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Concept: Astrobiology

26

Abstract Data from automated orbiters and landers have dashed humankind’s hopes of finding complex life-forms elsewhere in the Solar System. The focus of exobiological research was thus forced to shift from the detection of life through simple visual imaging to complex biochemical experiments aimed at the detection of microbial activity. Searching for biosignatures over interplanetary distances is a formidable task and poses the dilemma of what are the proper experiments that can be performed on-site to maximize the chances of success if extraterrestrial life is present but not evident. Despite their astonishing morphological diversity, all known organisms on Earth share the same basic molecular architecture; thus the vast majority of our detection and identification techniques are b(i)ased on Terran biochemistry. There is, however, a distinct possibility that life may have emerged elsewhere by using other molecular building blocks, a fact that is likely to make the outcome of most of the current molecular biological and biochemical life-detection protocols difficult to interpret if not completely ineffective. Nanopore-based sensing devices allow the analysis of single molecules, including the sequence of informational biopolymers such as DNA or RNA, by measuring current changes across an electrically resistant membrane when the analyte flows through an embedded transmembrane protein or a solid-state nanopore. Under certain basic assumptions about their physical properties, this technology has the potential to discriminate and possibly analyze biopolymers, in particular genetic information carriers, without prior detailed knowledge of their fundamental chemistry and is sufficiently portable to be used for automated analysis in planetary exploration, all of which makes it the ideal candidate for the search for life signatures in remote watery environments such as Mars, Europa, or Enceladus. Key Words: Astrobiology-Biopolymers-Biosignatures-Nucleic acids-Life detection. Astrobiology 14, xxx-xxx.

Concepts: DNA, Molecular biology, Chemistry, Solar System, Astrobiology, Extraterrestrial life, Planetary habitability, Europa

25

This paper draws a comparison between the 700-year-old historically reported will-o'-the-wisp phenomenon and the more recent discovery of extremophilic colonization of hostile environments; both have been observed as present in isolated, stressed environmental regions and originating from biological phenomena. However, whereas extremophilic activity can be understood in terms of a survival strategy based upon the synthesis of specific suites of protective biochemicals which are designed to control biogeologically the stressed habitats and to provide protection against the extreme environments, the analytical techniques that have proved so successful for the illumination of these survival strategies of extremophiles and which are now being miniaturized for in-field studies and for extraterrestrial exploration have not been applied to a clarification or evaluation of the phenomenon of will-o'-the-wisp. The reason is simply that the will-o'-the-wispsightings have now disappeared completely. Tantalizingly, all of the most reasonable physico-chemical and biological explanations for the will-o'-the-wisp phenomenon proved to be unsatisfactory in some respect and it is clear that, just as in the case of extremophilic colonization, will-o'-the-wisp would benefit from a modern rigorous analytical study which would produce the data from which the potentially novel biological behaviour could be characterized and which would help a better understanding to be made of our natural world.

Concepts: Natural environment, Greek loanwords, Observation, Aristotle, Extremophiles, Extremophile, Astrobiology, Extreme environment

21

Advances in planetary and space sciences, astrobiology, and life and cognitive sciences, combined with developments in communication theory, bioneural computing, machine learning, and big data analysis, create new opportunities to explore the probabilistic nature of alien life. Brought together in a multidisciplinary approach, they have the potential to support an integrated and expanded Search for Extraterrestrial Intelligence (SETI (1) ), a search that includes looking for life as we do not know it. This approach will augment the odds of detecting a signal by broadening our understanding of the evolutionary and systemic components in the search for extraterrestrial intelligence (ETI), provide more targets for radio and optical SETI, and identify new ways of decoding and coding messages using universal markers.

Concepts: Life, Fermi paradox, Extrasolar planet, Astrobiology, Extraterrestrial life, Planetary habitability, SETI, Panspermia

9

Decades of robotic exploration have confirmed that in the distant past, Mars was warmer and wetter and its surface was habitable. However, none of the spacecraft missions to Mars have included among their scientific objectives the exploration of Special Regions, those places on the planet that could be inhabited by extant martian life or where terrestrial microorganisms might replicate. A major reason for this is because of Planetary Protection constraints, which are implemented to protect Mars from terrestrial biological contamination. At the same time, plans are being drafted to send humans to Mars during the 2030 decade, both from international space agencies and the private sector. We argue here that these two parallel strategies for the exploration of Mars (i.e., delaying any efforts for the biological reconnaissance of Mars during the next two or three decades and then directly sending human missions to the planet) demand reconsideration because once an astronaut sets foot on Mars, Planetary Protection policies as we conceive them today will no longer be valid as human arrival will inevitably increase the introduction of terrestrial and organic contaminants and that could jeopardize the identification of indigenous Mars life. In this study, we advocate for reassessment over the relationships between robotic searches, paying increased attention to proactive astrobiological investigation and sampling of areas more likely to host indigenous life, and fundamentally doing this in advance of manned missions. Key Words: Contamination-Earth Mars-Planetary Protection-Search for life (biosignatures). Astrobiology 17, xxx-xxx.

