Dr Antonin Knizek

Abstract:

Hydrogen cyanide (HCN), a key molecule of significant importance in contemporary perspectives on prebiotic chemistry, originates in planetary atmospheres from various processes, such as photochemistry, thermochemistry, and impact chemistry, as well as from delivery by impacts. The resilience of HCN during periods of heavy bombardment, a phenomenon caused by an influx of material on unstable trajectories after accretion, remains relatively understudied. This study extensively investigates the stability of HCN under impact conditions simulated using a laboratory Nd:YAG laser in the ELISE experimental setup. High-resolution infrared spectroscopy was employed to monitor the gas phase composition during these simulations. Impact chemistry was simulated in bulk nitrogen atmospheres with varying mixing ratios of HCN and water vapor. The probed range of compositions spans from ∼0 to 1.8% of HCN and 0 to 2.7% of H₂O in a ∼1 bar nitrogen atmosphere. The primary decomposition products of HCN are CO and CO₂ in the presence of water and unidentified solid phase products in dry conditions. Our experiments revealed a range of initial HCN decomposition rates between 2.43 × 10¹⁵ and 5.17 × 10¹⁷ molec J^-1 of input energy depending on the initial composition. Notably, it is shown that the decomposition process induced by the laser spark simulating the impact plasma is nonlinear, with the duration of the irradiation markedly affecting the decomposition rate. These findings underscore the necessity for careful consideration and allowance for margins when applying these rates to chemical models of molecular synthesis and decomposition in planetary atmospheres.

Abstract:

Abstract A hollow cathode discharge with a CuNi (Cu50Ni50) cathode was operated in an Ar/N$$_2$$ 2 /O$$_2$$ 2 gas mixture. The energy distribution of plasma ions is investigated with the help of energy-resolved mass spectrometry. Formation of singly ionised Ar$$^+$$ + and of Cu$$^+$$ + and Ni$$^+$$ + ions is observed in pure argon. With the addition of N$$_2$$ 2 or O$$_2$$ 2 gas the additional formation of molecular N$$_2^ +$$ 2 + or O$$_2^+$$ 2 + ions is observed. The intensity of these ions is reduced in the Ar+N$$_2$$ 2 +O$$_2$$ 2 gas mixture and molecular NO$$^+$$ + ions become the most abundant ionic species. The formation of neutral NO molecules is confirmed by optical emission spectroscopy. Gas samples collected at the exhaust of the vacuum chamber confirm the formation of NO and, additionally, of NO$$_2$$ 2 molecules. Graphic Abstract Sample taken from the exhaust of the plasma chamber with a liquid-nitrogen-cooled glass container showing solid nitrogen oxide (blue).

Abstract:

Abstract Benzene is a simple neutral aromatic compound found in molecular clouds, comets, and planetary atmospheres. It has been confirmed on Jupiter, Saturn, Titan, and is expected on exoplanets. In this paper, the decomposition of benzene in a simulated asteroid or comet impact into an N2-dominated atmosphere was investigated. The impact plasma was simulated with laser-induced dielectric breakdown and the gas phase decomposition products were observed using high-resolution Fourier transform infrared spectroscopy. The gas phase decomposition products involve mainly HCN, C2H2, and smaller amounts of CH4 with yields of 3.1%–24.0%, 0–11.7%, and 0.5%–3.3%, respectively. Furthermore, in presence of water, benzene also produces CO and CO2 with yields of 2.4%–35.1% and 0.01%–4.8%, respectively. The oxidation state of the product mixture is proportional to the water content. Apart from that, a black-brownish solid phase is formed during the experiments, which makes up about 60% of the original carbon content. Our results therefore show that in anoxic N2-dominated planetary atmospheres, impacts might lead to the depletion of benzene and the formation of HCN, C2H2, and CH4 and, in the presence of water, to the formation of CO and CO2.

Abstract:

<div>Asteroids and comets are often rich in polyaromatic hydrocarbons (PAHs), which represent in general about 30 % of total carbon in space. The fate of these molecules in extraterrestrial delivery was investigated by experimentally simulating asteroid impacts – using laser induced breakdown (LIBD) – into rocky planetary atmospheres and surfaces. Experiments were performed with benzene, naphtalene and anthracene as simple members of PAHs. The effect of LIBD on chemistry of these compounds in various chemical environments, such as atmospheric composition and solid phase matrices, were also studied. The main gas phase products are acetylene and HCN, followed by CO and CO2, whose yields mainly depend on the content of water vapour in the atmosphere. A brownish solid product was also observed. Therefore, simple aromatic compounds, and likely also PAHs, can be a viable source of HCN – molecule with significant prebiotic importance– in planetary environments.</div> <div> </div>

Abstract:

<p>Several of the Venera, Vega and Pioneer probe data as well as ground based observation support the presence of so-called Redox Disequilibrium Pairs (RDPs) in atmosphere of so hostile world as Venus (Greaves 2021). State of the art chemical networks cannot explain origin of an important RDP, phosphine, in oxidized atmosphere of Venus by a conventional processes (Bains 2021). We used the hybrid Density Functional Theory (DFT) for investigation of a series of chemical reaction pathways leading to the reduction of phosphate monoxide to phosphine. Our calculations indicated that a reaction network similar to photochemical synthesis of methane from carbon monoxide over acidic surfaces suggested for Mars (Civiš 2019) can also occur in clouds of Venus. As a seminal step, we have explored – via state-of-the-art quantum-based calculations – the a priori energetic feasibility of the following reaction:</p><p>HCO(radical) + PO = CO2 + PH (biradical).</p><p>Our calculations have shown that chemical conversion is constituted of three steps. Two of them are energetically favoured, however, the final conversion to phosphine is hardened by a significant activation barrier. This barrier can be overcome by a reaction of OPH radical with hydrogen radical. For assessing the potential of the newly introduced reaction mechanisms, models of the Venus and early Earth atmospheres in ARGO code and modified STAND chemical network were created and verified. Comparison of reaction yields suggests that this pathway is potentially effective enough and could be the source of phosphine recently discovered on Venus. </p><p>We acknowledge the support provided by the Czech Science Foundation within the project reg. no. 21-11366S and by ERDF/ESF "Centre of Advanced Applied Sciences" (No. CZ.02.1.01/0.0/0.0/16_019/0000778). We acknowledge support of the Czech Academy of Sciences, Strategy AV21, project VP16. The Czech team is part of the VenSpec-H Consortium onboard the ESA EnVision mission.</p><p>References:</p><p>Civiš S. et al.: Formation of Methane and (Per)Chlorates on Mars. ACS Earth Space Chem. 2019, 3, 2, 221–232.</p><p>Bains W. et al.: Phosphine on Venus Cannot Be Explained by Conventional Processes. Astrobiology 2021, 10 (21), 1277-1304.</p><p>Greaves J. S. et al.:  Phosphine gas in the cloud decks of Venus. Nature Astronomy 2021, 5, 655–664.</p>