New Phase Function
"The previous model had two phase functions: above and below80 km. This abrupt change in aerosol properties at 80 km altitude is not consistent with the DLVS data near that altitude. Either the phase function changes more gradually, or it is constant with alti- tude. Furthermore, de Bergh et al. (2012) showed that the original aerosol model produces too much radiance at low phase angles compared to VIMS observations at 1600 nm (see e.g. their Fig. 11). This could be attributed to the enhanced backward peak in the original phase functions below 80 km. Those authors showed that applying the phase function above 80 km to all alti- tudes allowed them to restore an acceptable fit of VIMS data near the Huygens site. In our new model we find that a linear combina- tion of 85% of the phase function above 80 km and 15% of that below 80 km works well if applied for all altitudes"
Phase Functions Accuracy at 700nm and other WLs
"Small changes in the phase function would not be detectable in
our measurements of the radiance field at continuum wavelengths because there were several optical depths of aerosols above Huy- gens that smeared out features of the phase function. The system- atic effects suggest that our phase functions may be realistic around 700 nm wavelengths, but slightly off in different directions at shorter and longer wavelengths. We ran some sensitivity tests showing that the phase functions influence the strengths of methane absorptions. However, since there are so many choices of altering theoretically calculated phase functions, we did not attempt to perfectly reproduce the measured strengths of methane absorptions."\citep{Doose_2016}
Phase Functions does not fit VIMS observation.
"Fig. 11 shows that this model produces intensities about 40% larger than observed, both in the center and in the wings of the window, where the surface is no longer visible. This points to an inadequacy of the aerosol model. We found that a large fraction of the discrepancy can be removed by modifying the phase function below 80 km. In Tomasko et al. (2008b), this phase function differs from that above 80 km by the presence of a backward peak that was set to reproduce the downward-looking visible DISR measurements at 934 nm. No backward peak is required at 491 nm to fit the DISR visible data and, at wavelengths longer than 934 nm, the need for a backward peak has not been investigated by Tomasko et al. We think that introducing such a backward peak at 1.58 mm is questionable and we have chosen to apply the phase function of the aerosols above 80 km to all atmospheric layers. Doing so, the synthetic spectrum is only about 10% higher than observed. Matching the VIMS spectrum requires an additional small decrease of the single scattering albedo and/or of the particle optical depth. We chose to decrease the single scattering albedo at 1.58 mm by 0.02 and the optical depths of all three particle layers by 10%. These small adjustments are at the limits of the uncertainty of the DISR model parameters (Tomasko et al., 2008b, Table 3). As shown in Fig. 11, this modified aerosol model allows us to reproduce satisfactorily the VIMS spectrum (Model 1). An even better agreement (Model 2) is obtained with an overall 10% increase of the surface albedo derived from DISR by Jacquemart et al. (2008)."\citep{de_Bergh_2012}
Parameters of the Haze:\cite{Lavvas_2010}
Refracttive Index 1.5 times Khare et al.
In the past, analyses of Titan’s geometric albedo suggested that
the tholin refractive index used in the simulations has to be multi- plied by a factor of?1.5 in order to match the observations (McKay et al., 1989; Toon et al., 1992). This factor brings the tholin refrac- tive index (Khare et al., 1984) very close to the retrieved index from our work between 0.43 and 0.7 lm, but does not provide the increased absorption observed for longer wavelengths \citep{Lavvas_2010}
To Search For:
- Phase function constrains the refractive index as well as SSA. Explain it. Also Explain why forward scattering part of the phase function does not constraint the refractive index.