life Review Astrochemical Pathways to Complex Organic and Prebiotic Molecules: Experimental Perspectives for In Situ Solid-State Studies Daniele Fulvio 1,2,* , Alexey Potapov 3, Jiao He 2 and Thomas Henning 2 1 Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Naples, Italy 2 Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany; [email protected] (J.H.); [email protected] (T.H.) 3 Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany; [email protected] * Correspondence: [email protected] Abstract: A deep understanding of the origin of life requires the physical, chemical, and biological study of prebiotic systems and the comprehension of the mechanisms underlying their evolutionary steps. In this context, great attention is paid to the class of interstellar molecules known as “Complex Organic Molecules” (COMs), considered as possible precursors of prebiotic species. Although COMs have already been detected in different astrophysical environments (such as interstellar clouds, protostars, and protoplanetary disks) and in comets, the physical–chemical mechanisms underlying their formation are not yet fully understood. In this framework, a unique contribution comes from laboratory experiments specifically designed to mimic the conditions found in space. We present Citation: Fulvio, D.; Potapov, A.; He, a review of experimental studies on the formation and evolution of COMs in the solid state, i.e., J.; Henning, T. Astrochemical within ices of astrophysical interest, devoting special attention to the in situ detection and analysis Pathways to Complex Organic and techniques commonly used in laboratory astrochemistry. We discuss their main strengths and Prebiotic Molecules: Experimental weaknesses and provide a perspective view on novel techniques, which may help in overcoming the Perspectives for In Situ Solid-State current experimental challenges. Studies. Life 2021, 11, 568. https:// doi.org/10.3390/life11060568 Keywords: complex organic molecules; astrobiology; astrochemistry; interstellar medium; molecular ices; solid state; protoplanetary disks; star forming regions; comets Academic Editors: Michele Fiore and Emiliano Altamura Received: 28 May 2021 1. Introduction Accepted: 14 June 2021 Published: 17 June 2021 Over the next decades, it is expected that one of the main challenges in science will be to gain a clear understanding on the processes and phenomena linked to the concept of life Publisher’s Note: MDPI stays neutral how we know it, its origin and early evolution. Key questions, which need to be answered, with regard to jurisdictional claims in include: (i) How did life begin and evolve on Earth? (ii) What are the conditions for the published maps and institutional affil- origins of life? Does life exist elsewhere in the Universe? (iii) What is life’s future on Earth iations. and beyond? Answering these and similar questions requires the physical, chemical, and biological study of prebiotic systems and a deep understanding of the principles governing their evolution into more complex systems and, finally, into living matter. Astrochemistry combines astronomy and chemistry. It directly studies the aforementioned issues, with a Copyright: © 2021 by the authors. special focus on the so-called “Complex Organic Molecules” (COMs), a term referring to Licensee MDPI, Basel, Switzerland. those astrophysically relevant organic molecules consisting of six or more atoms, many This article is an open access article of which are considered possible precursors of prebiotic molecules, such as amino acids distributed under the terms and (in turn, precursors of proteins) and nucleobases (in turn, precursors of DNA and RNA). conditions of the Creative Commons Although COMs are considered of paramount importance from an astrochemical and Attribution (CC BY) license (https:// astrobiological point of view, the reader should keep in mind that they are only relatively creativecommons.org/licenses/by/ “complex” when studied from a chemical and biological perspective [1,2]. 