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

Abstract

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.

Low Temperature Plasma for Astrochemistry: Toward a Further Understanding with Continuous and Precise Temperature Control

“…In some laboratory simulations of the chemical processes occurring in the atmosphere of Titan, the abovementioned precursor ions, as predicted using calculated models, were detected. Dubois et al (2020) [94] shown in reaction 2. These laboratory data are consistent with the above-cited model calculations [93].…”

“…The results of Dubois et al (2020) [94] show that discharge plasma can be used in laboratory studies to explore the chemistry of the atmosphere of Titan. Although the recent improvements in photochemical modeling [34,95] have increased our understanding of the chemical processes that occur in the atmosphere of Titan, we still lack considerable information regarding the specific chemical pathways that result in the production of organic aerosols.…”

Ion-driven organic chemistry for Titan-like atmospheres: Implications for N-dominated super-Earth exoplanets

“…The species of the gas mixture were then irradiated by a windowless EUV photon source coupled to the photochemical reactor produced by a microwave plasma discharge of a lowpressure inert neon flux providing radiation at 73.6 nm (NeI resonance line), corresponding to a photon energy of 16.8 eV, for a flux of about 10 14 ph cm −2 s −1 (Tigrine et al 2016;Bourgalais et al 2019). The photoproducts present in the reactor were then sampled using a quadrupole m/z spectrometer (Hiden Analytical EQP 200 QMS) to measure neutral compounds and positive ions with high sensitivity below the parts per million (ppm) level (Bourgalais et al 2019;Dubois et al 2019Dubois et al , 2020. It is worth noting that the neutral molecules detected in this article are A171, page 2 of 9 ionized fragments.…”

Plasma Modeling and Prebiotic Chemistry: A Review of the State-of-the-Art and Perspectives

“…Several studies actually considered the chemical activation via plasma electron of planetary atmospheric compositions with a prebiotic perspective. For example, plasma chemistry has been studied in electric discharges fed with chemical compositions compatible with the present atmosphere of Titan [ 44 , 45 , 46 , 47 , 48 ] and for Venus and Jupiter atmospheres [ 49 ].…”