Photochemical Haze Formation on Titan and Uranus: A Comparative Review
Abstract
1. Introduction
- Section 2 will include a review of the atmospheric and radiative environments of the ice giants with an emphasis on Uranus and how they differ from Titan in the Saturnian system. In addition, I will survey the current state-of-the-art body of knowledge of their chemical inventories.
- Section 3 will include a review of the state of knowledge of low-energy (<50 eV) photochemistry-induced mechanisms on Titan and Uranus, covering observations and in situ measurements, experimental simulations of gas phase and condensed state chemistry, photochemical modeling, dicationic chemistry, and recent advances in quantum chemical calculations. Important aspects of branching ratio determination will also be discussed.
- Section 4 will be dedicated to negative ions. Discoveries pertaining to negative ion chemistry on Titan deserve their own section, as their participation in molecular and haze growth requiring photochemical and radiative processes has proven to be substantial.
- Section 5 will conclude with a summary of potential future investigations needed to probe Uranus and prepare for upcoming studies before future missions to the gas giants.
2. The Chemical and Radiative Environments of Titan and the Ice Giants
2.1. Ice Giants: General Considerations
2.2. The Atmosphere of Uranus
2.3. Chemical Inventory in Uranus
2.4. The Atmosphere of Titan
2.5. Chemical Inventory in Titan
2.6. From Dynamics to Haze Stratification and Evolution
3. Photochemistry vs. Radiative Chemistry: Competing Processes and Role in Haze Formation
3.1. Fundamental Processes
- photodissociation:
- quenching:
- luminescence:
- photoisomerization:
- bimolecular reaction:
- hydrogen abstraction:
- Ionization:
- Ion dissociation:
- Ion-molecule reaction:
- Electron attachment:
- Fluorescence:
- Excimer formation:
3.2. Observational Considerations: From Low-Mass to Intermediate-Mass Molecules
3.2.1. Low-Mass Species
3.2.2. Higher-Mass Species
3.2.3. Polycyclic Aromatic Hydrocarbons: Agents of Haze Growth?
3.3. Laboratory Experiments: Simulating Atmospheric Chemistry
- Atmospheric chemistry science: Multiple complementary laboratory experiments (Table 6) utilizing different sources of energy (plasmas, UV lamps, high-energy synchrotron beamlines) are substantial to probe specific chemical and photoionizing processes. In particular, questions surrounding the role of ion-molecule chemistry and haze growth on Titan have significantly benefitted from laboratory studies. Coupled with in situ or ex situ analyses such as high-resolution mass spectrometry, IR spectroscopy, electron microscopy, secondary ion mass spectrometry, X-ray photoemission spectroscopy, atmospheric-pressure photoionization, just to name a few, future measurements would provide much insights into the chemical composition of Uranian tholins and gas phase precursors. Furthermore, laboratory characterizations of photochemical products would help support in situ measurements by a future UOP and help quantify these precursors resulting from the photodissociation of CH4, NH3, etc.
- Cloud science: Laboratory measurements of the physical and optical properties of any Uranian laboratory-produced aerosols would directly provide valuable information to interpret cloud observations and the modeled scattering, nucleating, and size properties of the CCN. Their properties would then help address the role and interaction of clouds with condensable species. Moreover, studies of the photochemical evolution under low-energy (as well as of much higher-energy) photon irradiation remains critically unexplored.
- Chemical kinetics & thermodynamics: As outlined below, kinetic rates, branching ratios, and absorption cross-sections are fundamental properties that are needed to solve model degeneracies and inaccurate abundance retrievals (Table 7). Future theoretical calculations combined with experimental measurements are much needed.
