Hydrothermal gypsum deposits in the Haughton impact structure, Devon Island, Canada, contain microbial communities in an endolithic habitat within individual gypsum crystals. Cyanobacterial colonies occur as masses along cleavage planes, up to 5 cm from crystal margins. The crystals are transparent, so allow transmission of light for photosynthesis, while affording protection from dehydration and wind. The colonies appear to have modified their mineral host to provide additional space as they expanded. The colonies are black due to UV-screening pigments. The relative ease with which microbial colonization may be detected and identified in impact-generated sulphate deposits at Haughton suggests that analogous settings on other planets might merit future searches for biosignatures. The proven occurrence of sulphates on the Martian surface suggests that sulphate minerals should be a priority target in the search for life on Mars.

The lichen Xanthoria elegans has been exposed to space conditions and simulated Mars-analogue conditions in the lichen and fungi experiment (LIFE) on the International Space Station (ISS). After several simulations and short space exposure experiments such as BIOPAN, this was the first long-term exposure of eukaryotic organisms to the hostile space conditions of the low Earth orbit (LEO). The biological samples were integrated in the EXPOSE-E facility and exposed for 1.5 years outside the ISS to the combined impact of insolation, ultraviolet (UV)-irradiation, cosmic radiation, temperatures and vacuum conditions of LEO space. Additionally, a subset of X. elegans samples was exposed to simulated Martian environmental conditions by applying Mars-analogue atmosphere and suitable solar radiation filters. After their return to Earth the viability of the lichen samples was ascertained by viability analysis of LIVE/DEAD staining and confocal laser-scanning microscopy, but also by analyses of chlorophyll a fluorescence. According to the LIVE/DEAD staining results, the lichen photobiont showed an average viability rate of 71%, whereas the even more resistant lichen mycobiont showed a rate of 84%. Post-exposure viability rates did not significantly vary among the applied exposure conditions. This remarkable viability is discussed in the context of particular protective mechanisms of lichens such as anhydrobiosis and UV-screening pigments.

We interpret recent spectral data of Mars collected by the Mars Exploration Rovers to contain substantial evidence of sulfate minerals and aqueous processes. We present visible/near-infrared (VNIR), mid-IR and Mössbauer spectra of several iron sulfate minerals and two acid mine drainage (AMD) samples collected from the Iron Mountain site and compare these combined data with the recent spectra of Mars. We suggest that the sulfates on Mars are produced via aqueous oxidation of sulfides known to be present on Mars from Martian meteorites. The sulfate-rich rock outcrops observed in Meridiani Planum may have formed in an acidic environment similar to AMD environments on Earth. Because microorganisms are typically involved in the oxidation of sulfides to sulfates in terrestrial AMD sites, sulfate-rich rock outcrops on Mars may be a good location to search for evidence of life on that planet. Whether or not life evolved on Mars, following the trail of sulfate minerals is likely to lead to aqueous processes and chemical weathering. Our results imply that sulfate minerals formed in Martian soils via chemical weathering, perhaps over very long time periods, and that sulfate minerals precipitated following aqueous oxidation of sulfides to form the outcrop rocks at Meridiani Planum.

The effect of UV radiation on solutions of free and clay-adsorbed DNA has been investigated. It turns out that clay (montmorillonite/kaolinite) adsorbed nucleic acid undergoes less radiation damage than free nucleic acid. Our laboratory experiments have an astronomical counterpart in terms of solar irradiance on the Earth. An origin of life scenario is proposed where ubiquitous clay minerals lead the surface chemistry of the molecules relevant to the biological evolution and at the same time protect them from the deadly rainfall of UV photons.

Despite its amazing morphological diversity, life as we know it on Earth today is remarkably similar in its basic molecular architecture and biochemistry. The assumption that all life on Earth today shares these molecular and biochemical features is part of the paradigm of modern biology. This paper examines the possibility that this assumption is false, more specifically, that the contemporary Earth contains as yet unrecognized alternative forms of microbial life. The possibility that more than one form of life arose on Earth is consistent with our current understanding of conditions on the early Earth and the biochemical and molecular possibilities for life. Arguments that microbial descendents of an alternative origin of life could not co-exist with familiar life are belied by what we know of the complexity and diversity of microbial communities. Furthermore, the tools that are currently used to explore the microbial world – microscopy (with the aid of techniques such as DAPI staining and fluorescence in situ hybridization), cultivation and PCR amplification of rRNA genes – could not detect such organisms if they existed. Thus, the fact that we have not discovered any alternative life forms cannot be taken as evidence that they do not exist.