Concepts: Mars, Space exploration, NASA, Astrobiology, Life on Mars, Extraterrestrial life, Planetary habitability, Mariner 4

9

Abstract The climate and, hence, potential habitability of a planet crucially depends on how its atmospheric and ocean circulation transports heat from warmer to cooler regions. However, previous studies of planetary climate have concentrated on modeling the dynamics of atmospheres, while dramatically simplifying the treatment of oceans, which neglects or misrepresents the effect of the ocean in the total heat transport. Even the majority of studies with a dynamic ocean have used a simple so-called aquaplanet that has no continental barriers, which is a configuration that dramatically changes the ocean dynamics. Here, the significance of the response of poleward ocean heat transport to planetary rotation period is shown with a simple meridional barrier-the simplest representation of any continental configuration. The poleward ocean heat transport increases significantly as the planetary rotation period is increased. The peak heat transport more than doubles when the rotation period is increased by a factor of ten. There are also significant changes to ocean temperature at depth, with implications for the carbon cycle. There is strong agreement between the model results and a scale analysis of the governing equations. This result highlights the importance of both planetary rotation period and the ocean circulation when considering planetary habitability. Key Words: Exoplanet-Oceans-Rotation-Climate-Habitability. Astrobiology 14, xxx-xxx.

Concepts: Energy, Temperature, Planet, Thermodynamics, Entropy, Ocean, Astrobiology, Planetary habitability

5

Finding life on exoplanets from telescopic observations is an ultimate goal of exoplanet science. Life produces gases and other substances, such as pigments, which can have distinct spectral or photometric signatures. Whether or not life is found with future data must be expressed with probabilities, requiring a framework of biosignature assessment. We present a framework in which we advocate using biogeochemical “Exo-Earth System” models to simulate potential biosignatures in spectra or photometry. Given actual observations, simulations are used to find the Bayesian likelihoods of those data occurring for scenarios with and without life. The latter includes “false positives” wherein abiotic sources mimic biosignatures. Prior knowledge of factors influencing planetary inhabitation, including previous observations, is combined with the likelihoods to give the Bayesian posterior probability of life existing on a given exoplanet. Four components of observation and analysis are necessary. (1) Characterization of stellar (e.g., age and spectrum) and exoplanetary system properties, including “external” exoplanet parameters (e.g., mass and radius), to determine an exoplanet’s suitability for life. (2) Characterization of “internal” exoplanet parameters (e.g., climate) to evaluate habitability. (3) Assessment of potential biosignatures within the environmental context (components 1-2), including corroborating evidence. (4) Exclusion of false positives. We propose that resulting posterior Bayesian probabilities of life’s existence map to five confidence levels, ranging from “very likely” (90-100%) to “very unlikely” (<10%) inhabited. Key Words: Bayesian statistics-Biosignatures-Drake equation-Exoplanets-Habitability-Planetary science. Astrobiology 18, xxx-xxx.

Concepts: Scientific method, Philosophy of science, Conditional probability, Bayesian probability, Bayes' theorem, Astrobiology, Extraterrestrial life, Planetary habitability

5

The so-called Fermi paradox claims that if technological life existed anywhere else, we would see evidence of its visits to Earth-and since we do not, such life does not exist, or some special explanation is needed. Enrico Fermi, however, never published anything on this topic. On the one occasion he is known to have mentioned it, he asked “Where is everybody?”-apparently suggesting that we do not see extraterrestrials on Earth because interstellar travel may not be feasible, but not suggesting that intelligent extraterrestrial life does not exist or suggesting its absence is paradoxical. The claim “they are not here; therefore they do not exist” was first published by Michael Hart, claiming that interstellar travel and colonization of the Galaxy would be inevitable if intelligent extraterrestrial life existed, and taking its absence here as proof that it does not exist anywhere. The Fermi paradox appears to originate in Hart’s argument, not Fermi’s question. Clarifying the origin of these ideas is important, because the Fermi paradox is seen by some as an authoritative objection to searching for evidence of extraterrestrial intelligence-cited in the U.S. Congress as a reason for killing NASA’s SETI program on one occasion. But evidence indicates that it misrepresents Fermi’s views, misappropriates his authority, deprives the actual authors of credit, and is not a valid paradox. Key Words: Astrobiology-SETI-Fermi paradox-Extraterrestrial life. Astrobiology 15, 195-199.

