Atmospheric Chemistry of Titan Options

[Juramike]

Here is a "Benzene-O-Vision" graphic showing the amount of benzene and phenyl radicals at high altitudes on Titan. This is based on detections of benzene and phenyl radical (which recombined in the sample chamber to make benzene) using the INMS instrument during closest approach. The numbers are normalized to constant pressure altitude, roughly 1000 km.



The data was taken from Table 1 in: Vuitton et al, Journal of Geophysical Research 113 (2008) E05007. "Formation and distribution of benzene on Titan". doi: 10.1029/2007JE002997 [EDIT 5/24/10: Article freely available here] and overlaid on a map of Titan.

The authors mentioned that the errors in these measurements are 20%.

These detections are well above the detached haze layer. Most are at the same sun azimuth angle. (T23 observation had the lowest angle.) Assuming that the temporal difference is minimal (each dot is from a different flyby), there doesn't appear to be an obvious correlation with latitude.

This graphic does show that benzene is present even waaaay up in the thermosphere and ionosphere.

[Juramike]

Purple Haze or Where It’s At

A paper by Lavvas et al. (Lavvas et al. Icarus 201 (2009), 626-633. "The detached haze layer in Titan's mesosphere." doi: 10.1016/j.icarus.2009.01.004) examined the detached haze layer and reported that it is the major driver of Titan chemistry.

The graphic below shows that the detached haze layer (the image and plot are scaled to the same altitude) has a shift in the UV-Visible properties (187 and 338 nm) at about 520 km above Titan. This also corresponds to an inversion layer measured by the Huygens probe (solid line in plot). So what makes up this warm thicker layer?



The authors state that photochemical production occurs very high up in Titan’s atmosphere, near 1000 km. Some of these chemical reactions produce particles which slowly grow radially as they thicken downwards. At about 520 km, these monomeric particles reach their highest optical density forming the visible purple layer, and absorbing solar radiation and causing the local warm region in the upper atmosphere. But at this high density, the particles begin to glom together and stick to form even larger aggregates. Fractal growth begins, and the agglomerated particles fall through the upper atmosphere forming the larger globby particles that make up the lower haze layers. According to the authors, the agglomerated particles have a lower optical density than the individual monomers (many small particles = hi optical density; one biiig particle = lower optical density), so the region immediately below the detached haze layer is “clearer”.



The authors created a model with a photochemical production centered around 900 km altitude and a mass flux of 9E-14 g cm-2 s-1 and an average particle size at 520 km of 40 nm which fits the observations and fit the required influx rate for the Titan’s lower stratospheric haze layers from above.
It is very important to note that the saturation vapor pressure of simple organics (such as benzene) is not high enough to condense out at 500 km. The aerosol particles are made of bigger molecules.

The chemistry that occurs at these high altitudes is driven by solar flux of high-energy electron-stealing EUV photons around 145 nm. The solar flux also matches the modeled production rate.

Here are some numbers:
Overall photochemical mass flux (organics+aerosol particles): 9E-14 g cm-2 s-1
Aerosol production flux at 520 km: 2.7-4.6E-14 g cm-2 s-1 (30-50% aerosol particulate production)
Particulated Flux required to maintain stratospheric haze layer: 0.5-2.0 E-14 g cm-2 s-1.

The high-energy photochemical pathways up at 900 km drive much of Titan’s organic chemistry, making organics (such as benzene, seen at 1000 km) and particulates.

[rlorenz]

Purple Haze or Where It’s At

A paper by Lavvas et al. (Lavvas et al. Icarus 201 (2009), 626-633. "The detached haze layer in Titan's mesosphere." doi: 10.1016/j.icarus.2009.01.004) examined the detached haze layer and reported that it is the major driver of Titan chemistry.

A major thrust of the paper is that the detached haze layer is located by the chemical fluxes alone (i.e. they dismiss the
previously-proposed dynamical origin)

I disagree. The detached haze varies both with season and latitude, so I don' t think things are as simple as
their 1-D perspective claims.