Ethane in Titan’s Stratosphere from Cassini CIRS Far- and Mid-infrared Spectra

Journal Article · Mon Apr 01 04:00:00 EDT 2019 · The Astronomical Journal (Online)

DOI:https://doi.org/10.3847/1538-3881/AB0E07· OSTI ID:22897331

Lombardo, Nicholas A. ^[1]; Nixon, Conor A.; Jennings, Donald E.; Flasar, F. Michael ^[2]; Sylvestre, Melody; Teanby, Nicholas ^[3]; Irwin, Patrick J. G., ^[4]

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1. Center for Space Science and Technology, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD (United States)
2. Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD, 20770 (United States)
3. School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ (United Kingdom)
4. Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU (United Kingdom)

The Cassini Composite Infrared Spectrometer (CIRS) observed thermal emission in the far- and mid-infrared (from 10 to 1500 cm{sup −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 ν {sub 12} band, centered on 822 cm{sup −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 ν {sub 4} torsional band at 289 cm{sup −1} and the mid-infrared ν {sub 8} band centered at 1468 cm{sup −1} have recently been studied in the laboratory. We model CIRS observations of each of these bands (along with the ν {sub 12} band) separately and compare the retrieved mixing ratios from each spectral region. Nadir observations of the ν {sub 4} band probe the low stratosphere below 100 km. Our equatorial measurements at 289 cm{sup −1} show an abundance of (1.0 ± 0.4) × 10{sup −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 ν {sub 12} and ν {sub 8} bands are comparable to each other, with the ν {sub 12} band probing an altitude range that extends deeper in the atmosphere. We suggest that future studies of planetary atmospheres may observe the ν {sub 8} band, enabling shorter wavelength studies of ethane. There may also be an advantage to observing both the ethane ν {sub 8} band and nearby methane ν {sub 4} band in the same spectral window.

• OSTI ID: 22897331
• Journal Information: The Astronomical Journal (Online), Journal Name: The Astronomical Journal (Online) Journal Issue: 4 Vol. 157; ISSN 1538-3881
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
COMPARATIVE EVALUATIONS
COMPUTERIZED SIMULATION
ELEMENT ABUNDANCE
EMISSION
ETHANE
FAR INFRARED RADIATION
INFRARED SPECTRA
INFRARED SPECTROMETERS
INTERMEDIATE INFRARED RADIATION
METHANE
MIXING RATIO
MOLECULES
PHOTOCHEMISTRY
PLANETARY ATMOSPHERES
PLANETS
SATELLITE ATMOSPHERES
SATELLITES
STRATOSPHERE