • at the pole. Nixon et al (2019) summarise the CIRS coverage of Titan over the Cassini mission. Applying our criteria we have 18 observations available for this study, ranging from early 2007 through to late 2016 (late northern winter through mid northern summer on Titan).…”

“…The Cassini spacecraft toured the Saturnian system from 2004 through to 2017, performing 127 targeted fly-bys of Titan over this time period, providing almost half a Titan year of observations (Nixon et al, 2019). Titan's northern spring equinox occurred in August 2009, meaning Cassini observes seasonal changes on Titan as northern winter evolves into northern summer.…”