Stratospheric Haze Bands Observed in Cassini VIMS

Introduction: The Visual and Infrared Mapping Spectrometer (VIMS) aboard the Cassini satellite made observations of Titan from 2004-2017. Images acquired at wavelength shorter than 600nm reveal dark bands in Titan’s atmosphere, while images acquired at methane absorption channels show the bands as bright. We present our results on the seasonality, altitude, and production of these bands. Data: VIMS consisted of two imaging spectrometers with 352 spectroscopic channels between .35 and 5.13 μμμμ. Figure 1 (Right) shows the north polar annulus (NPA) observed on Titan flyby T96 (2013-12-01), and Figure 1 (Left) shows the equatorial annulus (EQA) observed on flyby T61 (2009-08-25). Both observations were taken at ~1.15 μμμμ (which is a methane absorption channel) and have been processed to highlight the contrast of the atmosphere and the bright bands. Both the NPA and EQA show the same spectral characteristics, and are locally brighter or darker than the surrounding atmosphere at the same VIMS channels. This is largely true for all observations and flybys, except for observations of the NPA towards the end of the mission; we will discuss the implications of this below in Seasonality. For every observation of the NPA or EQA we determined the relevant pixel’s location (latitude and longitude), viewing geometry (incidence, emission, phase, etc) and other ancillary information using the SPICE toolkit from NAIF. Using this information, we constructed transects along lines of longitude and analyzed the resulting curves of latitude vs brightness (I/F) for channels of interest. We found that the bands we observed in the images (Figure 1) presented as deviations in the otherwise featureless curves. We used the deviations to identify the bands spectrally. The definition used to identify the NPA or EQA is a concave deviation at ~500nm, and convex deviations at ~900nm and ~1150nm. Using these definitions, we made transects of Titan for the entire mission, binning the data every 10 degrees of longitude. Haze: We acquired differential spectra of the bands by first removing the deviations from the transects. We then fitted a straight line to the gap, and subtracted this line from the deviation for all 352 channels. The differential spectra are negative at wavelengths shorter than 600 nm, with the darkest point occurring at ~500 nm. This suggests that the bands are either more absorptive than the surrounding atmosphere, or obscuring a backlit source of illumination. The differential spectra steadily increase to a maximum positive value between 850 and 1000nm. We see multiple peaks of decreasing intensity at wavelengths where methane absorbs. We propose that this is consistent with a localized increase in high altitude haze. The dark feature short of 600nm is due to the haze obscuring Rayleigh scattering by the higher density nitrogen and methane in the lower atmosphere. As wavelength increases beyond 500nm, the effect of Rayleigh scattering from below diminishes and the contrast between annulus and atmosphere decreases. Further redwards, Mie scattering from the top of the haze dominates, inverts the contrast, and leads to a bright feature. The haze has the highest positive contrast at methane absorption wavelengths. There, most photons are absorbed in the atmosphere, especially at depth, while those photons that encounter the high altitude haze are scattered back to VIMS.