Astrobiology of Mars, Europa, Titan and Enceladus - Most Likely Places for Alien Life

The Cassini-Huygens mission has shown that Titan, the only moon known to have a dense atmosphere, is an unusual world in our solar system. Although it is far from the Sun and hence cold, solar radiation interacts with its methane rich atmosphere instigating the formation of complex organic molecules and aerosols that eventually deposit on Titan's surface, where further chemical reactions could take place. This may be leading to greater molecular complexity and eventually to the most complex organic building blocks of life. In addition, recent Cassini gravity data suggest that Titan's global subsurface ocean may be in contact with the rock core, potentially providing redox gradients and heavier elements critical for a habitable environment. In addition, at the surface, liquid water delivered from Titan's ocean through cryovolcanic activity could indicate the presence of habitable environments if not on the surface because of the extremely cold surface temperatures, so in the subsurface close to the surface.In this Research Topic on Astrobiology of Mars, Europa, Titan and Enceladus, our aim is to emphasize the importance of astrobiology of our own backyard, relevant our solar system bodies where we might find alien life beyond Earth.Marín-Yaseli, González-Toril and Ruiz-Bermejo investigated reactivity of Cyanide at Water-Ice Interfaces since Cyanide has been proposed that polymerization of NH 4 CN would be applicable to icy planetary environments. They, as an approach to understand the possible organic chemistry of icy worlds, explored the effect of water freeze-thaw cycles, UV-radiation and salts on the polymerization of NH 4 CN. They synthesized insoluble and soluble NH 4 CN polymers, under multiple conditions, and analyzed by GC-MS. Their analysis resulted in identification of a diverse set of amino acids, carboxylic acids, and several N-heterocycles. Utilizing a multivariate analysis technique to their GC-MS results indicated that the ice-water interfaces may be ideal places to develop complex organic chemistry from a carbon source as simple as cyanide.Pasek conducted a modeling study investigating plume sample modification during the ejection and its implications for biosignatures at Icy Moons such as Europa and Enceladus relevant to in situ direct sampling (with potential for sample return) of liquid water from these bodies. This is critical to determine how representative the collected plume samples are of their environment, determining biosignatures and habitability. His results indicate that water droplets comprising part of plume are frozen rapidly, with loss of about 17% of the water by evaporation and sublimation if the water is originally liquid and at 273 K. The 17% loss of water corresponds to a 20% increase in the concentration of non-volatile solutes such as salts, if the associated water vapor is not collected. Volatiles, such as gases and clathrates (Mousis et al., 2013), will be altered during sublimation, at least as far as their concentration is concerned. Mitigation of this alteration will be highly critical to successful sample collection mission.Poggiali et al. conducted a laboratory investigation on ultraviolet photoprocessing of Glycine adsorbed on various space-relevant minerals such as serpentine antigorite, olivine forsterite, oxide mineral spinel, and iron-sulfide mineral pyrite. Being the simplest of the 20 amino acids from which complex organic molecules might have originated in our evolved Solar System, Glycine has a significant importance. Possible changes of glycine functional groups due to UV irradiation through in situ infrared (IR) spectroscopic analysis were investigated. Results show that degradation of glycine occurs with a half-life of 0.5-2 h depending on the mineral substrate. They highlight a lower efficiency of antigorite and spinel in catalyzing GLY photoreaction compared with forsterite and pyrite. Appearance of new IR bands suggests the occurrence of catalytic reactions mediated by minerals and UV. This investigation shows mineral composition would be an important factor to be considered for positive detection of biomarkers.Fornaro et al. carried out a comprehensive investigation comprised of an experimental study combined with anharmonic computational spectroscopy calculations on UV irradiation and near infrared characterization of laboratory Mars soil analog samples. They reported changes of the NIR spectroscopic features due to molecule-mineral interactions, molecular concentration, presence of oxidants such as magnesium perchlorate, and UV irradiation, for molecules relevant in the context of life detection, such as adenosine 5'-monophosphate, L-glutamic acid, L-phenylalanine, and phthalic acid, adsorbed on montmorillonite, a clay mineral common on Mars. They observed that moleculemineral interactions and UV irradiation cause the most significant changes in NIR spectra. Their investigation emphasizes the importance of building spectroscopic databases for molecule-mineral complexes under martian-like conditions in order to assist interpretation of data collected by space instruments based on NIR spectroscopy.Neveu et al. provide a comprehensive review on the evidence on Enceladus' habitability and rationalize the value of sample return for life detection from its plume material. In doing so, they estimate the amount of sample that would have to be returned to Earth in order to perform a suite of measurements to assess the presence of signature of life in the samples. Then they provide a discussion on a range of sample return mission architectures and finally provide an outline on technical and policy related unresolved issues to achieve such a mission. Finally, they conclude their review by placing Enceladus sample return in the broader context of Solar System exploration.Fagliarone et al., conducted a laboratory investigation on biomarker preservation and survivability under the Mars-Like conditions (i.e. extreme dryness and UV flux) relevant to future life detection missions on Mars. For this study, they used desert Cyanobacterium (anhydrobiotic cyanobacterium Chroococcidiopsis) capable of Trehalose and Sucrose accumulation and investigated its survivability and biomarker preservation by performing an in silico analysis of the genome of the CCMEE 029 strain. This work provides a better understanding for the adaptation strategies to extremely dry conditions and suggests that sucrose and trehalose accumulation might reduce macromolecular susceptibility to chemical and physical degradation taking place after cell death which has implications in designing future life detection missions on Mars; moreover, the biomarker detectability strategies.