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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - PRIMCHEM (Primitive chemistry in planetary atmospheres: From the upper atmosphere down to the surface)

Teaser

Summary

The presence of organic compounds was essential to the emergence of life on Earth 3.5 to 3.8 billion years ago. Such compounds may have had several different origins; amongst them the ocean-atmosphere coupled system (the primordial soup theory), or exogenous inputs by meteorites, comets and Interplanetary Dust Particles.

Titan, the largest moon of Saturn, is the best known observable analogue of the Early Earth. A totally new source of prebiotic material was recently found in this system: the upper atmosphere. Nucleobases have been highlighted as components of the solid aerosols analogues produced in a reactor mimicking the chemistry that occurs in the upper atmosphere. The specificity of this external layer is that it receives harsh solar UV radiations enabling the chemical activation of molecular nitrogen N2, and involving a nitrogen rich organic chemistry with high prebiotic interest.

As organic solid aerosols are initiated in the upper atmosphere of Titan, a new question is raised that we are addressing: what is the evolution of these organic prebiotic seeds when sedimenting down to the surface? Aerosols will indeed undergo the bombardment of charged particles, further UV radiation, and/or coating of condensable species at lower altitudes. I expect possible changes on the aerosols themselves, but also on the budget of the gas phase through emissions of new organic volatiles compounds. The aerosols aging may therefore impact the whole atmospheric system.

An original methodology is being developed to address this novel issue. The successive aging sequences will be experimentally simulated in chemical reactors combining synchrotron and plasma sources. The interpretation of the experimental results will moreover be supported by a modelling of the processes. This complementary approach will enable to decipher the aerosols evolution in laboratory conditions and to extrapolate the impact on Titan atmospheric system.

Work performed

The proposed project will be the first extended vision of upper atmospheric aerosols, from their production in ionized reactive media, to their evolution in the lower atmospheric layers when sedimenting to the surface (Figure 1). The study of the aerosol production in the upper atmosphere is presently a highly competitive field. The possible aging of the aerosols has however never been considered yet, whereas it may impact the whole atmospheric system. Three successive tasks are considered in the present project to describe the possible evolution of the aerosols:

1/ their production in upper atmospheric conditions,
2/ their aging by bombardment of charged particles in the upper atmosphere,
3/ their aging by UV radiation, and/or coating of condensable species at lower altitudes. The novelty of the atmospheric integrated approach requires the development of a new methodology.

The successive sequences will be experimentally simulated in chemical reactors combining synchrotron and plasma sources. The interpretation of the experimental results will be moreover supported by a modelling of the processes.

During the first 18 months of the project,
- First we developed most part of the instrumentation required for the project: we developed a VUV la source to simulate Titan\'s photochemistry in its ionosphere (article Tigrine et al. 2016), and we bought and installed ion mass spectrometers on our two reactors,
- Secondly we worked on a subtask of task 1, the chemical composition of aerosols produced in a plasma reactor mimicking Titan\'s ionospheric chemistry. Several important results were found involving the first molecular identifications in this complex material (Da Cunha et al. 2016, Gautier et al. 2016).
-Then we performed preliminary experiments to adress Task 2 : we analyzed the chemical composition of two solid phases produced in the same plasma environment, but exposed to different plasma durations (Carrasco et al. 2016).
-Finally we optimized an analytical method for the analysis of Titan\'s aerosols in the framework of a future space mission to Titan (Morisson et al. 2016)

Final results

At this stage of the project, the most important result beyond the state of the art corresponds to the first molecular identifications in Titan\'s aerosols analogues.
Several molecules composing the aerosols were found with high potential for astrobiology, suggesting a prebiotic chemistry possible in Titan\'s atmosphere since the very top of its atmosphere, above 1000 km of altitude.