3.4. Condensed Phase
Molecule | Photochemical Products | br |
---|---|---|
+ H | 0.42 | |
1CH2 + | 0.48 | |
3CH2 + 2H | <0.1 | |
CH + + H or C + 2 | <0.1 | |
→ + h (fluorescence) | 0.8–0.9 | |
H + H (predissociation) | 0.1–0.2 | |
C2H2 | C2H + H | 0.3 |
C2 + H2 | 0.1 | |
C2H2* → C2H2 | 0.6 | |
C2H2+ +e− | 0.84 a | |
C2H4 | C2H2 + H2 | 0.58 b |
C2H2 + 2H | 0.42 b | |
C2H6 | C2H4 + H2 | 0.12 |
C2H4 + 2H | 0.30 | |
C2H2 → 2H2 | 0.25 | |
CH4 +1CH2 | 0.25 | |
2CH3 | 0.08 | |
CH3C2H | C3H3 + H | 0.56 c |
C3H2 + H2 | 0.44 c | |
C3H8 | C3H6 + H2 | 0.34 d |
C2H6 + 1CH2 | 0.09 d | |
C2H5 + CH3 | 0.35 d | |
C2H4 + CH4 | 0.22 d | |
C4H2 | C4H + H | 0.20 e |
2C2H | 0.03 e | |
C2H2 + C2 | 0.10 e | |
C4H2* | 0.67 e | |
CH3CN | CH3 + CN | 0.20 f |
CH2CN + H | 0.80 f | |
H2S | H2 + S(1D) | <0.12 g |
AROM | C6H6 + photoproducts | 0.1–0.3 h |
3.5. Branching Ratios
3.6. Dication Chemistry and Photo Double Ionization Processes
4. Negative Ion Chemistry and Haze Growth
4.1. Anions on Titan
Species | Reaction | Rates (cm3 s−1) | Ref. |
---|---|---|---|
H− | CH4 + e− → H− + CH3 | () | [285] |
H− + h → H + e | () | Miller-threshold Law | |
CN− | H− + HCN → CN− + H2 | [286] | |
CN− + H → HCN + e | [286] | ||
C2H− | + → + | [282] | |
H + H → C2H2 + e | [287] | ||
C3N− | C3N + e → C3N− + h | [288] | |
C3N− + H →HC3N | [289] | ||
C5N− | CN− + HC5N → C5N− + HCN | Su-Chesnavich | |
C5N− + H → HC5N | [289] | ||
C4H− | C4H + e → C4H− + h | [288] | |
C4H− + H → C2H2 + e | [287] | ||
C6H− | H− + C6H2 → C6H− + H2 | Langevin | |
C6H− + H → Products | [287] | ||
OH− | H− + H2O → OH− + H2 | [282] | |
OH− + h → OH + e | Miller-threshold Law | ||
O− | H2O + e → O− + H2 | () | [290] |
O− + h → O + e | () | Miller-threshold Law |
4.2. Anions on Uranus
4.3. Summary: Dissociative Electron Attachment (<20 eV)
5. Summary: Opportunities for Future Studies
Funding
Acknowledgments
Conflicts of Interest
Abbreviations
UOP | Uranus Orbiter Probe |
GCR | Galactic Cosmic Rays |
UVS | Ultraviolet Spectrometer |
NUV | Near Ultraviolet |
FUV | Far Ultraviolet |
VUV | Vacuum Ultraviolet |
EUV | Extreme Ultraviolet |
LIPM | Local Interplanetary Medium |
ISRF | Interstellar Radiation Field |
ISM | Interstellar Medium |
JWST | James Webb Space Telescope |
CRIR | Cosmic Ray Ionization Rate |
INMS | Ion and Neutral Mass Spectrometer |
CAPS | Cassini Plasma Spectrometer |
SSI | Solar Spectrum Irradiance |
CCN | Cloud Condensation Nuclei |
DEA | Dissociative Electron Attachment |
TNI | Temporary Negative Ions |
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Region 1 | Region 2 | Region 3 | |
---|---|---|---|
Near/Far-UV (400–121.6 nm) | Lyman- (121.6 nm) | EUV/VUV (121.6–25 nm) | |
Energy range (eV) | 3.1–10.2 | 10.2 | 10.2–49.6 |
Mass () | Solar Constant (W m−2) | EUV Intensity (kR), 90–110 nm | T (1 bar Level) in Kelvin | Mean Molecular Weight | |
---|---|---|---|---|---|
Titan | 0.023 | 14.8 | 0.21 | 94 | 27.8 |
Saturn | 95.2 | 14.8 | 0.21 | 134–145 | 2.0 |
Uranus | 14.5 | 3.7 | 0.05 | 76–86 | 2.3 |
Atoms | C | N | O | Other | ||||
---|---|---|---|---|---|---|---|---|
Titan | Uranus | Titan | Uranus | Titan | Uranus | Titan | Uranus | |
1 | CH4 | CH4 | HCN, HNC, CH3CN, HC3N, C3H3N, C3H5N, C4H3N | - | H2O, CO | CO | - | H2, H2S |
2 | C2H2, C2H4, C2H6 | C2H2, C2H6 | N2, C2N2 | - | CO2 | CO2 | - | - |
3 | C3H2, C3H4, C3H6, C3H8 | C3H4 | - | - | - | - | - | - |
4 | C4H2 | C4H2 | - | - | - | - | - | - |
5 | - | - | - | - | - | - | - | - |
6 | C6H6 | - | - | - | - | - | - | - |
Stratosphere | ||||
---|---|---|---|---|
Species | Titan | Saturn | Uranus | Ref. |
CH4 | 1–2% | 16 ppm | [17,65,66] | |
C2H2 | 0.25 ppm | [67,68,69,70] | ||
C2H4 | < | [17,70,71] | ||
C2H6 | 0.13 ppm | [17,67,68,69,70] | ||
C3H4 | 0.36 ppb | [17,70,72] | ||
C4H2 | 0.13 ppb | [17,70,72] | ||
CO2 | 0.08 ppb | [17,70,73] | ||
