David Dubois

David Dubois

Junior Professor at the Université Caen Normandie - GANIL

Publications

You can also find my articles on my Google Scholar profile.

Journal Articles

Modeling Low-temperature Plasmas Simulating Titan’s Atmosphere

Published in Planetary Science Journal, 2025

In the study presented here, we model the gas phase chemistry induced by plasma discharge at low temperatures (150 K) in the NASA Ames COSmIC Simulation Chamber (COSmIC) using a 1D multifluid plasma model named COSmIC Plasma Reactivity and Ionization Simulation Model. Our model incorporates an extensive chemical reaction network to simulate the neutral─neutral and ion─neutral reactions occurring in the COSmIC experiments when using N2─CH4-based gas mixtures relevant to Titan’s atmosphere. Our reaction network now includes crucial reactions involving the first electronically excited state of atomic nitrogen, recent electron collision cross sections, and radical chemistry. In particular, we have investigated the influence of C2H2 on the gas phase polymeric growth and the elemental composition of the chemical products, and we have compared our findings to recently published solid phase analyses. The modeling results are consistent with experimental measurements of N2─CH4─C2H2 plasmas on COSmIC, showing the production of C6Hx intermediates and precursors of larger organics, as well as methanimine in small concentration. Our numerical results point to cationic pathways enabling efficient intermediate-sized and nitrogen-rich C2H2-driven chemistry driving tholin production. Comparison of the modeled gas phase elemental composition with the elemental composition of the solid phase samples produced in COSmIC reveals similar trends, with C/N increasing when C2H2 is present in the gas mixture. Finally, our results demonstrate the importance of such synergistic studies using low-temperature plasma chemistry experiments combined with modeling efforts to improve our understanding of cold planetary environments.

Recommended citation: Dubois et al. (2025). PSJ 6:241
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Photochemical Haze Formation on Titan and Uranus: A Comparative Review

Published in International Journal of Molecular Sciences, 2025

The formation and evolution of haze layers in planetary atmospheres play a critical role in shaping their chemical composition, radiative balance, and optical properties. In the outer solar system, the atmospheres of Titan and the giant planets exhibit a wide range of com-positional and seasonal variability, creating environments favorable for the production of complex organic molecules under low-temperature conditions. Among them, Uranus-the smallest of the ice giants-has, since Voyager 2, emerged as a compelling target for future exploration due to unanswered questions regarding the composition and structure of its atmosphere, as well as its ring system and diverse icy moon population (which includes four possible ocean worlds). Titan, as the only moon to harbor a dense atmosphere, presents some of the most complex and unique organics found in the solar system. Central to the production of these organics are chemical processes driven by low-energy photons and electrons (<50 eV), which initiate reaction pathways leading to the formation of organic species and gas phase precursors to high-molecular-weight compounds, including aerosols. These aerosols, in turn, remain susceptible to further processing by low-energy UV radiation as they are transported from the upper atmosphere to the lower stratosphere and troposphere where condensation occurs. In this review, I aim to summarize the current understanding of low-energy (<50 eV) photon-and electron-induced chemistry, drawing on decades of insights from studies of Titan, with the objective of evaluating the relevance and extent of these processes on Uranus in anticipation of future observational and in situ exploration.

Recommended citation: Dubois et al. (2025). IJMS 26, 7531
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Investigating the condensation of benzene (C6H6) in Titan’s South polar cloud system with a combination of laboratory, observational, and modeling tools

Published in Planetary Science Journal, 2021

We have combined laboratory, modeling, and observational efforts to investigate the chemical and microphysical processes leading to the formation of the cloud system that formed at an unusually high altitude (>250 km) over Titan’s south pole after the northern spring equinox. We present here a study focused on the formation of C6H6 ice clouds at 87°S. As the first step of our synergistic approach, we have measured, for the first time, the equilibrium vapor pressure of pure crystalline C6H6 at low temperatures (134–158 K) representative of Titan’s atmosphere. Our laboratory data indicate that the experimental vapor pressure values are larger than those predicted by extrapolations found in the literature calculated from higher-temperature laboratory measurements. We have used our experimental results along with temperature profiles and C6H6 mixing ratios derived from observational data acquired by the Cassini Composite Infrared Spectrometer (CIRS) as input parameters in the coupled microphysics radiative transfer Community Aerosol and Radiation Model for Atmospheres (CARMA). CARMA simulations constrained by these input parameters were conducted to derive C6H6 ice particle size distribution, gas volume mixing ratios, gas relative humidity, and cloud altitudes. The impact of the vapor pressure on the CIRS data analysis and in the CARMA simulations was investigated and resulted in both cases in benzene condensation occurring at lower altitude in the stratosphere than previously thought. In addition, the stratospheric C6H6 gas abundances predicted with the new saturation relationship are ∼1000× higher than previous calculations between 150–200 km, which results in larger particle sizes.

Recommended citation: Dubois et al. (2021). PSJ 121, 2
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Positive ion chemistry in an N2-CH4 plasma discharge: Key precursors to the growth of Titan tholins

Published in Icarus, 2020

Titan is unique in the solar system as it hosts a dense atmosphere mainly made of molecular nitrogen N2 and methane CH4. The Cassini-Huygens Mission revealed the presence of an intense atmospheric photochemistry initiated by the photo-dissociation and ionization of N2 and CH4. 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 CH4 mixing ratios (1%, 5% and 10%) showing a variability in ion population. Low methane concentrations result in an abundance of amine cations such as NH4+ whereas aliphatic compounds dominate at higher methane concentrations. In conditions of favored tholin production, the presence of C2 compounds such as HCNH+ and C2H5+ 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 CH4 amounts is consistent with modeling work simulating aerosol growth in Titan’s ionosphere, which includes mass exchange primarily between HCNH+ and C2H5+ and negatively charged particles. These results also confirm the prevalent role of C2 cations as precursors to molecular growth and subsequent mass transfer to the charged aerosol particles as the CH4 abundance decreases towards lower altitudes.

