Capacitively coupled radio-frequency discharges in nitrogen at low pressures

Jovanović

et al. 2020

Icarus

Titan is unique in the solar system as it hosts a dense atmosphere mainly made of molecular nitrogen N 2 and methane CH 4 . The Cassini-Huygens Mission revealed the presence of an intense atmospheric photochemistry initiated by the photo-dissociation and ionization of N 2 and CH 4 . In the upper atmosphere, Cassini detected signatures compatible with the presence of heavily charged molecules which are precursors for the solid core of the aerosols. These observations have indicated that ion chemistry has an important role for organic growth. However, the processes coupling ion chemistry and aerosol formation and growth are still mostly unknown. In this study, we investigated the cation chemistry responsible for an efficient organic growth that we observe in Titan's upper atmosphere, simulated using the PAMPRE plasma reactor. Positive ion precursors were measured by in situ ion mass spectrometry in a cold plasma and compared with INMS observations taken during the T40 flyby. A series of positive ion measurements were performed in three CH 4 mixing ratios (1%, 5% and 10%) showing a variability in ion population. Low methane concentrations result in an abundance of amine cations such as NH 4 + whereas aliphatic compounds dominate at higher methane concentrations. In conditions of favored tholin production, the presence of C 2 compounds such as HCNH + and C 2 H 5 + is found to be consistent with copolymeric growth structures seen in tholin material. The observed abundance of these two ions particularly in conditions with lower CH 4 amounts is consistent with modeling work simulating aerosol growth in Titan's ionosphere, which includes mass exchange primarily between HCNH + and C 2 H 5 + and negatively charged particles. These results also confirm the prevalent role of C 2 cations as precursors to molecular growth and subsequent mass transfer to the charged aerosol particles as the CH 4 abundance decreases towards lower altitudes.

Section: Comparisons With the Inms T40 Measurements And Group Patternsmentioning

confidence: 74%

Positive ion chemistry in an N2-CH4 plasma discharge: Key precursors to the growth of Titan tholins

Dubois

Jovanović

et al. 2020

Icarus

“…In plasmas, N 2 dissociation mostly occurs via (i) electron impact at electron energies above the dissociation threshold of N 2 (9.8 eV) and (ii) heavy particles collisions (Fridman, 2008, Alves et al, 2012. As electron energy of impacting electrons is continuous and not quantified as for photons, several excited states of atoms and molecules can be reached by electron impact in plasmas.…”

Section: Discussion and Implications 41 Origin Of The Nitrogen Isotomentioning

confidence: 99%

Nitrogen isotopic fractionation during abiotic synthesis of organic solid particles

Kuga

Earth and Planetary Science Letters

Marty

et al. 2014

The formation of organic compounds is generally assumed to result from abiotic processes in the Solar System, with the exception of biogenic organics on Earth. Nitrogenbearing organics are of particular interest, notably for prebiotic perspectives but also for overall comprehension of organic formation in the young solar system and in planetary atmospheres. We have investigated abiotic synthesis of organics upon plasma discharge, with special attention to N isotope fractionation. Organic aerosols were synthesized from N 2 -CH 4 and N 2 -CO gaseous mixtures using low-pressure plasma discharge experiments, aimed at simulating chemistry occurring in Titan's atmosphere and in the protosolar nebula, respectively. The nitrogen content, the N speciation and the N isotopic composition were analyzed in the resulting organic aerosols. Nitrogen is efficiently incorporated into the synthesized solids, independently of the oxidation degree, of the N 2 content of the starting gas mixture, and of the nitrogen speciation in the aerosols. The aerosols are depleted in 15 N by 15-25 ‰ relative to the initial N 2 gas, whatever the experimental setup is. Such an isotopic fractionation is attributed to mass-dependent kinetic effect(s).Nitrogen isotope fractionation upon electric discharge cannot account for the large N isotope variations observed among solar system objects and reservoirs. Extreme N isotope signatures in the solar system are more likely the result of self-shielding during N 2 photodissociation, exotic effect during photodissociation of N 2 and/or low temperature ionmolecule isotope exchange. Kinetic N isotope fractionation may play a significant role in the

“…This is in agreement with the modeling of the electron energy distribution functions (EEDF) in our plasma discharge. Indeed, the calculated EEDF realized in pure nitrogen presents a maximum at 2 eV and a relatively populated tail for electron energy above 4 eV (Alves et al 2012). Thus, the plasma produces electrons with enough energy to produce the above Reactions (R1).…”

Section: Water Formation and Quantificationmentioning

confidence: 96%

Water Formation in the Upper Atmosphere of the Early Earth

Fleury

Marcq

et al. 2015

ApJ

The water concentration and distribution in the early Earthʼs atmosphere are important parameters that contribute to the chemistry and the radiative budget of the atmosphere. If the atmosphere above the troposphere is generally considered as dry, photochemistry is known to be responsible for the production of numerous minor species. Here we used an experimental setup to study the production of water in conditions simulating the chemistry above the troposphere of the early Earth with an atmospheric composition based on three major molecules: N 2 , CO 2 , and H 2. The formation of gaseous products was monitored using infrared spectroscopy. Water was found as the major product, with approximately 10% of the gas products detected. This important water formation is discussed in the context of the early Earth.