CO | 6 ppb | [17,70,74] | ||
H2O | 1.1 ppb | 3.8 ppb | [20,70,75] | |
D/H (in H2/C2H2) | [20,76,77] |
Characteristics | Photochemistry | Radiation-Induced Chemistry | Examples |
---|---|---|---|
Energy source | Ly-; UV continuum | EUV/X-rays; energetic particles | 100–400 nm; secondary electrons; ions |
Primary effect | Photodissociation; photoionization; electronic excitation | Ionization; radiolysis; dissociative electron attachment | + → + H → H− + CH3+ |
Key products | Radicals; small hydrocarbons | Ions (e.g., ); complex organics; electrons | H2+, CH3, CH3+, C2H3+, C3H4+ |
Timescales | ns–hours (daylight-driven) | fs–ns (instantaneous, flux-dependent) | O → H• + OH• (spur reactions) |
Temperature dependence | Strong (Arrhenius kinetics) | Weak (governed by particle flux) | CH4 + H → CH3 + H2 |
Electron transfer | Charge transfer | Ionization cascades; secondary electron emission | O+ + CH4 |
Quantum effects | Electronic transitions; spin-forbidden pathways | Ro-vibrational excitation; plasmon resonances (ices) | singlet-triplet absorption |
Observables | Dayglow emissions; gas abundances | Auroral X-rays; Lyman-Werner band emissions; mass spectra of ices | mass spectra, IR-UV spectra |
Altitude/region | Stratosphere; ionosphere (day side) | Thermosphere; polar auroral zones; interstellar ices | + → + H |
Desorption yields | Low–moderate (UV-photon dependent) | High (sputtering by >100 eV electrons) | CO + e− → CO(ads) → CO(g) |
Multiphoton effects | Rare | Dominant (ionization cascades; track formation) | → + e− (15.6 eV) |
Category 1 | Main Processes | Energy Source | References |
---|---|---|---|
Gas phase | Ionization, dissociation, radical chemistry | Plasma discharges | [206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226] |
Photolysis, radical, excitation, SE | FUV–Ly-–EUV | [220,227,228,229,230,231,232,233,234,235,236,237,238,239,240] | |
Tholins/ice | Condensation, solid-state photochemistry, SE | FUV/VUV | [104,119,120,121,178,198,241,242,243,244,245,246,247] |
Synchrotron | Ionization, dissociation, excitation, SE | EUV-VUV Target wavelength | [232,233,236,248,249,250,251] |
Reaction | Photochemical Pathway | Quantum Yield () |
---|---|---|
N+ + CH4 | CH3+ + NH | 0.50 |
CH4+ + N | 0.05 | |
H2CN+ + H2 | 0.10 | |
HCN+ + NH + H | 0.36 | |
N2+ + CH4 | CH2+ + N + H2 | 0.09 |
CH3+ + N + H | 0.91 | |
N2H+ + CH3 | - | |
CH3N2+ | N2CH2+ + H | 0.01 |
Atom | H production channels | H atom yield |
H | CH + CH4 | 1.00 |
CH + C2H6 | 0.22 | |
CH + C2H6 | 0.14 | |
CH + C3H8 | 0.19 | |
CH + C4H10 | 0.14 |
Parent | Fragment | Resonance Position (eV) | Cross-Section Peak (cm2) | Refs. |
---|---|---|---|---|
CH4 | CH2− | 10.4 | [127] | |
H− | 9.8 | [127] | ||
H2 | H− | 4.0 | [299] | |
14 | [299] | |||
D2 | D− | 14.0 | [299] | |
C2H2 | C2H− | 2.8 | [300] | |
C2− | 8.3 | [300] | ||
H− | 7.9 | [300] | ||
C2H4 | H− | 10.5 | [301,302] | |
CH− | 9.8 | ion yield | [302] | |
C2H− | 9.8 | ion yield | [302] | |
C2H2− | 1.6 | ion yield | [302] | |
C2H3− | 7.0 | ion yield | [302] | |
C2H6 | H− | 9.2 | ion yield | [301] |
C3H4 | C3H3− | 3.4 | [303] | |
C3H8 | H− | 8.6 | ion yield | [301] |
C4H2 | C4H− | 2.5 | [304] | |
5.3 | [304] | |||
C4H6 | H− | 4.0 | [303] | |
C6H2 | C6H− | 2.8 | , est. | [278,298,304] |
HCN | CN− | 1.9 | [305] | |
DCN | CN− | 1.9 | [305] | |
NH3 | H− | 5.7 | [285] | |
NH2− | 5.9 | [285] | ||
CH2N2 | CN− | 6.4 | [306] | |
C2H4N2 | CN− | 1.9 | [307] | |
H2S | HS− | 1.6 | [308,309] | |
S− | 9.7 | [308,309] |
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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
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 StyleDubois, 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 StyleDubois, 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