The chemistry of prebiotic N-containing compounds in the atmosphere of Titan and primitive Earth beyond a holistic approach

<p> </p> <p>How did life emerge from inanimate matter? The processes that led from complex organic molecules to the first self-replicating systems are no longer at play and we cannot easily reconstruct them because we do not have a geological record of the period when the transition from simple molecules to the very first forms of “life” have occurred. The presence of stable hydrosphere is considered as the first milestone in the timeline of the abiotic origin of life theory, with the second milestone being the massive accumulation of organic compounds necessary for the transition from organic chemistry to the biochemistry of life. But how Earth became so rich in complex organic molecules – up to the point that life spontaneously evolved from them - is still a matter of debate. At that stage, the abundance of liquid water, indeed, represents an obstacle for organic synthesis. Two theories have been suggested to solve this paradox, which are usually referred to as <em>endogenous synthesis</em> and <em>exogenous synthesis</em> scenarios [1]. But in both cases, prebiotic molecules (that is, molecules which are simple to be formed in abiotic processes but contain the functional groups typical of biological molecules or have the capability to easily evolve into them) are formed in gaseous media. Indeed, gas-phase prebiotic molecules have been observed in the upper atmosphere of Titan, the massive moon of Saturn, as well as in the interstellar clouds and cometary comae.</p> <p>The comprehension of the chemical processes that lead from simple atomic/diatomic species to prebiotic complex chemicals is an important part of the study on the origin of life. The study of these preliminary steps might seem relatively simple compared to the characterization of the other unknown phenomena that have led to the first living organisms. Nevertheless, the formation mechanisms of many of the prebiotic molecules that we observe nowadays in proto-stellar clouds or comets/meteorites or planetary atmospheres are far from being understood, while a comprehension of those processes can certainly help to set the stage for the emergence of life to occur.</p> <p>For this reason, in our laboratory we have started a systematic investigation of gas-phase reactions leading to simple prebiotic molecules within the Italian National Project of Astrobiology—Life in Space—Origin, Presence, Persistence of Life in Space, from Molecules to Extremophiles [2].</p> <p>In particular, by combining an experimental and theoretical approach, we have investigated a series of bimolecular reactions under single collision conditions. The aim is to provide detailed information on the elementary reactions which are employed in photochemical models of planetary atmosphere and cometary comae [3]. In particular, we have investigated several reactive systems leading to the formation of nitriles (such as dicyanoacetylene) and imines (such as ethanimine), as well as reactive radicals that can further react in subsequent reactions. We have also investigated reactions involving nitrogen atoms and aromatic compounds (benzene, pyridine, toluene) to address the role of these compounds in the growth of N-containing aromatic compounds, a proxy of DNA and RNA bases. In this contribution, the main results concerning the reactions involving atomic nitrogen, N, or cyano radicals, CN, and cyanoacetylene, acrylonitrile, benzene, toluene and pyridine will be illustrated and the implications for prebiotic chemistry noted.</p> <p>[1] C. Chyba and C. Sagan. Nature 1992, 355, 125.</p> <p>[2] S. Onofri, N. Balucani, V. Barone et al. Astrobiology 2020, 20, 580. DOI: 10.1089/ast.2020.2247</p> <p>[3] N. Balucani. Physics of Life Reviews 2020, 34–35, 136. DOI: 10.1016/j.plrev.2019.03.0061571-0645</p>

In the context of Comet Interceptor: Unexpected polarimetric properties of some dust particles in cometary comae and on small bodies surfaces

<p>The ESA-JAXA Comet Interceptor mission is expected to flyby a dynamically new comet (or an interstellar one) and better reveal the properties of its dust particles and nucleus surface. We therefore tentatively compare polarimetric properties of dust released by some comets, as well as present on surfaces of some small bodies.</p><p>Phase curves of the linear polarization of cometary dust particles (observed in equivalent wavelength ranges) show analogous trends. Some unique dynamically new comets or fragmenting comets (e.g. C/1995 O1 Hale-Bopp, C/1999 S4 LINEAR) may nevertheless present a higher positive branch than Halley-type or Jupiter-family comets (e.g. 1P/Halley, 67P/Churyumov-Gerasimenko). Such differences are clues to differences in the properties (sizes, morphologies, complex optical indices) of the dust particles. Dust particles, ejected by nuclei frequently plunging in the inner Solar System, might indeed partly come from quite dense a surface layer, as detected on the small lobe of comet 67P by Rosetta [1].</p><p>Although polarimetric observations of surfaces of cometary nuclei are almost impossible, observations of the rather quiescent nucleus of 1P/Encke have been obtained [2].  Similarities between polarimetric properties of 1P/Encke and atypical small bodies (e.g. Phaeton and particularly Bennu [3]), and of dust in cometary comae may be pointed out. Numerical and laboratory simulations could represent a unique tool to better understand such similarities. It may also be added that dust particles originating from comets, with emphasis on those of Jupiter-family, may survive atmospheric entry, as CP-IDPs collected in the Earth’s stratosphere, and that dust found in debris disks of stellar systems shows levels of polarization similar to those of highly-polarized comets [4].</p><p> </p><p>[1] Kofman et al., MNRAS, 497, 2616-2622, 2020, [2] Boehnhardt et al., A&A, 489, 1337-1343, 2008. [3] Cellino et al., MNRAS, 481, L49-L53, 2018. [4] Levasseur-Regourd et al., PSS, 186, 104896, 2020,</p><p> </p>

Iron and nickel atoms in comet atmospheres

When sufficiently close to the Sun, ices in cometary nuclei sublimate, ejecting in space dust and gases whose compositions can be derived by the remote spectral analysis of the cometary atmospheres. Those very rich spectra reveal a host of constituents from simple radicals like OH and CN in the optical range, to relatively complex organic molecules in the infrared and sub-millimeter domain. The majority of these molecules are made of C, H, O and N atoms. Iron, nickel and a few other siderophile atoms have only been detected in two exceptional sungrazer comets in a century and a half. Here we report that free atoms of iron and nickel are ubiquitous in cometary atmospheres as revealed by high-resolution spectra obtained in the near-ultraviolet with the ESO Very Large Telescope for a large sample of comets of various dynamical origins. The emissions of NiI and FeI in cometary comae have been overlooked until now and, surprisingly, are even detected at large heliocentric distances. The abundances of both species appear to be of the same order of magnitude, contrasting with the typical solar system abundance and providing clues about their origins in comet nuclei.