Recommended citation: Dubois et al. (2020). 338, 113437
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C6H6 condensation on Titan’s stratospheric aerosols: An integrated laboratory, modeling and experimental approach

Published in Proceedings of the IAU, 2020

Saturn’s moon Titan was explored by the Cassini mission for nearly 13 years. Important discoveries made during the Cassini mission include the observations of stratospheric clouds in Titan’s cold polar regions in which spectral features or organic molecules were detected in the infrared (<100 um). In particular, benzene (C6H6) ice spectral signatures were recently detected at unexpectedly high altitudes over the South Pole. The combined experimental, modeling and observational effort presented here has been devised and executed in order to interpret these high altitude benzene observations. Our multi-disciplinary approach aims to understand and characterize the microphysics of benzene clouds in Titan’s South Pole.

Recommended citation: Dubois et al. (2020). Proceedings of the IAU 350 5-8
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C6H6 condensation on Titan’s stratospheric aerosols: An integrated laboratory, modeling and experimental approach

Published in Proceedings of the IAU, 2020

We have combined laboratory, modeling, and observational efforts to investigate the chemical and microphysical processes leading to the formation of the cloud system that formed at an unusually high altitude (>250 km) over Titan’s south pole after the northern spring equinox. We present here a study focused on the formation of C6H6 ice clouds at 87°S. As the first step of our synergistic approach, we have measured, for the first time, the equilibrium vapor pressure of pure crystalline C6H6 at low temperatures (134-158 K) representative of Titan’s atmosphere. Our laboratory data indicate that the experimental vapor pressure values are larger than those predicted by extrapolations found in the literature calculated from higher-temperature laboratory measurements. We have used our experimental results along with temperature profiles and C6H6 mixing ratios derived from observational data acquired by the Cassini Composite Infrared Spectrometer (CIRS) as input parameters in the coupled microphysics radiative transfer Community Aerosol and Radiation Model for Atmospheres (CARMA). CARMA simulations constrained by these input parameters were conducted to derive C6H6 ice particle size distribution, gas volume mixing ratios, gas relative humidity, and cloud altitudes. The impact of the vapor pressure on the CIRS data analysis and in the CARMA simulations was investigated and resulted in both cases in benzene condensation occurring at lower altitude in the stratosphere than previously thought. In addition, the stratospheric C6H6 gas abundances predicted with the new saturation relationship are ~1000× higher than previous calculations between 150-200 km, which results in larger particle sizes.

Recommended citation: Dubois et al. (2020). Proceedings of the IAU 350 5-8
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In Situ Investigation of Neutrals Involved in the Formation of Titan Tholins

Published in Icarus, 2019

The Cassini Mission has greatly improved our understanding of the dynamics and chemical processes occurring in Titan’s atmosphere. It has also provided us with more insight into the formation of the aerosols in the upper atmospheric layers. However, the chemical composition and mechanisms leading to their formation were out of reach for the instruments onboard Cassini. In this context, it is deemed necessary to apply and exploit laboratory simulations to better understand the chemical reactivity occurring in the gas phase of Titan-like conditions. In the present work, we report gas phase results obtained from a plasma discharge simulating the chemical processes in Titan’s ionosphere. We use the PAMPRE cold dusty plasma experiment with an N2eCH4 gaseous mixture under con- trolled pressure and gas influx. An internal cryogenic trap has been developed to accumulate the gas products during their production and facilitate their detection. The cryogenic trap condenses the gas-phase precursors while they are forming, so that aerosols are no longer observed during the 2 h plasma discharge. We focus mainly on neutral products NH3, HCN, C2H2 and C2H4. The latter are identified and quantified by in situ mass spec- trometry and infrared spectroscopy. We present here results from this experiment with mixing ratios of 90–10% and 99–1% N2-CH4, covering the range of methane concentrations encountered in Titan’s ionosphere. We also detect in situ heavy molecules (C7). In particular, we show the role of ethylene and other volatiles as key solid- phase precursors.

Recommended citation: Dubois et al. (2019). 317, 182-196
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Nitrogen-containing Anions and Tholin Growth in Titan’s Ionosphere : Implications for Cassini CAPS-ELS Observations

Published in Astrophysical Journal Letters, 2019

The Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) instrument on board Cassini revealed an unexpected abundance of negative ions above 950 km in Titan’s ionosphere. In situ measurements indicated the presence of negatively charged particles with mass-over-charge ratios up to 13,800 u/q. At present, only a handful of anions have been characterized by photochemical models, consisting mainly of CnH− carbon chain and Cn−1N− cyano compounds (n = 2–6); their formation occurring essentially through proton abstraction from their parent neutral molecules. However, numerous other species have yet to be detected and identified. Considering the efficient anion growth leading to compounds of thousands of u/q, it is necessary to better characterize the first light species. Here, we present new negative ion measurements with masses up to 200 u/q obtained in an N2:CH4 dusty plasma discharge reproducing analogous conditions to Titan’s ionosphere. We perform a comparison with high- altitude CAPS-ELS measurements near the top of Titan’s ionosphere from the T18 encounter. The main observed peaks are in agreement with the observations. However, a number of other species (e.g., CNN−, CHNN−) previously not considered suggests an abundance of N-bearing compounds, containing two or three nitrogen atoms, consistent with certain adjacent doubly bonded nitrogen atoms found in tholins. These results suggest that an N-rich incorporation into tholins may follow mechanisms including anion chemistry, further highlighting the important role of negative ions in Titan’s aerosol growth.

Recommended citation: Dubois et al. (2019). ApJL 872:L31
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