Radiation Astrochemistry in Low Temperature Ices: From Dense Interstellar Clouds to our Cosmic Neighbourhood
Dr. Duncan Mifsud
HUN-REN Institute for Nuclear Research (ATOMKI), Debrecen, Hungary
In the 1920s and 1930s, evidence emerged for the presence of molecules in interstellar environments.1,2 This proved challenging to rationalise, particularly since these environments are characterised by low temperatures and pressures which should, in principle, preclude two atoms from coming together and surmounting an activation energy barrier to produce a molecule. Furthermore, space is a radiation-rich environment, and so any nascent molecule should be readily destroyed by the interstellar radiation field. Nevertheless, mounting evidence for the existence of interstellar molecules seemed to be incontrovertible.
The solution to this apparent paradox lies in the fact that interstellar environments are not homogeneous, and large cloud-like structures (i.e., nebulae) exist that have particle den- sities which are sufficiently high (~10–6 cm–3) as to effectively block the ultraviolet component of the interstellar radiation field. The net result is that the temperature within the core of the cloud drops to circa 10-20 K, and gas-phase atoms and molecules condense onto the surfaces of nanoscale dust grains. These dust grains act as heterogeneous catalysts in the formation of new molecular species via a variety of mechanisms,3 such as atom or radical additions and low-temperature thermal reactions. Importantly, dense interstellar clouds do not attenuate galactic cosmic rays, and thus they may trigger chemistry within the so-called ‘interstellar icy grain mantles’. This chem- istry may be triggered directly (i.e., radiolysis of ice-phase molecules as a result of interaction of the ice mantle with the cosmic ray) or indirectly, in which the interaction of cosmic rays with gas-phase H2 results in the emission of a low flux of Lyman-α photons that can trigger photochemical reactions within the ice.
Nowadays, it is recognised that the Universe is a molecular one; and more than 320 individual molecules have been detected in various interstellar environments, with another 70 or so having been detected in extra-galactic sources.4 Most of these molecules are thought to be formed in the solid phase within interstellar icy grain mantles. These molecules range in complexity from simple di- and triatomic molecules to complex polycyclic aromatic hydrocarbons and fullerenes. Moreover, laboratory experiments seeking to simulate the chemistry occurring in interstellar space have demonstrated that the formation of biologically consequential molecules (e.g., sugars, amino acids, nucleobases) is plausible.5–7 These results have important implications for astrobiology, as they imply that the seeds of life may have been formed in space and then subsequently transported to the early Earth (and, by extension, any other planetary body amenable to life) where they may have contributed to the emergence of life.
Interestingly, many of the reaction mechanisms that engender chemistry within interstellar icy grain mantles (i.e., atom and radical additions, thermal processes, radiolysis, and photolysis) are also the same processes driving chemistry on icy bodies in the outer Solar System, such as the moons of giant planets, dwarf planets, and comets. On moons such as Europa and Enceladus, which are considered to be among the most likely extraterrestrial abodes of life, such chemistry may contribute to geochemical cycles that may help or hinder putative biochemical processes. The detailed study of planetary astrochemistry may therefore help contribute to constraining the habitability of such icy outer Solar System bodies.
In this presentation, I shall provide a brief overview of astrochemistry as a scientific discipline and how experimental astrochemical research is conducted, before then proceeding to highlight some recent results of the research in which I have been involved. In doing so, I shall describe the role of ionising radiation in the form of galactic cosmic rays, stellar winds, and magnetospheric plasmas in contributing to the diversity of chemical species in both interstellar and outer Solar System (i.e., lunar or planetary) environments, with a particular emphasis on the production of molecules relevant to biology or prebiotic chemistry and their implications for the emergence of life in the Universe.
1. P Swings, L Rosenfeld (1937). Astrophys. J. 86, 483.
2. A McKellar (1940). Publ. Astron. Soc. Pac. 52, 187.
3. CR Arumainayagam, et al. (2019). Chem. Soc. Rev. 48, 2293.
4. BA McGuire (2022). Astrophys. J. Supp. Ser. 259, 30.
5. C Meinert, et al. (2016). Science 352, 208.
6. Y Oba, et al. (2019). Nature Commun. 10, 4413.
7. S Ioppolo, et al. (2021). Nature Astron. 5, 197.
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