Laboratory studies of organic chemistry in planetary atmopheres: From simulation experiments to spectroscopic determinations

Abstract

Possible approaches to the study of organic chemistry in planetary atmospheres are threefold: they comprise theoretical modeling, simulation experiments, and observational programs. Because of their respective merits and limitations, these approaches are quite complementary, and their simultaneous improvement is the way to progress further in the field. All three ask for laboratory work, and the lack, or limited accuracy, of laboratory data is the main restriction to future improvement. Together with the development of theoretical modeling (based on chemical kinetics and depending on laboratory studies of reaction pathways and rate constants) laboratory simulation remains a powerful technique. Despite the inaccurate reproduction of all planetary conditions, this experimental approach yields precious information on the nature of middle and higher order molecular weight organics that can be expected in an atmosphere of a given overall composition; and there is, in general, good agreement between the data obtained from simulations and those derived from observations. Indeed, several of the organic species highlighted in such experiments, and their relative abundances, are compatible with those detected in related planetary atmospheres. This is shown in the particular case of Titan. Thus experimental results furnish information on the nature of organics to be searched for in planetary atmospheres, while, in turn, the detection of such candidates and possible indications of their concentration profiles, or the setting of upper limits to their abundancies, constrain the kinetic approach. Given the lists of candidates from simulation experiments, experimental programs for a systematic determination of spectroscopic characteristics, including frequencies and band or line intensities, of the likely organics in planetary atmospheres, have to be developed. As an example, experimental requirements and current results, both in the IR and the UV range, are presented concerning Titan's atmosphere in view of the Cassini‐Huygens mission.

The 2009 edition of the GEISA spectroscopic database

“…UV absorption cross-sections (cm 2 molecule À 1 ) of five gaseous aromatic hydrocarbons have been measured with a FTS Bruker IFS120M at the resolution of 1 cm À 1 (0.9 cm MOPD and boxcar apodization) over the 30,000-42,000 cm À 1 spectral range (238-333 nm). The molecules, benzene (C 6 H 6 ), toluene or methylbenzene (C 7 H 8 ), and the three isomers of dimethyl-benzene (C 6 H 4 (CH 3 ) 2 ) also called meta-, ortho-, and para-xylene, were chosen for their importance in the chemistry of tropospheric ozone [485], in urban air quality problems [486] and in astronomical studies [317,318,402,487].…”

The HITRAN2012 molecular spectroscopic database

“…The need for laboratory spectroscopic data to study organic chemistry in planetary atmospheres was highlighted in Refs. [367][368][369], in particular the low-temperature dependence in the UV range. This band system becomes more allowed and therefore more intense as the D 6h symmetry is broken in methyl and dimethyl-substituded benzene, i.e toluene and xylene respectively.…”

UV Fourier transform absorption cross sections of benzene, toluene, meta-, ortho-, and para-xylene

“…required for those modeling studies. The need for laboratory spectroscopic data to study organic chemistry in planetary atmospheres was highlighted in [22][23][24]. In particular, low temperature dependence in the UV range is an important concern.…”