Understanding Titan’s Chemical Factory: An Analysis of VIMS Occultation Data

Background/Motivation:

Titan’s atmosphere is an organic factory, producing high-order hydrocarbons from the photolysis of methane by solar ultraviolet radiation [Lavvas et al. 2008]. These organics aggregate together to form tholins, which then sediment out of the atmosphere to the surface. Constraining the temporal evolution and distribution of Titan’s aerosols is required to understand the roles of haze production in the Titan system including radiative heating, dynamics, atmospheric chemistry, and surface erosion/dune formation. Studies of Titan’s hazes have demonstrated significant variability in haze abundance over the course of the Cassini mission and beyond [Rannou et al. 2010, West et al. 2018, Nicols-Fleming et al. 2021]. We seek to expand on this work by considering newly analyzed data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS) in combination with state-of-the-art radiative transfer (RT) models.

Here, we present an analysis of all the VIMS solar occultation data over the course of the Cassini mission. These unique data offer the ability to sample fine vertical structure in both gaseous composition and aerosol abundance. Past studies have leveraged the occultation data occurring in the first half of the mission [Bellucci et al. 2009, Hayne et al. 2014, Maltagliati et al. 2015, Rannou et al. 2022]. Now, we double the dataset, including data from the second half of the mission, providing both a longer temporal baseline and greater spatial coverage through which to understand the formation and distribution of Titan’s aerosols. The final profile of derived vertical abundance permits both a study of the spatio-temporal variability in Titan’s aerosols and more accurate atmospheric profiles for RT modelling, which generally assume constant profiles as derived from the Huygens probe. Accurate RT is critical for atmospheric compensation and interpretation of retrieved surface properties.

Methods:

Occultation observations were attempted on 10 different flybys of Titan over the Cassini mission, with 6 flybys offering data on both the ingress and egress of the flyby for a total of 16 datasets. However, of these 16 datasets, one resulted in no data of the Sun, which was like result of bad VIMS pointing while an additional 7 datasets suffered from significant variability because of Cassini’s thrusters being activated during the acquisition resulting in significant drifts in intensity as the Sun moved about the focal plane. As the occultation requires an accurate baseline of the solar flux prior to the event to determine relative transmission, these observations were discarded from the analysis.

Of the remaining 8 datasets, all were analyzed with a similar approach built around the methodology as described in [Maltagliati et al. 2015]. First, a 2D Guassian was fit to each wavelength-dependent image from the occultation and removed to determine the background residuals. For cases where stray light from Titan was present through the non-solar port, a 2D plane was used to fit and remove the background residuals. After, the 2D Gaussian was refit and the total flux summed across the focal plane. Transmission was then determined for each timestep through division by the average solar flux taken from all altitudes greater than 1000 km. As a final correction, a linear polynomial was fit to all transmission data to account for any additional slow variability in source drift over the course of the acquisition.

Preliminary Results:

Figure 1 displays the results of the occultation data reduction. As evident in Figure 1, significant variability in sampling resolution is observed between the datasets. This results from differences in the integration time, image frame size, and distance of Cassini from Titan at the time of acquisition. Resolutions range from 5-40 km on average. All plots have been color coded to correspond to similar altitudes ranging from 50-500 km. Strong absorptions are observed around 2.3- and 3.3-µm from methane, with additional weaker absorptions around 1.2- and 1.4-µm. Also evident in most observations is the strong carbon monoxide (CO) doublet at 4.8-µm. However, this signature appears to be significantly weaker in later observations, suggesting the potential for spatio-temporal variability in CO on Titan. Variability in the spectral slopes in the continuum from 1.0 - 2.2-µm also demonstrate changes in vertical aerosol abundance over the course of the observations. Most notable is decreased transmission on the T103/T116 ingress datasets, corresponding to similar latitudes at ~30°N, at ~150km altitudes. These data suggest a significant increase in aerosol abundance at mid-latitudes later in the Cassini mission and will be used to place constraints on seasonal circulation of Titan’s atmosphere.

Figure 1: Derived spectral transmission files from eight occultation datasets range from the T10 flyby (Jan. 2006) to the T116 flyby (Feb. 2016). The color corresponds to the sampled altitude for each profile. Strong absorptions from CH₄ and CO are observable in the spectra. Variability in spectral slopes are indicative at changes to the vertical aerosol abundance between flybys.