The perplexity of how the building blocks of life formed from simple and abundantly-available precursors on early earth has baffled scientific community for centuries. In order to get a clearer picture, the present study proposes and investigates plausible ammonia-assisted, free radical pathways for nucleobase formation from starting precursors such as cyanoacetaldehyde, urea, cyanoacetylene and cyanamide. Particularly, density functional theory is used to obtain optimized geometries and zero-point vibrational energy corrected electronic energies of reactants, transition states, intermediates and products along the reaction pathways in the gas phase at B3LYP/6–311G(d,p) level, as well as in the water (dielectric constant of 78.3) and ammonia (dielectric constant of 22.4) using the IEFPCM framework. Our proposed mechanisms are characterized by a smaller number of precursors and relatively lower barriers compared to previously reported reactions with other prebiotic precursors [1-3]. Features such as barrier-less formation of imidazole intermediate and favorable contribution of prebiotic enolate chemistry highlight the plausibility of the presently proposed pathways. The pathways are most suitable to environments like prebiotic earth (for purine formation) and present-day Titan (for purine and pyrimidine formation) where radical reactions are rendered feasible by continuous influx of UV and cosmic radiations. Overall, our analysis proposes kinetically accessible routes to nucleobases formation, and will hopefully contribute towards understanding the relevance of these precursors in prebiotic reactions. References: Jeilani, Y. A. Williams, P. N. Walton, S., Nguyen, M. T. (2016) Phys. Chem. Chem. Phys.,18, 20177-20188. Jeilani, Y. A. Fearce, C. and Nguyen, M. T. (2015) Phys. Chem. Chem. Phys.,17, 24294-24303. Nguyen, H. T. Jeilani, Y. A., Hung, H. M. and Nguyen, M. T. (2015) J. Phys. Chem. A, 119, 8871-8883.
Considering the theme for AbSciCon 2019: “Understanding and Enabling the Search for Life on Worlds Near and Far”, it is worth to set the emphasis on ferric minerals and show that their formation in the absence of oxygen does not require the necessary presence of microorganisms but can occur during the alkaline interaction of ferrous silicates rocks with water in conditions of temperature and pressure near the critical point. The results show that molecules of life can form in a path which is concomitant to this specific water-rock interaction and that organic matter of biological interest can form inside inclusions in the produced minerals. The knowledge about the formation of ferric iron in anoxic alkaline conditions may be important for the understanding of the Earth oxygenation and of extraterrestrial objects such as Enceladus. It is concluded that the search for the molecules of life may be connected to the search of amorphous silica, quartz, ferric oxides, amorphous and crystalline ferric silicates, in association with siderite. The observation of ferric minerals on early Earth and extraterrestrial objects does not mean that life had already emerged at the time of formation of the minerals.
Oxygen first arose in Earth’s atmosphere 2.3 billion years ago, but geochemical evidence suggests that small pockets of oxygen may have arisen earlier than the atmospheric rise in oxygen. Cyanobacteria, a modern phylum of bacteria, are believed to have been the driving force behind the oxygenation of Earth’s atmosphere, and there are two basic hypotheses about how they caused this major geologic event. There is a hypothesis, called the ‘ecological’ hypothesis, that suggests cyanobacteria were unable to live in most environments initially, and thus we see the evidence for pockets of oxygen earlier than the atmospheric rise in oxygen. Specifically, the ‘ecological’ hypothesis says that cyanobacteria originally were unable to swim and couldn’t live in saline water, meaning seawater. However, the data for this only considers two possible states for the levels of salinity: freshwater and seawater. We used data from the literature and from experiments to show that the gradient of salinity matters to the ability of cyanobacteria to live in environments, and that we cannot say what salinity levels a cyanobacteria can tolerate based on where they were found alone. See supplemental file for full abstract.
For years the debate about the possible contamination of space and other planets with microbes from Earth has been a hot topic. Furthermore, the discovery of sulfate minerals on the Martian surface make this planet suitable to colonization by microorganisms adapted to survive and grow under earthly extreme conditions. One of these microorganisms is Desulfotalea psychrophila, a microbe able to generate cellular energy by means of an enzyme known as the dissimilatory sulfate reductase. As all bacterial enzymes are encoded within the bacterium nucleic acids, we have designed experiments to study the ability of this microbe to survive, grow and metabolize under simulated Martian conditions of pressure, temperature and different concentrations of sulfate compounds.
