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Review

Photochemical Haze Formation on Titan and Uranus: A Comparative Review

1
NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035, USA
2
Bay Area Environmental Research Institute, Moffett Field, CA 94035, USA
Int. J. Mol. Sci. 2025, 26(15), 7531; https://doi.org/10.3390/ijms26157531 (registering DOI)
Submission received: 12 June 2025 / Revised: 26 July 2025 / Accepted: 1 August 2025 / Published: 4 August 2025
(This article belongs to the Special Issue Chemistry Triggered by Low-Energy Particles)

Abstract

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 compositional 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.
Keywords: solar system; Titan; Uranus; giant planets; atmospheres; ion-neutral reactions; haze formation; astrochemistry solar system; Titan; Uranus; giant planets; atmospheres; ion-neutral reactions; haze formation; astrochemistry

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MDPI and ACS Style

Dubois, D. Photochemical Haze Formation on Titan and Uranus: A Comparative Review. Int. J. Mol. Sci. 2025, 26, 7531. https://doi.org/10.3390/ijms26157531

AMA Style

Dubois D. Photochemical Haze Formation on Titan and Uranus: A Comparative Review. International Journal of Molecular Sciences. 2025; 26(15):7531. https://doi.org/10.3390/ijms26157531

Chicago/Turabian Style

Dubois, David. 2025. "Photochemical Haze Formation on Titan and Uranus: A Comparative Review" International Journal of Molecular Sciences 26, no. 15: 7531. https://doi.org/10.3390/ijms26157531

APA Style

Dubois, D. (2025). Photochemical Haze Formation on Titan and Uranus: A Comparative Review. International Journal of Molecular Sciences, 26(15), 7531. https://doi.org/10.3390/ijms26157531

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