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]

Titan's organic chemistry is driven by photochemical reactions that originate high in the atmosphere, near 1000 km, where the pressure is only 1.5E-5 Pa. (This is 1E-10 atmosphere, or 1/10 billionth Earth's pressure, much much weaker pressure than I got in my graduate school "vaccum" line. This is about the same range as a typical high-vacuum molecular turbopump.)

Taking 1.5E-5 Pa altitude as an upper limit for stuff to happen, here is a graphic that shows the "chemically available" atmospheric volume for each of the planets:

(full res here: http://www.flickr.com/photos/31678681@N07/4612160825/)

As expected, Jupiter dominates, but Saturn may actually be more impressive, with a lower gravity, the scale height is larger - it has a fluffier atmosphere. For similar reasons, smaller Neptune has more atmosphere. Values for Saturn, Uranus, and Neptune are based on scale height - which is probably underestimating the 1.5E-5 limit. The Jovian estimate is based on Galileo probe data.

Same graphic but detail for the silicate and icy bodies:

(full res here: http://www.flickr.com/photos/31678681@N07/4613181600/)

Titan has more atmosphere, and it extends farther out, so it's chemically-available volume is larger than Earth's. With more mass and higher density, both Earth and Venus hold their atmosphere in closer. With a low surface pressure, but a much weaker gravity field, Pluto has a large but thin atmospheric volume.

Ganymede, Callisto, the Moon, and Mercury have a surface atmosphere well below the 1.5E-5 Pa cutoff and so are not shown.

The atmosphere chemistry that happens on Titan depends on amount of atmosphere, amount of solar flux, and the exact mix of the methane-containing atmosphere (spoiler alert: H2 is bad, N2 and Ar are good).

For other methane-containing atmospheres, it might be possible to scale Titan's atmospheric chemistry to derive photochemically derived organic fluxes on those surfaces as well. (Hint: Pluto!)