We investigate the Spectral Kurtosis (SK) statistical signature of a signal observed with the Robert C. Byrd Green Bank Telescope Breakthrough Listen back-end that was transmitted by the Voyager 1 spacecraft, which is up to date the only known artificial signal originating from outside our solar system, and we demonstrate the ability of the SK estimator to perform real-time detection and discrimination against natural astronomical transients of deep-space Voyager 1-like technological signatures of alien origin. We use the same approach to investigate the yet controversial nature of the FRB 180301 signal detected during the Breakthrough Listen observations with the Parkes telescope.
A radio transmitter which is accelerating with a non-zero radial component with respect to a receiver will produce a signal that appears to change in frequency over time. This effect, commonly produced in astrophysical situations where orbital and rotational motions are ubiquitous, is called a drift rate. In radio SETI (Search for Extraterrestrial Intelligence) research, it is unknown a priori which frequency a signal is being sent at, or even if there will be any drift rate at all besides motions in the solar system. Therefore a range of potential drift rates need to be individually searched, and a maximum drift rate needs to be chosen. The middle of this range is zero, indicating no acceleration, but the absolute value for the limits remains unconstrained. A balance must be struck between computational time and the possibility of excluding a signal from ETI. In this work, we examine physical considerations that constrain a maximum drift rate and highlight the importance of this problem in any narrowband SETI search. We determine that a normalized drift rate of 200 nHz (e.g. 200 Hz/s at 1 GHz) is a generous, physically motivated guideline for the maximum drift rate that should be applied to future narrowband SETI projects if computational capabilities permit.
Venus is commonly known as the beautiful Evening Star of the night sky. Despite its bulk composition and size being similar (Interior ESI 0.98) to those of Earth, Venus is an extremely hot and dry planet, with temperatures ranging from 630 - 740K. With the current physical conditions, a potential areal biosphere could exist on the sulphuric acid-dominated clouds, providing moderate temperatures and pressures due to high altitudes. A theorised Iron or Sulphur metabolism could support exotic life in these extremely acidic (pH 0) conditions. The past habitability for regular life could be connected to the present habitability for exotic life through possible evolution, extending the possible Habitability timeline. Due to its smaller orbital distance (0.7 AU), Venus could have been the first habitable planet in the solar system, with optimistic models showing that Venus may have had a complex atmosphere and a liquid ocean for around 2 billion years, before the moist runaway greenhouse effect transformed the planet. High values for the D/H ratio and carbon abundance, combined with Venus’s slow rotation rate indicate the potential for biological life developing in Past Venus’s possible Earth-like conditions. These models were analysed from a perspective of Habitability, along with probable evolution into the present exotic life.
The microorganisms that evolved during the Archean era had extraordinary impacts on this planet. If not for them, Earth would not have developed the oxygen-rich atmosphere needed to support the evolution of multicellular organisms. However, our direct observations of life from that time come from only fifteen known fossiliferous Archean rock formations, and the exploration of these formations is not complete. As a result, study of these formations can yield new insights into the communities of microfossils that lived in the Archean era and previously unobserved microfossil morphologies. Here we present spheroid microfossils, as well as unusually large microfossils with clublike morphologies not previously observed in Archean microorganisms. These microfossils were three-dimensionally preserved in black chert from the Gamohaan Formation, Griqualand West Basin, Kaapvaal Craton, South Africa. These microfossils were discovered in a small, domal stromatolite that formed in a shallow marine setting on a carbonate shelf system at 2.52 billion years ago (Sumner and Bowring, 1996), just one to two hundred million years before the Great Oxidation Event.
Detecting biosignature gases on exoplanet atmosphere with near-future space telescopes is one of the most promising methods of detecting life beyond Earth. However, only a handful of biosignature gases are discussed today, and some can also be made by non-living, geological processes. Life, however, produces thousands of gases for a wide variety of purposes. Here we present isoprene, C5H8, as a potential biosignature gas. On Earth, isoprene is made at a comparable rate to methane (~500 million tonnes per year) and solely by living organisms. Remarkably, isoprene is produced by many organisms; plants, bacteria, and animals. Unfortunately, isoprene is rapidly destroyed on Earth by oxygen and OH, so for modern Earth isoprene is a poor biosignature, but on a world without oxygen, could this abundant gas be a sign of life? We evaluated the observation time required to detect isoprene in various anoxic atmospheres and found that detection is possible using JWST if life on that world made only one third as much isoprene as Earth life does. Despite the observational challenges, isoprene should be considered as a potential biosignature gas because of wide and abundant production by life on Earth and no false positives in any planetary scenario.
