New experimental constraints on the composition and structure of tholins

Understanding the composition, formation processes and precise location of aerosols developed within the planetary atmosphere of Titan still represents an enigma and a challenge for spacecraft-based observational studies and terrestrial laboratory experiment. The CASSINI mission demonstrated that these materials contain substantial hydrogen, carbon and nitrogen, with no detectable oxygen, but none of the instruments aboard the CASSINI or HUYGENS probes could succeed in revealing their chemical structure. However, an unexpected breakthrough for understanding the formation and nature of Titan’s tholins was achieved recently by a series of mass spectrometry measurements (INMS) of the neutral and ionic species in the upper atmosphere, that demonstrated that at least part of the aerosols are formed at very high altitudes (i.e., >1000 km) through various ion-molecule reactions, and not only in the stratosphere, if at all.

Several tholin species were produced with the PAMPRE experiment, located at the Service d’Aéronomie (Verrières le Buisson, France). This experiment allows us to synthesize tholins as submicrometric grains under conditions of electrostatic levitation, thus preventing any wall or substrate effects. A series of samples was produced using three CH4/N2 ratios within the initial gas mixture. Other tholins were obtained using a DC-discharge experimental set-up; an HCN “polymer” material, and several coal samples were also studied for comparison. Here we present and discuss new insights on the molecular composition and structure of the PAMPRE tholins, and compare them with tholins produced in other groups, and also investigate the effect of the composition of the initial gas mixture on the structure and properties of the resulting tholins.

TEM bright field images of PAMPRE tholins. Left: SA98 (1.1×1.1 µm²). Right: SA99 (1×1 µm²). Particles tend to form clusters, with some of them being strongly bound together with others sticking together more loosely during the formation process (see arrows).

Elemental compositions of tholins produced with the PAMPRE experiment (SA90, SA98 and SA99), with the DC-discharge system located at LISA and a HCN polymer.

X-ray diffraction patterns of tholins SA90, SA98 and the graphitic carbon nitride g-CN. They demonstrate the highly crystalline nature of the g-CN compound, and the disordered structure of both tholins.

Infrared spectra of samples studied. K84 represents the tholin studies by Khare et al. (1984), whose optical constants have been widely used in the literature up to now. The dot-dash lines point to the following wavenumbers: 2240, 2180, 1630, 1555, 1450, 1415, 1380, 990 and 815 cm^-1. The asterisk * indicates the residual absorption of atmospheric CO2.

The symmetric and antisymmetric C–H stretching bands of CH2 and CH3 chemical groups in a terrestrial kerogen, SA90 and series of N-rich photolysis products (Tran et al., 2003, data files were kindly provided by Drs. Tran and Ferris). The figure reveals a systematic shift towards higher wavenumber as a function of N abundance in the samples. This trend suggests that a fraction of CH2 and CH3 are connected to nitrogen atoms, however, the chemical group to which this N atom belongs is not yet identified. Amine functions are discarded regarding IR data. Note the higher intensity of CH2 bands in the terrestrial type III kerogen, which are known to contain long alkyl chains.

Infrared spectra of SA90, SA90-F, SA98 and SA98-F. The spectra reveal some differences between powders and thin films, in particular a larger amount of hydrogen contained in the film samples. Note measurements on thin films were performed under ultravacuum (P ~ 10^-8 mbars), eliminating the presence of any atmospheric adsorbed water.

UV Raman spectra of: (a) poly-HCN—244 nm; (b) TF90—229 nm; (c) TF90—244 nm; (d) SA98—244 nm; (e) g-CN—244 nm; (f) g-CN—229 nm; (g) g-CN—1064 nm; (h) melamine; (i) triazine—1064 nm. Main bands: A1 [C3N3]; A2 [C3N3]; D; G; N1 [N=C=N] and N2 [–CN]; * [N2 atmospheric stretching vibration]. Note the weak band at 980 cm^-1 in spectrum (g).

Results of fitting to UV Raman G and D bands. I_D /I_G : ratio of peak intensity of D and G bands; FWHM-G: width of G band; ω-G: peak position of G band.

HRTEM images of SA90 (left), an amorphous carbon film (center) and SA98 (right) [16 ×16 nm]. Images reveal SA90 and SA98 are mainly amorphous.

Cumulative abundance versus HRTEM fringe length for SA90, SA98, series of amorphous carbon films and a low volatile bituminous coal, which do contain larger polycyclic aromatic units (sample PSOC880 provided by the Pennsylvania State Coal Data Bank). Tholins exhibit cumulative distributions ranging between the end-members of amorphous carbon samples. No significant differences are observed between SA98 and SA90 samples.