Second only to H2, protonated molecular hydrogen, H3+, is the most abundantly produced interstellar molecule. Owing to its high reactivity and acidity, it plays the pivotal role in initiating interstellar chemical reactions, something which also reduces its steady-state concentration. Interstellar H3+ is not only destroyed in chemical reactions but also in dissociative recombination with electrons. The rate constant and mechanism of recombination have long been controversial, but great advances have been made during recent years, with the important consequence that the cosmic ray ionization rate in diffuse clouds is now believed to be higher by an order of magnitude than previously assumed.

Rock coatings have been observed on Mars by Mars Pathfinder, Viking and the Mars Exploration Rovers. Although rock varnish has been studied for its potential as a biosignature, other types of rock coating have been largely ignored. In Kärkevagge, Swedish Lapland, sulphate crusts, aluminium glazes and Fe/Mn films occur with mineralogies mimicking those observed on the surface of Mars. Molecular analysis and scanning electron microscopy (SEM) were used to investigate the bacterial communities associated with these rock coatings. Molecular techniques revealed differences in community structure and metabolisms associated with the production of secondary minerals between the three coating types. SEM analysis showed evidence of encrustation in mineral coatings in the Fe/Mn films and aluminium glazes, and evidence of abundant microbial communities in all three coating types. These observations provide evidence for bacterial participation in the genesis of rock coatings. For astrobiology on Mars, rock coatings are an attractive biosignature target scientifically and logistically: they are surface environments easily accessible by rovers, endoliths are afforded protection from surface conditions, and evidence of life could potentially be preserved through biomineralization and lithification. This study describes the bacterial communities from rock coatings compatible with martian mineralogy, explores the potential for biologically facilitated rock-coating formation, and supports rock coatings as targets of astrobiological interest on Mars.

Silicic acid increased numbers of both aerobic and facultatively anaerobic bacteria in ultrapure water incubated under strict oligotrophic conditions; soil extracts acted as the bacterial inoculum. The results are discussed in relation to the possibility that silicic acid, produced by the hydrolysis of silicates on the early Earth, could have stimulated the growth of the first bacterium, thereby allowing it to become established in the then prevailing conditions (presumed to be oligotrophic).

This paper develops a detailed quantitative model which uses the Drake equation and an assumption of an average maximum radio broadcasting distance by an communicative civilization. Using this basis, it estimates the minimum civilization density for contact between two civilizations to be probable in a given volume of space under certain conditions, the amount of time it would take for a first contact, and the question of whether reciprocal contact is possible.

Mars is considered to be one of the most favourable places in the Solar System to search for past and present life. In the past Mars was warmer and wetter, so terrestrial halophiles can be regarded as analogues of hypothetical ancient Martian halophiles. In this study we used microorganisms from unique Altai region (Russia) to estimate the capability of terrestrial bacteria and archaea to survive at low temperatures and high concentration of salts and metals, similar to the Martian environment. The current report demonstrates that both halophilic archaea and halotolerant bacteria from saline lakes of the Altai region may be considered as analogues of ancient Martian organisms, since they are able to withstand conditions that hypothetically existed in subsurface layers of the early Mars (low temperatures, salt solutions with a high content of NaCl) with only slight decrease in viability. We also found that the studied microorganisms can use some organic substances found in meteorites. We consider that transfer of unicellular halophiles from Earth to Mars was possible, and, moreover, they could successfully survive and grow on early Mars. Adjusting our growth media to the chemical composition of the lakes, from which the studied strains were isolated, resulted in significant increase in survival and growth rates. Certain strains could survive several freeze–thaw cycles at −70 °C typical for Martian nights.

One of the criteria for the concept of a galactic habitable zone (GHZ) is that the pattern speed of the stars in the GHZ should be close to the pattern speed of the spiral arms. Another criteria is that the stars in it should have a high enough metallicity. In a barred galaxy, the GHZ will be more complicated to define since the bar can change stellar orbits. Many disc galaxies, including the Milky Way, are barred galaxies. The stars in the bar move in a number of fairly complicated orbits. However, the bar will also influence the orbits of stars in the whole galaxy. Stars passing close to the bar can either gain or lose angular momentum, due to a positive or negative torque by the bar. Some stars will therefore be captured by the bar while some stars eventually may reach the escape velocity from the galaxy. The bar will hence be able to relocate stars, and stars with low or high metallicity could be found far away from their original orbits. The ordinary evolution of a bar is to grow in length out to the co-rotation radius for the pattern speed of the bar. As the galaxy ages, and the bar grows in length, the bar will influence a larger part of the galaxy. The effect of moving stars inwards or outwards is greatest just outside the bar, and this region can eventually lose a high percentage of the stars.

