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Seasonal Evolution of Titan’s Stratospheric Composition as a Probe of Atmospheric Dynamics at the Equator

Presentation #204.03 in the session Titan II: Atmospheres (Poster + Lightning Talk)

Published onOct 23, 2023
Seasonal Evolution of Titan’s Stratospheric Composition as a Probe of Atmospheric Dynamics at the Equator

The equatorial region of Titan’s atmosphere has intrigued astronomers ever since the Voyager spacecraft flew past Titan in the 1980s. Voyager 1 & 2 observed a sharp change in brightness at Titan’s equator [1, 2], often referred to as the North-South Asymmetry (NSA). The NSA has since been observed by the Hubble Space Telescope [3-5] and in Cassini ISS images (e.g., [6]). The NSA has been shown to vary seasonally [3, 7] and is attributed to the distribution of aerosols in Titan’s stratosphere. There is a sharp brightness contrast near the equator, hinting that there is a barrier to mixing of aerosols over the equator. Stratospheric HCN has also been shown to exhibit a sharp abundance change near the equator [8, 9]. HCN has a lifetime similar in length to a Titan year [10, 11], so can probe seasonal changes in global circulation. Here, we map the latitudinal distribution of some key stratospheric hydrocarbon and nitrile species, including HCN, to further probe the dynamics taking place at Titan’s stratospheric equator.

We use infrared spectra acquired by Cassini’s Composite Infrared Spectrometer (CIRS) instrument [12] which observed Titan for 13 years (2004—2017). Low-spectral-resolution (FWHM ~ 14.5 cm-1) CIRS nadir observations have both high spatial resolution and extensive spatial coverage throughout the entire Cassini mission [13]. We analyse the full CIRS FP3/4 low-resolution nadir data set, which covers the mid-infrared spectral range 580—1500 cm-1. We perform atmospheric retrievals using the Non-linear Optimal Estimator for MultivariatE Spectral AnalySIS (NEMESIS) [14] radiative transfer and retrieval tool. This provides maps of the latitudinal distribution of key hydrocarbon and nitrile species in the mid-stratosphere, at ~5 mbar pressure level. We find that stratospheric HCN has a sharp latitudinal gradient over the equator, in agreement with [8, 9]. We average spectra in 2o latitude bins to improve signal-to-noise: a significant improvement in spatial resolution compared to previous studies (e.g., [8, 9]) which average over latitude bins of size 10o —20o. We find that the HCN gradient varies seasonally, flattening after the northern spring equinox. Towards the end of the Cassini mission, around northern summer solstice, the HCN gradient appears to reverse.

References: [1] Sromovsky et al., 1981; [2] Smith et al., 1981; [3] Lorenz et al., 1997; [4] Lorenz et al., 2001; [5] Roos-Serote, 2005; [6] Roman et al., 2009; [7] Kutsop et al., 2022; [8] Teanby et al., 2006; [9] Teanby et al., 2010; [10] Wilson & Atreya, 2004; [11] Vuitton et al., 2019; [12] Flasar et al., 2004; [13] Nixon et al., 2019; [14] Irwin et al., 2008

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