Effect of Cosmic Radiation on the Chemistry of a Simulated Titan Atmosphere

1. Introduction

The photochemistry in Titan’s atmosphere begins with the dissociation and ionization of its main atmospheric constituents N₂ and CH₄, therefore, the nitrogen chemistry, aerosol formation, and atmospheric loss energy sources are driven from external energy sources such as Solar UV, Saturn´s magnetosphere, solar wind and galactic cosmic rays [1].

Cosmic particles, composed mainly of protons and α particles, possess a higher penetration power, which makes cosmic‑ray radiation the main mechanism responsible for ionizing Titan’s lower atmosphere [2]. For this reason, studying its effect on Titan’s atmosphere is of great importance, especially because the connection between Titan’s surface and atmosphere is unique in our Solar System: atmospheric chemistry produces materials that are deposited onto the surface and subsequently modified by surface–atmosphere interactions [3].

2. Experimental Methods

2. 1. Preparation of Titan simulated atmosphere.

The gas mixture (10% methane in nitrogen) used to simulate Titan’s atmosphere was prepared using a gas‑blending system (Figure 1) located at the Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México (UNAM). For this purpose, high‑purity gases supplied by Linde were used (N₂ = 4.8 and CH₄ = 3.7).

The gas‑blending system is a Linde FM 4660 model equipped with eight channels, each fitted with its corresponding mass‑flow controller. These controllers provide a maximum flow rate of 20 mL min⁻¹ and a minimum of 2 mL min⁻¹, allowing precise regulation of the gas flows and, consequently, the required proportions for simulating Titan’s atmosphere.

Likewise, this gas‑blending unit is connected to a high‑vacuum system, which allows the system to reach pressures below 2 × 10⁻³ mbar.

Once prepared, the gas mixture was allowed to equilibrate inside the gas containers for at least 24 hours to ensure proper homogenization. Subsequently, the reactors each with an average internal volume of 1.16 ± 0.04 L were connected to the vacuum line, and their interiors were evacuated to a pressure of 2.4 × 10⁻² mbar before introducing the gas mixture. After filling, an internal pressure of 1000 mbar was reached, corresponding approximately to the atmospheric pressure at an altitude of 10 km above Titan’s surface, where cosmic rays are expected to have a stronger effect. Once this pressure was reached, the reactors were sealed.

Figure 1. Diagram illustrating the system used for the preparation of the simulated Titan atmosphere, as well as the filling of the reactors through the vacuum line. Provided by M.C. José de la Rosa Canales, personal communication.

 

2. 2. Irradiation system.

The irradiation was carried out in the Unidad de Irradiación, Instituto de Ciencias Nucleares, UNAM, which is equipped with a Gammabeam 651PT deep‑pool irradiator, where the simulated atmosphere was exposed to different levels of accumulated gamma‑radiation dose, generated by ⁶⁰Co sources.

2. 3. Analysis system.

The analysis after irradiation was performed using the coupled analytical technique of Gas Chromatography–Mass Spectrometry (GC–MS), which allows the separation, identification, and quantification of the compounds generated. Each analysis began at 50 °C with a five‑minute isothermal hold, followed by a temperature ramp of 10 °C min⁻¹ until reaching a final temperature of 240 °C. These conditions were maintained until the end of the run, resulting in a total analysis time of 30 minutes.

 

3. Results and discussion

3. 1. Identification of the Compound

The chromatograms revealed the formation of a total of seven compounds (Figure 2). The analyses indicated that all the products generated were saturated hydrocarbons (linear and branched), including ethane, propane, butane, and isobutane, which exhibited the highest response in the coupled analytical system.

The identification of these compounds was carried out by comparing their mass spectra with those from reference libraries and by injecting analytical standards.

Figure 2. Separation by gas chromatography of the compounds produced after irradiating at 483 kGy a mixture of methane (10%) in nitrogen at 298 K and a pressure of 1000 mbar. Peaks: 1 = nitrogen; 2 = methane; 3 = ethane; 4 = propane; 5 = isobutane; 6 = butane; 7 = 2‑methylbutane; 8 = 2,2‑dimethylbutane; and 9 = 2,3‑dimethylbutane.

 

3.2. Determination of Hydrocarbon Production Rate as a Function of Energy Dose

The abundance of each generated product was determined and converted to the corresponding number of molecules by interpolating the abundance values on the appropriate calibration curve (Figure 3).

 

Figure 3. Abundance curve of the compounds generated by gamma radiation.

 

In Table 1 it summarizes the molecular production rates obtained for the simulated Titan atmosphere exposed to different gamma-radiation doses.

Table 1. Hydrocarbon production rate under gamma‑radiation incidence.

 

3. 3. Estimation of hydrocarbon production per terrestrial year on Titan
Using the cosmic ray flux reaching Titan as measured by the Voyager 1 mission (9.0 × 10-6 J/(m^2*s)) [4], an estimate was made of the potential annual formation rate of these hydrocarbons directly in Titan’s atmosphere, per terrestrial years (Table 2). 

Table 2. Hydrocarbon production rate due to cosmic ray incidence occurring in Titan’s atmosphere, expressed in tonnes (t) per terrestrial year

   

 4. Conclusion

Under the experimental conditions employed in this study gamma radiation exclusively promotes the formation of saturated hydrocarbons, both linear and branched, which include ethane, propane, isobutane, and butane, with ethane emerging as the most abundant species, exhibiting concentrations at least one order of magnitude higher than propane and two orders of magnitude higher than isobutane and butane. In contrast, gamma radiation did not contribute to the synthesis of unsaturated hydrocarbons (alkenes, alkynes, or aromatic compounds) or nitriles.

Furthermore, estimations of hydrocarbons per Earth year on Titan suggest that, over time, this hydrocarbons could accumulate in the satellite’s atmosphere and on its surface, potentially serving as precursors to other organic compounds of astrobiological relevance.

References

[1] Sagan, C., et al. (1992). Titan: a laboratory for prebiological organic chemistry. Accounts of Chemical Research, 25(7), 286–292. 

[2] Molina-Cuberos., et al  (1999). Ionization by cosmic rays of the atmosphere of Titan. Planetary and space science, 47(10-11), 1347-1354.

[3] Hörst, S. (2017). Titan's atmosphere and climate. Journal of Geophysical Research: Planets, 122(3), 432-482.

[4] Sagan, C., & Reid Thompson, W. (1984). Production and condensation of organic gases in the atmosphere of Titan. Icarus, 59(2), 133–161.