For detecting life from reflection spectra on extrasolar planets, trace of photosynthesis is one of the indicators. However, it is not yet clear what kind of radiation environments is acceptable for photosynthesis. Light absorption in photosystems on the Earth occurs using limited photosynthetic pigments such as chlorophylls (Chls) and bacteriochlorophylls (BChls). Efficiencies of light absorption for the pigments were evaluated by calculating the specific molecular absorption spectra at the high accuracy-quantum mechanical level. We used realistic stellar radiation spectra such as F, G, K and M-type stars to investigate the efficiencies. We found that the efficiencies are increased with the temperature of stars, from M to F star. Photosynthetic pigments have two types of absorption bands, the Qy and Soret. In higher temperature stars like F star, contributions from the Soret region of the pigments are dominant for the efficiency. On the other hand, in lower temperature stars like M stars, the Qy band is crucial. Therefore, differences on the absorption intensity and the wavelength between the Qy and Soret band are the most important to characterize the photosynthetic pigments. Among photosynthetic pigments, Chls tend to be efficient in higher temperature stars, while BChls are efficient for M stars. Blueward of the 4000 Å break, the efficiencies of BChls are smaller than Chls in the higher temperature stars.

We suggest testing S isotopes as biomarkers for Mars. An analogous robust biosignature has recently been proposed for the forthcoming exploration of the icy surface of Europa, and in the long term for the exploration of the surfaces of other icy moons of the outer solar system. We discuss relevant instrumentation for testing the presence of life itself in some sites, whether extinct or extant in order to complement a set of other independent biosignatures. We pay special attention to the possible early emergence of sulphate-metabolizing microorganisms, as it happened on the early Earth. Fortunately, possible sites happen to be at likely landing sites for future missions ExoMars and Mars 2020, including Oxia Planum and Mawrth Vallis. We suggest how to make additional feasible use of the instruments that have already been approved for future missions. With these instruments, the proposed measurements can allow testing S isotopes on Mars, especially with the Mars Organic Molecule Analyzer.

The appellation ‘habitable zone’ in astrobiology in sooth evinces an overlooked and winding history that can be traced back to the 19th century. This paper sketches how this term from geography was generalized to encompass planetary habitability. The people involved in this narrative are numerous, but the bulk of their musings were rather nebulous. Yet, during this period appear the first true insights, although sadly this saga is not altogether sans blights.

The idea that life migrates naturally between planetary bodies has grown in strength in recent years. This idea (panspermia) is believed to be possible via the mechanism of impact events. Previous research on this topic has concentrated on small meteoroids (micrometres to centimetres in diameter), with giant objects (metres to kilometres in diameter) being relatively ignored. This is due to the common belief that the larger objects vaporize on impact with the Earth's surface, which in most studies is taken as rock. Here we examine experimentally whether hypervelocity impacts into water result in significant survival of the impactors. For this study the University of Kent's two-stage light gas gun was used to accelerate millimetre-sized shale projectiles obliquely into a relatively deep water layer, at approximately 5 km s−1. Two shots have been made with surviving fragments being recovered from each. The surviving fragments appear highly shocked and display clear signs of cracking. The fragments that have been isolated contribute to a significant percentage (~10%) of the original unfired projectile mass and are as large as ~20% of the original projectile diameter. This indicates that oceanic hypervelocity impact events of large asteroids may deliver significant volumes of solid material to the Earth and thus provide a possible mechanism for successful panspermia.

