Browsing by Author "Jennings, Donald E."
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Item Cassini Composite Infrared Spectrometer (CIRS) Observations of Titan 2004–2017(The American Astronomical Society, 2019-09-11) Nixon, Conor A.; Ansty, Todd M.; Lombardo, Nicholas A.; Bjoraker, Gordon L.; Achterberg, Richard K.; Annex, Andrew M.; Rice, Malena; Romani, Paul N.; Jennings, Donald E.; Samuelson, Robert E.; Anderson, Carrie M.; Coustenis, Athena; Bezard, Bruno; Vinatier, Sandrine; Lellouch, Emmanuel; Courtin, Regis; Teanby, Nicholas A.; Cottini, Valeria; Flasar, F. MichaelFrom 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 yr mission. CIRS observed on both targeted encounters (flybys) and more distant opportunities, collecting 8.4 million spectra from 837 individual Titan observations over 3633 hr. 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 intercomparison of CIRS results with those of other Cassini instruments and ground-based observations.Item Ethane in Titan’s Stratosphere from Cassini CIRS Far- and Mid-infrared Spectra(The American Astronomical Society, 2019-04-02) Lombardo, Nicholas A.; Nixon, Conor A.; Sylvestre, Melody; Jennings, Donald E.; Teanby, Nicholas; Irwin, Patrick J. G.; Flasar, F. MichaelThe Cassini Composite Infrared Spectrometer (CIRS) observed thermal emission in the far- and mid-infrared (from 10 to 1500 cm⁻¹), enabling spatiotemporal studies of ethane on Titan across the span of the Cassini mission from 2004 through 2017. Many previous measurements of ethane on Titan have relied on modeling the molecule’s mid-infrared ν₁₂ band, centered on 822 cm⁻¹. Other bands of ethane at shorter and longer wavelengths were seen, but have not been modeled to measure ethane abundance. Spectral line lists of the far-infrared ν₄ torsional band at 289 cm⁻¹ and the mid-infrared ν₈ band centered at 1468 cm⁻¹ have recently been studied in the laboratory. We model CIRS observations of each of these bands (along with the ν12 band) separately and compare the retrieved mixing ratios from each spectral region. Nadir observations of the ν₄ band probe the low stratosphere below 100 km. Our equatorial measurements at 289 cm⁻¹ show an abundance of (1.0 ± 0.4) × 10⁻⁵ at 88 km from 2007 to 2017. This mixing ratio is consistent with measurements at higher altitudes, in contrast to the depletion that many photochemical models predict. Measurements from the ν₁₂ and ν₈ bands are comparable to each other, with the ν₁₂ band probing an altitude range that extends deeper in the atmosphere. We suggest that future studies of planetary atmospheres may observe the ν₈ band, enabling shorter wavelength studies of ethane. There may also be an advantage to observing both the ethane ν₈ band and nearby methane ν₄ band in the same spectral window