4.0/). Life 2021, 11, 568. https://doi.org/10.3390/life11060568 https://www.mdpi.com/journal/life Life 2021, 11, 568 2 of 22 From the observational point of view, the main detection tool for molecules in space, in various astrophysical environments, is gas-phase radio astronomy supported by mi- crowave, millimeter-wave, and terahertz spectroscopy. Complementary, solid-state ma- terials are characterized by infrared (IR) spectroscopy. To date, more than 200 species have already been detected in the gas phase while only about 10 molecules have been spectroscopically identified in the solid state (e.g., Cologne Database for Molecular Spec- Life 2021, 11, x FOR PEER REVIEW 3 of 22 troscopy and [3]). The variety, abundance, and distribution of gas-phase and solid-state COMs already detected or tentatively detected in space have been increasing in the last decades, touching various astronomical environments, from interstellar clouds, protostars, aand detailed protoplanetary discussion disks of these (e.g., [processes1–19]) to the an outerd mechanisms solar system, is beyond where COMs the scope and prebiotic of the cur- rentspecies review have and already we refer been the detected interested in several reader comets to dedicated (e.g., [20– papers/reviews31]). To give the ( reader[49–56 an] and referencesidea, the list therein). of these COMs includes but is not limited to: acetaldehyde (CH3CHO), ethanol (CH CH OH), formamide (HCONH ), glycine (NH CH COOH), and urea (H NCONH ). The3 2discussion so far indicates 2the importance2 of 2a comprehensive understanding2 2 of Their structural formulas are shown in Figure1. The detection of COMs in the solid state, the physical–chemical mechanisms and processes underlying COM formation and evolu- i.e., in the icy component present in cold cosmic regions, such as dense molecular clouds of tion,the interstellarespecially in medium molecular (ISM), ices. protostellar Most processes envelopes and andmechanisms protoplanetary are not disks yet fully (beyond under- stood,the snowline), and in this and framework, on the surface laboratory of minor studies bodies ofspecifically the solar systemdesigned (comets, to simulate asteroids, astro- physicallysatellites of relevant planets, trans-Neptunianconditions play objects,a unique... ),role is made in improving difficult by our the intrinsiccomprehension. prob- Thankslems related to dedicated to IR spectroscopy. experiments, Simply realistic said, thescenarios difficulties towards in identifying molecular COMs complexity in ices by can beIR established. spectroscopy are due to the unspecific nature of the signal coming from overlapping featuresIn this belonging contribution, to different after speciesa brief havingreview the of samethe main functional formation groups. and Nevertheless, evolution pro- cessesfor solar of COMs system in studies, space space (Section missions 2), we typically focus on combine a key aspect IR spectroscopy of COM studies, and mass often spec- not comprehensivelytrometry (MS). This covered allows in the reviews: identification the main of COMsstrengths in situ and (e.g., weaknesses [27,28]), althoughof in situ MSdetec- tioncarries and its analysis own intrinsic techniques critical commonly issues as well, used such in asexp theerimental difficulty astrochemistry to distinguish molecules (Section 3). Theof thecurrent same experimental mass and chiral challenges isomers. and A detailed novel techniques discussion ofto theseovercome techniques present can limita- be tionsfound will in thebe the following focus of sections Section of 4. this review. Figure 1. Structural formulas of some among the most astrophysically relevant COMs discussed in this review. The present Figure 1. Structural formulas of some among the most astrophysically relevant COMs discussed in this review. The present figure was created by adapting original graphics taken from Wikipedia, the free encyclopedia (in particular, the graphic of figure was created by adapting original graphics taken from Wikipedia, the free encyclopedia (in particular, the graphic acetaldehyde was created by UAwiki). of acetaldehyde was created by UAwiki). While most of the early astrochemistry models presumed that COM formation in the 2.aforementioned Formation of astrophysicalCOMs in the environments Solid State occurs through gas-phase reactions (e.g., [2,32–34]), the majorityChemical of processes current models leading assume to the that formation COM formation, of molecules especially in interstellar the more and complex circum- stellarones, occursenvironments mainly incan the be solid divided state, into onto two or groups, within molecular gas-phase ices and found solid- onstate cosmic surface reactions.dust grains Gas and-phase planetary formation surfaces. routes For to the COMs reader are interested out of the in understandingscope of the present the nature review andand we evolution refer the of intereste cosmic dustd reader grains to and a recent their rolereview in astrochemistry,paper on this topic we recommend [57]. In the a fol- lowing,few review we will papers focus (e.g., on [solid35–42-state]). Moving reactions. from earlier models to current ones has been favoredIn colder by the astrophysical comparison ofenvironmen theoreticalts, simulations such as diffuse with new and laboratory dense clouds experiments