A planet may be defined as habitable if it has an atmosphere and is warm enough to support the existence of liquid water on its surface. Such a world has the basic set of conditions that allow it to develop life similar to ours, which is carbon-based and has water as its universal solvent. While this definition is suitably vague to allow a fairly broad range of possibilities, it does not address the question as to whether any life that does form will become either complex or intelligent. In this paper, we seek to synthesize a qualitative definition of which subset of these ‘habitable worlds’ might develop more complex and interesting life forms. We identify two key principles in determining the capacity of life to breach certain transitions on route to developing intelligence. The first is the number of potential niches a planet provides. Secondly, the complexity of life will reflect the information density of its environment, which in turn can be approximated by the number of available niches. We seek to use these criteria to begin the process of placing the evolution of terrestrial life in a mathematical framework based on environmental information content. This is currently testable on Earth and will have clear application to the worlds that we are only beginning to discover. Our model links the development of complex life to the physical properties of the planet, something which is currently lacking in all evolutionary theory.

Robotic astrobiology involves the remote projection of intelligent capabilities to planetary missions in the search for life, preferably with human-level intelligence. Planetary rovers would be true human surrogates capable of sophisticated decision-making to enhance their scientific productivity. We explore several key aspects of this capability: (i) visual texture analysis of rocks to enable their geological classification and so, astrobiological potential; (ii) serendipitous target acquisition whilst on the move; (iii) continuous extraction of regolith properties, including water ice whilst on the move; and (iv) deep learning-capable Bayesian net expert systems. Individually, these capabilities will provide enhanced scientific return for astrobiology missions, but together, they will provide full autonomous science capability.

The astrobiological relevance of carbonaceous chondrites is reviewed. It is argued that the primitive meteorites called carbonaceous chondrites provide a unique source of information about the materials and conditions in the Solar System during the earliest phases of its history, and its subsequent evolution. Presolar dust grains extracted from the carbonaceous chondrites provide direct information on the previous generations of stars that provided the materials present for planet formation. The organic material found in carbonaceous chondrites consist of amino acids, carboxylic acids and sugar derivatives. Part of the amino acids found show L-enantiomeric excesses, which indicates that homochirality on Earth could be a direct result of input from meteoritic material to the early Earth.

Batch experiments of komatiite–H2O–CO2 system with temperatures from 200 to 450°C were performed to simulate the interactions between the newly formed ultramafic crust and the proto-atmosphere on Earth before the formation of its earliest ocean. Particularly, magnetite nanocrystals were observed in the experiment carried out at 450°C that are characterized by their hexagonal platelet-like morphology and porous structure. Exactly the same set of lattice fringes on the two opposite sides of one pore suggests post-crystallization erosion. The results demonstrate that magnetite could be produced by the direct interactions between the ultramafic rocky crust and the atmosphere before the formation of the ocean on the Hadean Earth. These magnetite nanoparticles could serve as a catalyst in the synthesis of simple organic molecules during the organochemical evolution towards life.

How prebiotic metabolic pathways could have formed is an essential question for the origins of life on early Earth. From the abiogenetic point of view, the emergence of primordial metabolism may be postulated as a continuum from Earth's geochemical processes to chemoautotrophic biochemical procedures on mineral surfaces. In the present study, we examined in detail the reversible amination of α-ketoglutarate on UV-irradiated ZnS particles under variable reaction conditions such as pH, temperature, hole scavenger species and concentrations, and different amino acids. It was observed that the reductive amination of α-ketoglutarate and the oxidative amination of glutamate were both effectively performed on ZnS surfaces in the presence and absence of a hole scavenger, respectively. Accordingly, a photocatalytic mechanism was proposed. The reversible photochemical reaction was more efficient under basic conditions but independent of temperature in the range of 30–60 °C. SO32− was more effective than S2− as the hole scavenger. Finally, we extended the glutamate dehydrogenase-like chemistry to a set of other α-amino acids and their corresponding α-oxo acids and found that hydrophobic amino acid side chains were more conducive to the reversible redox reactions. Since the experimental conditions are believed to have been prevalent in shallow water hydrothermal vent systems of early Earth, the results of this work not only suggest that the ZnS-assisted photochemical reaction can regulate the redox equilibrium between α-amino acids and α-oxo acids, but also provide a model of how prebiotic metabolic homoeostasis could have been developed and regulated. These findings can advance our understanding of the establishment of archaic non-enzymatic metabolic systems and the origins of autotrophy.

