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

Abstract

The 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