Aerosol properties in the atmosphere of Titan from Cassini/UVIS observations of haze scattering

The opaque haze layers of Titan directly influence the global temperature and photochemistry, and chemical reactions might also be driven by heterogeneous processes involving haze particles [Lavvas et al., 2008, 2011]. Moreover, haze is an essential component for understanding cloud formation and the following meteorological and chemical cycles [McKay et al., 2001, Barth and Toon, 2003, Luz et al., 2003, Lebonnois et al., 2012, Larsonet al., 2014].

Haze extinction can be derived from different Cassini measurements such as stellar occultation realised by VIMS and UVIS instruments onboard the Cassini spacecraft [Bellucci et al., 2009, Koskinen et al., 2011] or by direct observation by CIRS [Vinatier et al., 2010] and ISS [Seignovert et al., 2021], and they provide a good insight on haze distribution. Extinction, however, depends on both the density and size of aerosol particles, and determining these two quantities remains a challenge, particularly for validating microphysics models. Retrieved density profiles based on extinction have to assume a fixed size for particles [Bellucci et al., 2009, Vinatier et al., 2010] with values based on Huygens/DISR observations from Tomasko et al., 2008, so this assumption is valid for a narrow range of altitude and location. In parallel, determining density and size distributions can be achieved through numerical simulations including several physical processes (aerosol growth within a microphysics model, ion coupling) but they rely on multiple assumptions regarding thermal structure, dynamics, and chemistry [Lavvas et al., 2011, 2013].

We present a retrieval analysis from UVIS data aiming to retrieve both size and density by fitting a forward model on the data from multiple observation angles. This analysis accounts for forward-scattering effect induced by the aggregate structure of haze particles [West, 1991]. We use data from the UVIS FUV channel where haze scattering is evident at wavelengths longer than about 1600 Å. The observation covers several fly-bys of the moon among the entire spacecraft's lifetime between 2005 and 2017. UVIS spatial resolution allows the analysis to be performed on different parts of the atmosphere and above different surface locations. In order to maximize the signal-to-noise ratio, all observed spectra for each fly-by dataset are averaged in bins of altitude, latitude and longitude.

For a given line of sight geometry, the output UV emission is computed based on the processes happening at each segment of the line of sight: the scattering from the aerosols, the airglow emission, and the Rayleigh scattering by the gas, modulated by the gas and haze extinction. Therefore, simulated emission depends on the mixing ratios of gaseous absorbers given as input, and the profiles of haze density and mean radius that are introduced as two free parameters. Inversion is performed using the maximum a priori approach [Rodgers, 2000] performed on several Cassini observations simultaneously to exploit a large phase angle coverage.

Preliminary results reveal the altitude profiles of Titan's haze density and particle size derived from direct measurements of scattered light. We present the haze properties and distribution for different locations above Titan and for different years of observations. We also discuss interesting spatial variability with respect to latitude, and differences between the day and night sides.

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