A never-ending story in the sky: The secrets of chemical evolution

Cosmic evolution is the tale of progressive transition from simplicity to complexity. The newborn universe started with the simplest atoms formed after the Big Bang and proceeded toward the formation of the so-called ‘astronomical complex organic molecules’ (aCOMs), most of them showing a clear prebiotic character. Understanding the chemical evolution of the universe is one of the main aims of Astrochemistry, with the starting point being the knowledge whether a molecule is present in the astronomical environment under consideration and, if so, its abundance. However, the interpretation of astronomical detections and the identification of molecules are not at all straightforward. Indeed, the extraterrestrial chemical inventory has been obtained by means of astronomical observations based on spectroscopic signatures determined in laboratory (either experimental or computational) studies. Even though the presence of aCOMs has been known for decades, the processes that lead to their synthesis are still hotly debated or even unknown. It is often assumed that aCOMs are mostly synthesized on grain surfaces during the so-called warm-up phase, when various radicals trapped in the grain mantles acquire mobility and recombine into large molecules. However, recent detections of aCOMs in cold environments have challenged this exclusive role of grain-surface chemistry. Clearly, gas-phase chemistry is at work in cold environments. Moving to Titan's atmosphere, prior to the Cassini-Huygens arrival in the Saturn system, it was generally believed that Earth and interstellar space are the two places where organic molecules are/were synthesized extensively. However, the experimental measurements by the instruments on board the Cassini orbiter spacecraft and the Huygens probe lander have changed this view. To disclose the “secrets” of chemical evolution across space, the first step is the understanding of how small prebiotic species are formed and how the chemical complexity can further increase. This review indeed addresses the chemical evolution in space, focusing – in particular – on the role played by molecular spectroscopy and quantum-chemical computations. To summarize, in this review we will first of all present how the signatures of molecules can be found in space. Then, we will address, from a computational point of view, the derivation of the molecular spectroscopic features, the investigation of gas-phase formation routes of prebiotic species in the ISM, and the evolution of chemical complexity, from small molecules to haze, in Titan's atmosphere. Finally, an integrated strategy, also involving high-performance computers and virtual reality, will be discussed.