Infrared limb sounding of Titan with the Cassini Composite InfraRed Spectrometer: effects of the mid-IR detector spatial responses

et al. 2019

The Cassini mission performed 127 targeted flybys of Titan during its 13-year mission to Saturn, culminating in the Grand Finale between April-September 2017. Here we demonstrate the use of the Atacama Large Millimeter/submillimeter Array (ALMA) to continue Cassini's legacy for chemical and climatological studies of Titan's atmosphere. Whole-hemisphere, interferometric spectral maps of HCN, HNC, HC 3 N, CH 3 CN, C 2 H 3 CN, C 2 H 5 CN and C 3 H 8 were obtained using ALMA in May 2017 at moderate (≈ 0.2 , or ≈ 1300 km) spatial resolution, revealing the effects of seasonally-variable chemistry and dynamics on the distribution of each species. The ALMA sub-mm observations of HCN and HC 3 N are consistent with Cassini infrared data on these species, obtained in the same month. Chemical/dynamical lifetimes of a few years are inferred for C 2 H 3 CN and C 2 H 5 CN, in reasonably close agreement with the latest chemical models incorporating sticking of C 2 H 5 CN to stratospheric aerosol particles. ALMA radial limb flux profiles provide column density information as a function of altitude, revealing maximum abundances in the thermosphere (above 600 km) for HCN, HNC, HC 3 N and C 2 H 5 CN. This constitutes the first detailed measurement of the spatial distribution of HNC, which is found to be confined predominantly to altitudes above 730 ± 60 km. The HNC emission map shows an east-west hemispheric asymmetry of (13 ± 3)%. These results are consistent with very rapid production (and loss) of HNC in Titan's uppermost atmosphere, making this molecule an effective probe of short-timescale (diurnal) ionospheric processes.

Section: Cassini Observationsmentioning

confidence: 99%

ALMA Spectral Imaging of Titan Contemporaneous with Cassini's Grand Finale

Cordiner

Teanby

et al. 2019

“…The spectral modeling and fitting used the NEMESIS computer code (Irwin et al 2008), that has previously been widely utilized to model outer planet infrared spectra, including Titan (e.g., de Kok et al 2007ade Kok et al , 2007bTeanby et al 2007Teanby et al , 2009Nixon et al 2009aNixon et al , 2009bNixon et al , 2010Nixon et al , 2012Cottini et al 2012). Details can be found in these publications; only a brief overview of the modeling is given here, emphasizing changes to previous work.…”

Section: Radiative Modeling and Inversionmentioning

confidence: 99%

Detection of Propene in Titan's Stratosphere

Jennings

Bézard

et al. 2013

ApJ

The Voyager 1 flyby of Titan in 1980 gave a first glimpse of the chemical complexity of Titan's atmosphere, detecting many new molecules with the infrared interferometer spectrometer (IRIS). These included propane (C 3 H 8 ) and propyne (CH 3 C 2 H), while the intermediate-sized C 3 H x hydrocarbon (C 3 H 6 ) was curiously absent. Using spectra from the Composite Infrared Spectrometer on Cassini, we show the first positive detection of propene (C 3 H 6 ) in Titan's stratosphere (5σ significance), finally filling the three-decade gap in the chemical sequence. We retrieve a vertical abundance profile from 100-250 km, that varies slowly with altitude from 2.0 ± 0.8 ppbv at 125 km, to 4.6 ± 1.5 ppbv at 200 km. The abundance of C 3 H 6 is less than both C 3 H 8 and CH 3 C 2 H, and we remark on an emerging paradigm in Titan's hydrocarbon abundances whereby alkanes > alkynes > alkenes within the C 2 H x and C 3 H x chemical families in the lower stratosphere. More generally, there appears to be much greater ubiquity and relative abundance of triple-bonded species than double-bonded, likely due to the greater resistance of triple bonds to photolysis and chemical attack.

“…The FP1 detector is held at 170 K, identical to the rest of the instrument optics and mechanical assembly, while FP3/FP4 are cooled to an operating temperature of ∼75 K via a 30-cm radiator pointed at cold space. For calibration therefore, the FP1 detector needs only one temperature reference target (space at 2.73 K), while FP3/FP4 require both reference scans of deep space and also an internal warm shutter at ∼170 K. A more detailed overview of the instrument can be found in the literature [8,10], while more detailed descriptions of the FP1 far-IR interferometer and the FP3/FP4 mid-IR interferometer have been separately published [11,12].…”

Section: The Composite Infrared Spectrometer (Cirs)mentioning

confidence: 99%

Nitrogen in the Stratosphere of Titan from Cassini CIRS Infrared Spectroscopy

Astrophysics and Space Science Proceedings

Teanby

Anderson

et al. 2013

Self Cite

In this chapter we describe the remote sensing measurement of nitrogenbearing species in Titan's atmosphere by the Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft. This instrument, which detects the thermal infrared spectrum from 10-1500 cm −1 (1000-7 µm) 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 N 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 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.