Ethane in Titan's Stratosphere From Cassini CIRS Far- and Mid-Infrared Spectra

The Cassini Composite Infrared Spectrometer (CIRS) observed thermal emission in the far- and mid-infrared (from 10 cm−1 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 ay 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 retrieved mixing ratios from each spectral region. Nadir observations of 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 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.

Nicholas A Lombardo, Conor A Nixon, Melody Sylvestre, Donald E Jennings, Nicholas Teanby, Patrick G J Irwin, F Michael Flasar
(Submitted on 6 Aug 2019)

Comments: 19 pages, 10 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Atmospheric and Oceanic Physics (physics.ao-ph)
Journal reference: AJ 157 160 (2019)
DOI: 10.3847/1538-3881/ab0e07
Cite as: arXiv:1908.01926 [astro-ph.EP] (or arXiv:1908.01926v1 [astro-ph.EP] for this version)
Submission history
From: Nicholas Lombardo
[v1] Tue, 6 Aug 2019 01:58:29 UTC (312 KB)
https://arxiv.org/abs/1908.01926
Astrobiology