Reaction of Atomic Oxygen with Thiophene: Implications for Satellite Polymers in Low Mars Orbit and Chemistry of Mars

Aromatic compounds, with their stable cyclic structure and [4n+2]π electrons, are resistant to chemical attack and degradation. This stability makes them prevalent in celestial bodies and valuable in designing polymers that withstand harsh space conditions [1-4].
In interstellar space, aromatic molecules make up ~20% of atomic carbon and are key to forming complex organic molecules [1]. Cyanopyrene, cyanonaphthalene, and indene have been identified in the TMC-1 molecular cloud [5]. Aromatic molecules are also found in Solar System objects, including Martian soil from Gale Crater mudstones [7-10].
Thiophene, an aromatic molecule, was detected by NASA’s Curiosity rover in the Glen Torridon clay unit, where high-temperature pyrolysis (~850°C) revealed sulfur-bearing organics, including alkyl derivatives, likely from Martian organic materials [9]. Atomic oxygen (O) in its ground state (³P) is a strong oxidant that degrades aromatic compounds like benzene and pyridine, releasing CO [10-13]. Recent models show O(³P) is present in small amounts on Mars’s surface and abundant in low orbit [13]. This presents a dual challenge: degrading thiophene-based polymers used in spacecraft and explaining Mars's organic scarcity [16].
Using quantum chemistry methods, we examined thiophene fragmentation from O(³P) interactions. Our results matched experimental data from the crossed molecular beam (CMB) scattering technique [10], showing that the reaction forms thioacrolein and CO, attacking the sulfur atom and breaking the aromatic ring. This ISC-enhanced mechanism may destabilize sulfur-containing polymers and contribute to organic compound loss on Mars.

Additionally, the photodissociation of O₃ on Mars generates highly reactive atomic oxygen in the excited ¹D state, which likely accelerates organic degradation [13]. While photodissociation degrades complex organics, residual organic matter remains unless converted to volatile species. These findings are pivotal for developing space-resilient materials and understanding atomic oxygen's role in Mars's chemical evolution. Furthermore, the degradation products, including released carbon, may contribute to forming prebiotic molecules, enriching the diversity of planetary systems and interstellar chemistry.

References
[1] A.G.G.M. Tielens, Rev. Mod. Phys. 85, 1021.


[2] D.A.F.T.W. Strganac, et al., J. Spacecr. Rocket 1995, 32,502–506


[3] K.K. De Groh, et al., High Perform. Polym. 2008, 20, 388–409


[4] T. K. Minton, et al., ACS Appl. Mater. Interfaces 2012, 4, 492−502


[5] G. Wenzel, et al., Science, 2024, 386,810-813.


[6] M.A. Sephton, Nat. Prod. Rep., 2002,19, 292-311


[7] C. Sagan, et al., Astrophys. J., 414, 1, 399-405

[8] J. L. Eigenbrode, et al., Science, 2018, 360, 1096–1101


[9] M. Millan, et al., J. Geophys. Res. Planets, 2022, 127, e2021JE007107


[10] Vanuzzo G., et al., J.  Phys. Chem. A, 2021, 125, 8434–8453


[11] Recio P., et al., Nat. Chem., 2022, 14, 1405–1412


[12] J. Lasne, et al., Astrobiology, 2016, 16, 977


[13] G. M. Paternò, et al., Scientific Reports, 2017, 7, 41013


[14] S. A. Benner, et al., PNAS, 2000, 97, 6, 2425–2430