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2013 Annual Science Report

NASA Jet Propulsion Laboratory - Titan Reporting  |  SEP 2012 – AUG 2013

Task Titan Photochemical Model

Project Summary

To develop a comprehensive model of the chemistry in Titan’s atmosphere including condensation of molecules onto grains, and sublimation back to the gas.

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

This year’s work has focused on refining and testing the sublimation and condensation code that has been introduced into the KINETICS model. The code is numerically stable and provides a substantial improvement over previous ad hoc methods for treating these processes in Titan’s atmosphere.

There have been computational difficulties in implementing the condensation scheme, which have taken some time to sort out. These have related to the correct treatment of the sublimation rate. The code now correctly replicates the atmospheric mixing ratios expected from consideration of the saturation vapor pressures.

Figures 1 – 3 illustrate how the inclusion of condensation and sublimation processes affect the calculated mixing ratios in Titan’s atmosphere of three molecules: CH4, HCN and HC3N. In the case of CH4 (Figure 1) the mixing ratios are always lower than the saturation densities, resulting in little change in the predictions of models including condensation and sublimation compared to those which consider the gas phase only. For HCN (Figure 2) (and other molecules) the mixing ratios now track the values expected from consideration of the saturation vapor pressure. In the case of HCN, condensation and sublimation result in a lower abundance at altitudes below 50 km. HC3N (Figure 3) shows large changes in mixing ratio compared to the gas only model, and below 700 km the calculated mixing ratios either match or fall well below the expected saturation densities. Above 700 km the inclusion of condensation reduces the mixing ratios markedly over the gas only model, but they still exceed the saturation values. This is because the formation of HC3N in this region is very efficient, and exceeds the rate of condensation of HC3N.

Figure 1
Figure 1 The mixing ratio of CH4 in (a) a gas only model without condensation (dashed black line), (b) the expected saturation density calculated from the vapor pressure (red line) and (c) the results from the sublimation/condensation model (blue line). The mixing ratios for both the gas only and the condensation models are always lower than the saturation value.
Figure 2.
Figure 2. As for figure 1 but showing the mixing ratio of HCN. The mixing ratio in the gas only model exceeds the saturation density at altitudes below 50km but in the condensation/sublimation model the mixing ratio follows the altitude variation in the saturation density.

Figure 3. As for figure 1 but this time showing the mixing ratio of HC3N. At altitudes below 100 km the condensation/sublimation model follows the saturation density whereas the gas only model predicts much higher mixing ratios. Further up in the atmosphere between 700 and 900 km the addition of condensation results in much lower abundances than in the gas only model. The agreement with the saturation density is not so good here as the formation rate of HC3N in this region of the atmosphere is much more rapid than the rate at which it is removed from the atmosphere by condensation.

    Mark Allen Mark Allen
    Project Investigator
    Karen Willacy

    Objective 2.2
    Outer Solar System exploration

    Objective 3.1
    Sources of prebiotic materials and catalysts