Concepts: Enrico Fermi, Fermi paradox, Astrobiology, Extraterrestrial life, Fermi problem, Carl Sagan, SETI, Drake equation

4

While it is anticipated that future human missions to Mars will increase the amount of biological and organic contamination that might be distributed on that planet, robotic missions continue to grow in capability and complexity, requiring precautions to be taken now to protect Mars, and particularly areas of Mars that might be Special Regions. Such precautionary cleanliness requirements for spacecraft have evolved over the course of the space age, as we have learned more about planetary environments, and are the subject of regular deliberations and decisions sponsored by the Committee on Space Research (COSPAR). COSPAR’s planetary protection policy is maintained as an international consensus standard for spacecraft cleanliness that is recognized by the United Nations Committee on the Peaceful Uses of Outer Space. In response to the paper presented in this issue by Fairén et al. (2017), we examine both their concept of evidence for possible life on Mars and their logic in recommending that spacecraft cleanliness requirements be relaxed to access Special Regions “before it is too late.” We find that there are shortcomings in their plans to look for evidence of life on Mars, that they do not support their contention that appropriate levels of spacecraft cleanliness are unaffordable, that there are major risks in assuming martian life could be identified by nucleic acid sequence comparison (especially if those sequences are obtained from a Special Region contaminated with Earth life), and that the authors do not justify their contention that exploration with dirty robots, now, is preferable to the possibility that later contamination will be spread by human exploration. We also note that the potential effects of contaminating resources and environments essential to future human occupants of Mars are both significant and not addressed by Fairén et al. (2017). Key Words: Mars-Special Region-Mission-Life detection-Planetary protection. Astrobiology 17, xxx-xxx.

Concepts: Life, Mars, Space exploration, Astrobiology, Life on Mars, Extraterrestrial life, Martian, United Nations Committee on the Peaceful Uses of Outer Space

4

The inability to unambiguously distinguish the biogenicity of microfossil-like structures in the ancient rock record is a fundamental predicament facing Archean paleobiologists and astrobiologists. Therefore, novel methods for discriminating biological from nonbiological chemistries of microfossil-like structures are of the utmost importance in the search for evidence of early life on Earth. This, too, is important for the search for life on Mars by in situ analyses via rovers or sample return missions for future analysis here on Earth. Here, we report the application of synchrotron X-ray fluorescence imaging of vanadium, within thermally altered organic-walled microfossils of bona fide biological origin. From our data, we demonstrate that vanadium is present within microfossils of undisputable biological origin. It is well known in the organic geochemistry literature that elements such as vanadium are enriched and contained within crude oils, asphalts, and black shales that have been formed by diagenesis of biological organic material. It has been demonstrated that the origin of vanadium is due to the diagenetic alteration of precursor chlorophyll and heme porphyrin pigment compounds from living organisms. We propose that, taken together, microfossil-like morphology, carbonaceous composition, and the presence of vanadium could be used in tandem as a biosignature to ascertain the biogenicity of putative microfossil-like structures. Key Words: Microfossils-Synchrotron micro-X-ray fluorescence-Vanadium-Tetrapyrrole-Biosignature. Astrobiology 17, xxx-xxx.

Concepts: DNA, Gene, Biology, Organism, Life, Species, Porphyrin, Astrobiology

3

To the astrobiologist, Enceladus offers easy access to a potential subsurface biosphere via the intermediacy of a plume of water emerging directly into space. A direct question follows: If we were to collect a sample of this plume, what in that sample, through its presence or its absence, would suggest the presence and/or absence of life in this exotic locale? This question is, of course, relevant for life detection in any aqueous lagoon that we might be able to sample. This manuscript reviews physical chemical constraints that must be met by a genetic polymer for it to support Darwinism, a process believed to be required for a chemical system to generate properties that we value in biology. We propose that the most important of these is a repeating backbone charge; a Darwinian genetic biopolymer must be a “polyelectrolyte.” Relevant to mission design, such biopolymers are especially easy to recover and concentrate from aqueous mixtures for detection, simply by washing the aqueous mixtures across a polycharged support. Several device architectures are described to ensure that, once captured, the biopolymer meets two other requirements for Darwinism, homochirality and a small building block “alphabet.” This approach is compared and contrasted with alternative biomolecule detection approaches that seek homochirality and constrained alphabets in non-encoded biopolymers. This discussion is set within a model for the history of the terran biosphere, identifying points in that natural history where these alternative approaches would have failed to detect terran life. Key Words: Enceladus-Life detection-Europa-Icy moon-Biosignatures-Polyelectrolyte theory of the gene. Astrobiology 17, xxx-xxx.

Concepts: Natural selection, Amino acid, Molecular biology, Life, Molecule, Polymer, Astrobiology, Extraterrestrial life