Energetics of Titans Tholin haze

Question

So, I've googled and wiki'd on this, but no clue, so I turn my question to the community:

Given Titan's nice orange haze that is supposedly composed of Tholins and strangely coincides with its stratosphere (taken from wikipedia)

I got to think whether the haze takes the role of clouds on earth when it comes to energetics. From this a bunch of questions arose:

• Do we know whether the haze is responsible for heat trapping and thus, the formation of a stratosphere on Titan?
• What about radiation transport modelling with those particles, is this successful?
• Also I'd be interested in what we know about the stability of Tholins. Could they survive harsh irradiation that atmospheres on Hot Jupiters do encounter?

I would like to be able to make a statement such as "Dust" or "Haze-controlled" energetics for the stratosphere of Titan, if this is so. It would be nice if you helped to clear this up.

Answer 1

This is a part answer to your question, based on some recent research of the photochemical behaviour modelled and observed for Titan's tholin haze and modelling of Titan's stratosphere.

The process appears to begin, according to the paper Ice condensation layers in Titan’s Stratosphere (Barth, 2012) (Abstract only - paywalled), with

Photochemical destruction of methane along with the destruction of nitrogen molecules from energetic electrons in Titan’s upper atmosphere result in the production of a number of hydrocarbon and nitrile compounds which may be capable of condensing at the colder temperatures of Titan’s lower stratosphere.

then, according to the paper Laboratory experiments of Titan tholin formed in cold plasma at various pressures: implications for nitrogen-containing polycyclic aromatic compounds in Titan haze (Imanaka et al. 2004), in particular in reference to Titan's stratosphere,

. In the stratosphere (100– 300 km), further chemical reactions are induced by the catalytic $CH_4$ dissociation by such molecules as $C_2H_2$ and $C_4H_2$ absorbing the long UV (> 155 nm) irradiation

The significance of these UV absorbing molecules is explained in the article Photochemical activity of Titan’s low-altitude condensed haze (Gudipati et al. 2013) (Abstract only - paywalled), they state that tholin haze could form on condensed aerosols in Titan's atmosphere, demonstrating that, at least part of Titan's atmosphere is photochemically active. Through modelling, they found that

Detected in Titan’s atmosphere, dicyanoacetylene ($C_4N_2$) is used in our laboratory simulations as a model system for other larger unsaturated condensing compounds. We show that $C_4N_2$ ices undergo condensed-phase photopolymerization (tholin formation) at wavelengths as long as 355 nm pertinent to solar radiation reaching a large portion of Titan’s atmosphere, almost close to the surface.

and evidence of these ices is suggested in the article Titan’s aerosol and stratospheric ice opacities between 18 and 500 μm: Vertical and spectral characteristics from Cassini CIRS (Anderson and Samuelson, 2011) stating that the ices and aerosols

appear to be located over a narrow altitude range in the stratosphere centered at ∼90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S.

Answer 2

In Corlies et al. (2020), which analyses instrumentational requirements for a possible future Titan orbiter mission, a modelled transmission function for Titans atmosphere as function of wavelength and chemical species is shown in their Fig. 1 (Can plots from papers on the arXiv legally be used here?).

That figure clarifies that Titan's tholin hazes possess significant absorption opacities in the optical, overlapping with Rayleigh and Mie-scattering, but stronger in effect. This effect is weakening strongly the deeper into the infrared one progresses, with a small high-absorption bump at 3 $\rm\mu m$. The weak have continuum opacity however reaches deep into the near-infrared, trapping very minute amounts of the planets emitted infrared heat. The latter process is however dominated by methane.

Additionally, Doose et al. (2016) had a look at the single scattering albedo $\omega_0$ of Hazes, which gives rise to reflection of incident sunlight. In the entire atmosphere this quantity is very high, $\omega_0>0.9$ at $750$ nm, but with a complex behaviour, dropping to smaller values above 0.9 at $500$ nm.

Together with the previous answer in this thread, I synthesize the following picture for the role of Titan's hazes onto its atmospheric energetics:
1.) Hazes readily absorb UV radiation, making it available for atmospheric photochemistry. Photochemical products rain out downwards into the troposphere, but barely contribute to further energetic activity.
2.) In the optical hazes scatter and absorb both strongly. Both factors lead to a blockage of sunlight towards the surface and hence the conclusion must be that Titan's hazes force an anti-greenhouse effect in the stratosphere.
3.) In the infrared, Hazes are unimportant for the energy balance. Here Methane keeps the planet warm-ish with a mini-greenhouse effect using the remainder of sunlight left by the hazes.