On Titan, Saturn's largest moon, chemical reactions occur at extremely low temperatures, challenging our understanding of chemistry. However, recent simulations suggest that quantum tunnelling may play a crucial role in enabling these reactions in space. In this astrobit we tell you how quantum chemistry simulations are changing the models we use to study planetary atmospheres and the search for extraterrestrial life.
Paper details:
Title: Quantum tunnelling could enable proton transfer reactions on Titan
Authors: Henry W. Longo, Richard C. Remsing
First-author Institution: Department of Chemistry and Chemical Biology, Rutgers University, USA
Status: Open acces on arXiv
Titan, Saturn's largest moon, shares some characteristics with early Earth, such as a dense atmosphere, stable surface liquids, and a wide range of organic molecules, including liquid hydrocarbons. These similarities make Titan an ideal candidate to study what prebiotic chemistry might have been like, or even to study the potential for hosting life that other bodies beyond Earth have.
However, the analogy has a limit, since Titan's icy environment does not, in principle, provide the thermal energy necessary to promote chemical reactions. In other words, the temperatures are so low (~94 K) that the molecules cannot react with each other. But what if temperature is not the only factor that facilitates chemistry in environments like Titan's? Well, in this astrobito we tell you how quantum tunnelling could allow chemical reactions under these very adverse conditions.
Quantum tunnelling is a phenomenon predicted by quantum mechanics in which particles pass through energy barriers rather than over them (Figure 1). For example, for two molecules to react, they need to overcome an energy barrier in which they break and form atomic bonds to finally reach the reaction product. This process is facilitated at high temperatures, since the molecules can reorganise more easily and therefore overcome the energy barrier.
At low temperatures, however, molecules move more slowly and do not have enough energy to break and form new bonds. This is where quantum tunnelling plays a key role by allowing molecules, instead of wanting to overcome the energy barrier, to cross it, giving way to the formation of products as seen in Figure 1. In other words, quantum tunnelling is a "shortcut" for chemical reactions that is facilitated at low temperatures. This quantum phenomenon is particularly important for reactions involving light nuclei, such as hydrogen, which can exhibit pronounced tunnelling effects.
Figure 1. Schematic energy diagram of a chemical reaction. The red line represents the energy change in a classic chemical reaction in which the reactants have to overcome an energy barrier to reach the products. The blue line corresponds to the alternative path that makes quantum tunnelling possible.
In space, quantum tunnelling could enable chemical reactions in cold environments, such as interstellar clouds and icy moons like Titan. This has profound implications for astrochemistry, suggesting that complex organic molecules could form under conditions previously considered too extreme for chemical activity.
To explore the potential of quantum tunnelling as a facilitator of chemical reactions on Titan, the research team in this paper employed advanced quantum chemistry simulations. This allowed them to investigate how much proton transfer is facilitated in Titan-like environments. Proton transfer are chemical reactions that involve the movement of protons (positively charged hydrogen ions) between molecules, which is crucial in many biological systems, facilitating processes such as cellular respiration and photosynthesis.
Normally, protons must overcome an energy barrier to move from one molecule to another (like the barrier in Figure 1). In other words, they need a certain “minimum energy” to carry out the reaction. At Earth-like temperatures, thermal energy is sufficient to help protons overcome these barriers. However, in colder environments like Titan, traditional thermal activation is insufficient.
Simulations performed in the paper demonstrate that quantum tunnelling significantly facilitates proton transfer even at Titan's frigid temperatures. Figure 2 shows the probability distribution diagrams of proton transfer between reactants and products at 94 K (left) and 300 K (right), temperatures that simulate standard conditions on Titan and Earth, respectively. The lines in the figure represent the molecules that have the proton that will be transferred throughout the reaction. In the yellow line, the reactant has the proton, in the blue line the product, and the red line represents the transition of the proton from one molecule to another.
At 300 K, the unimodal distribution of the red line in the middle of the figure indicates a clear transition state in which the yellow and blue molecules are forming and breaking bonds to transfer the proton (in red). This is the general process in which temperature-driven reactions occur. In contrast, at 94 K, the left panel shows a bimodal distribution, suggesting that the proton is transferred from one molecule to another without the formation of the transition state present in the diagram at 300 K. This evidences the presence of quantum tunnelling in the reaction since it shows that the proton is in both molecules “at the same time”.
Figure 2. Probability distribution (red line) for a proton transfer reaction at 94 K (left, Titan conditions) and 300 K (right, Earth conditions). The yellow and blue lines correspond to the reactants and products of the reaction, respectively, and the red lines the states where the proton transfer occurs. Credit: adapted from Figure 3 of the main article.
These results provide strong evidence that quantum tunnelling can occur at Titan's low temperatures, potentially facilitating core chemical reactions that allow the formation of more complex molecules.
The implications of these findings are important for astrobiology and planetary science. The presence of quantum tunnelling-driven reactions on Titan could suggest that other cold environments in the solar system and beyond could also host complex chemistry. This expands the potential habitability of these regions.
Furthermore, these findings highlight the need to incorporate quantum tunnelling effects into atmospheric models of planets or moons, as traditional models may underestimate the reaction rates of key processes involving light nuclei. Therefore, the inclusion of quantum tunnelling in these models could significantly improve our understanding of the atmospheric chemistry on Titan and other cold astronomical bodies, leading to more accurate predictions of their chemical compositions and their potential to support life.
In turn, the study of quantum tunnelling in Titan's chemistry opens new avenues for research into the origins of life and the development of more precise planetary models. As we continue to explore our Universe, these insights could help not only better understand the chemical processes that occur under extreme conditions but also guide our search for extraterrestrial life.