A prediction of panspermia is that organic material should be abundant throughout the universe. The recovery of micrometre-sized biotic particles at altitudes of around 40 km above the Earth is a key indicator of the validity of panspermia. A common criticism of experiments to capture atmospheric particles is that the particles could have originated from the Earth. Theoretical models of the variation of temperature with altitude above the Earth show that 10 μm diameter particles can withstand rapid deceleration without significant ablation. Experimental evidence shows that small particles containing bacteria can survive the temperature regimes imposed by entry into the Earth's lower atmosphere. This is an important step in demonstrating that life on Earth did not evolve in isolation from the remainder of the universe.

The most extremely osmotolerant microbial isolates are fungi from high-sugar environments that tolerate the lowest water activity (0.61) for growth yet reported. Studies of osmotolerant bacteria have focused on halotolerance rather than sucretolerance (ability to grow in high sugar concentrations). A collection of salinotolerant (≥10% NaCl or ≥50% MgSO4) bacterial isolates from the Great Salt Plains of Oklahoma and Hot Lake in Washington were screened for sucretolerance in medium supplemented with ≥50% fructose, glucose or sucrose. Tolerances significantly differed between solutes, even though water activities for saline media (0.92 and 0.85 for 10 and 20% NaCl Salt Plains media, respectively) were comparable or lower than water activities for high-sugar media (0.93 and 0.90 for 50 and 70% sucrose artificial nectar media, respectively). These specific solute effects were differentially expressed among individual isolates. Extrapolating the results of earlier food science studies with yeasts at high sugar concentrations to bacteria in salty environments with low water activity should be done with caution. Furthermore, the discussion of habitable Special Regions on Mars and the icy worlds should reflect an understanding of specific solute effects.

Recent work in developing self-replicating machines has approached the problem as an engineering problem, using engineering materials and methods to implement an engineering analogue of a hitherto uniquely biological function. The question is – can anything be learned that might be relevant to an astrobiological context in which the problem is to determine the general form of biology independent of the Earth. Compared with other non-terrestrial biology disciplines, engineered life is more demanding. Engineering a self-replicating machine tackles real environments unlike artificial life which avoids the problem of physical instantiation altogether by examining software models. Engineering a self-replicating machine is also more demanding than synthetic biology as no library of functional components exists. Everything must be constructed de novo. Biological systems already have the capacity to self-replicate but no engineered machine has yet been constructed with the same ability – this is our primary goal. On the basis of the von Neumann analysis of self-replication, self-replication is a by-product of universal construction capability – a universal constructor is a machine that can construct anything (in a functional sense) given the appropriate instructions (DNA/RNA), energy (ATP) and materials (food). In the biological cell, the universal construction mechanism is the ribosome. The ribosome is a biological assembly line for constructing proteins while DNA constitutes a design specification. For a photoautotroph, the energy source is ambient and the food is inorganic. We submit that engineering a self-replicating machine opens up new areas of astrobiology to be explored in the limits of life.

Whale strandings occur in many places worldwide and numerous possible explanations for this phenomenon have been proposed, including the effects of astronomical events such as Solar eruptions on the Earth's magnetic field. Whales use the geomagnetic field for navigation, and its distortion can therefore result in whale strandings in certain regions. However, Solar storms do not have the same impact on the geomagnetic field across the whole of the Earth's surface, and positions nearer to the equator are less exposed to this phenomenon. It is therefore plausible that Solar storms can explain whale strandings at high latitude at least, but not necessarily worldwide. This review considers strandings in relation to the geographical and geomagnetic properties of locations at higher latitudes and to changes in the magnetic field over recent centuries. It also focuses on a Solar storm in December 2015. These considerations suggest that navigation errors due to Solar storms are more likely to occur at higher latitudes, particularly in sea areas where the animals might subsequently swim into a geographic trap and become stranded. For sperm whales (Physeter macrocephalus), the southern Norwegian Sea in conjunction with the shallow North Sea represents such an area.

We proffer a contemporary solution to the so-called Fermi Paradox, which is concerned with conflict between Copernicanism and the apparent paucity of evidence for intelligent alien civilizations. In particular, we argue that every community of organisms that reaches its space-faring age will (1) almost immediately use its rocket-building computers to reverse-engineer its genetic chemistry and (2) self-destruct when some individual uses said technology to design an omnicidal pathogen. We discuss some of the possible approaches to prevention with regard to Homo sapiens’ vulnerability to bioterrorism, particularly on a short-term basis.