Chemical disequilibrium, or the long- term coexistence of two or more incompatible species, may be a useful metric for finding life. The presence of CH4 and O2 (that ought to react) in Earth’s atmosphere is an example and indicates biogenic sources of these gases. It is reasonable to think that life on an exoplanet or an icy moon would influence chemical disequilibrium because terrestrial life influences chemical disequilibrium by cycling almost all the bulk atmospheric gases. A chemical disequilibrium biosignature is appealing because it does not make assumptions about underlying biochemistry, unlike a search for biomolecules (e.g. DNA). Krissansen-Totton et al. (2016) calculated the atmosphere or atmosphere-ocean chemical disequilibrium of several planets and moons in our solar system. The metric used was the Gibbs free energy released when all chemical species are reacted together to an equilibrium state. They found that Earth’s atmosphere-ocean system has significantly more disequilibrium than any other planet due to biogenic fluxes. They propose high atmosphere-ocean chemical disequilibrium as a biosignature for exoplanets similar to the modern Earth, with photosynthetic biospheres. While disequilibrium is promising for detecting life on photosynthetic worlds, it remains to be determined how this metric applies to oceans in icy moons such as Europa and Enceladus. Indeed, an argument exists that purely chemosynthetic life will tend to destroy disequilibrium through its metabolism and produce anomalous equilibrium (Sholes et al., 2018). Thus, disequilibrium may have different interpretations: (1) High disequilibrium (uneaten food) on a dead world is an anti-biosignature. (2) High disequilibrium on a photosynthetic world would come from biogenic gases. (3) Low disequilibrium on a chemosynthetic world would be caused by biological consumption of chemical energy. We investigate the chemical disequilibrium biosignature for oceans on icy moons using analog environments: Antarctic subglacial lakes. First, we compute the disequilibrium in an observed “living” and modeled “dead” Antarctic subglacial lake. For a “living” subglacial lake, we use the aqueous composition of Subglacial Lake Whillans (SLW), located in Western Antarctica (Christner et al., 2014). For a “dead” subglacial lake, we model the steady-state chemistry of SLW if there was not life influencing chemical cycling. The disequilibrium calculation of both environments indicate that the “dead” lake has more available Gibbs energy than the “living” lake, suggesting that in purely chemosynthetic environments, anomalous chemical equilibrium is a sign of life, or inversely, that large chemical disequilibrium is an anti-biosignature. Our work on subglacial lakes can be considered within the context of measurements of Enceladus’ plumes by the Cassini Spacecraft. Plume measurements indicate relatively high available Gibbs energy in Enceladus’ ocean which may indicate low biomass, if life exists.
Stone Aerospace is developing the SUNFISH® autonomous underwater vehicle (AUV) which addresses many of the challenges of remote, autonomous exploration in unstructured environments such as the Ocean Worlds of the outer Solar System. The descendant of larger AUVs developed at Stone Aerospace (e.g. the NASA-funded DEPTHX, ENDURANCE, and ARTEMIS vehicles), SUNFISH is miniaturized, person-portable, six-degree-of-freedom (6-DOF) hovering vehicle with built-in precision navigation and control, multibeam mapping, imaging, and CTD capabilities. It was designed to be a highly-capable platform for operating in a wide variety of complex 3D spaces, ranging from man-made (e.g. piers) to natural (e.g. reefs, caves, and under ice). Building on these base functionalities, we have developed high-level capabilities for performing simultaneous localization and mapping (SLAM), exploration, path planning, and precision return home and docking. We describe these capabilities, and present a demonstration in the unstructured labyrinthine 3D environment of Peacock Springs in Florida, USA. SUNFISH and its technologies have direct application in several astrobiology-relevant goals, including furthering exploration and life search strategies for Ocean Worlds, as well as enabling Earth-analog fieldwork in structurally complex, remote aqueous environments such as caves, under ice shelves, or in sub-glacial lakes.
Basaltic lava caves are important Earth analogs in our search for life on Mars and other planets. Terrestrial lava caves exhibit morphologically diverse secondary mineral deposits (speleothems) often associated with liquid water. The detailed geochemical characterization of cave water and speleothems can provide valuable insights on potential biotic or abiotic mechanisms that lead to formation of these features. Our results showed that the cave water chemistry is consistent with basaltic host rock chemical composition. The water contained high levels of dissolved organic carbon and nitrogen, which could support microbial growth. The dissolved organic matter showed macromolecular structure and appears to be plant-derived, highly humified and microbially processed. Elevated nitrate in cave water may be due to agriculturally influenced regional surface water source or in situ oxidation of ammonia or organic N. Speleothems contained 29-79 wt% of crystalline, cryptocrystalline, or amorphous SiO2, and secondary minerals containing biosignature elements (Ca, Mg, Fe, Mn, S and V). This work complements the ongoing NASA BRAILLE (Biologic and Resource Analog Investigations in Low Light Environments) project to study basaltic lava tube caves as Earth analogs and ultimately provide insights for planning future missions to search for biosignature on Mars and other planetary bodies.