The lichen symbiotic organisms Fulgensia bracteata and Xanthoria elegans as well as their isolated photobionts and mycobionts were exposed to conditions simulating the extreme parameters of outer space in order to assess their limits of survival, e.g. during a potential interplanetary transfer or on the surface of an extraterrestrial body. Using the space simulation facilities at DLR, the test parameters under investigation were vacuum (10−3 Pa) and ultraviolet (UV) radiation at wavelength ranges from vacuum-UV to UV-A, applied separately or in combination. An analysis of vitality was made using confocal laser scanning microscopy and LIVE/DEAD staining using FUN I or SYTOX green. Intact lichens were extremely resistant to vacuum exposure as well as to UV radiation up to doses of about 160 kJ m−2 (200<λ<400 nm). Removal of the upper-cortex structure significantly increased the sensitivity of the lichens to the space parameters: already a relatively short treatment of about 5 h and a UV dose of about 50 kJ m−2 reduced the vitality rate to nearly 50%; however, the dose effect curves levelled off and continued treatment did not reduce the vitality further. Similar survival curves levelling off at higher UV doses were observed for the isolated photobionts; however, in the latter cases, the saturation occurred at five times lower doses (full UV spectrum). Also spores of the mycobionts showed a remarkable UV (254 nm) resistance up to doses of about 3 kJ m−2. The data suggest that the symbiotic features peculiar to lichens allow them to cope with the extreme conditions of outer space or even with Martian surface conditions provided suitable niche habitats are available to serve as refuges and complementary endogenous or exogenous protection mechanisms are established.

The asteroids are a major source of potential impactors on the Earth today. It has long been assumed that the giant planet Jupiter acts as a shield, significantly lowering the impact rate on the Earth from both cometary and asteroidal bodies. Such shielding, it is claimed, enabled the development and evolution of life in a collisional environment, which is not overly hostile. The reduced frequency of impacts, and of related mass extinctions, would have allowed life the time to thrive, where it would otherwise have been suppressed. However, in the past, little work has been carried out to examine the validity of this idea. In the first of several papers, we examine the degree to which the impact risk resulting from a population representative of the asteroids is enhanced or reduced by the presence of a giant planet, in an attempt to understand fully the impact regime under which life on Earth developed. Our results show that the situation is far less clear cut that has previously been assumed, that is, the presence of a giant planet can act to enhance the impact rate of asteroids on the Earth significantly.

EXPOSE-R flew as the second of the European Space Agency (ESA) EXPOSE multi-user facilities on the International Space Station. During the mission on the external URM-D platform of the Zvezda service module, samples of eight international astrobiology experiments selected by ESA and one Russian guest experiment were exposed to low Earth orbit space parameters from March 10th, 2009 to January 21st, 2011. EXPOSE-R accommodated a total of 1220 samples for exposure to selected space conditions and combinations, including space vacuum, temperature cycles through 273 K, cosmic radiation, solar electromagnetic radiation at >110, >170 or >200 nm at various fluences up to GJ m−2. Samples ranged from chemical compounds via unicellular organisms and multicellular mosquito larvae and seeds to passive radiation dosimeters. Additionally, one active radiation measurement instrument was accommodated on EXPOSE-R and commanded from ground in accordance with the facility itself. Data on ultraviolet radiation, cosmic radiation and temperature were measured every 10 s and downlinked by telemetry and data carrier every few months. The EXPOSE-R trays and samples returned to Earth on March 9th, 2011 with Shuttle flight, Space Transportation System (STS)-133/ULF 5, Discovery, after successful total mission duration of 27 months in space. The samples were analysed in the individual investigators laboratories. A parallel Mission Ground Reference experiment was performed on ground with a parallel set of hardware and samples under simulated space conditions following to the data transmitted from the flight mission.

One of the current goals of Martian exploration is to find evidence for extinct (or even extant) life. Carbon (an essential ingredient of life on Earth) is known to occur on Mars as CO2 in the atmosphere and frozen in the polar caps; it is inferred to be present as carbonates in the Martian crust and soils. We are attempting to define and quantify the different carbon reservoirs on Mars, so that we can follow Mars' carbon cycle. This paper discusses a primordial magmatic component that could be the starting point of such a cycle. The nature, distribution and isotopic composition of carbon was measured in a suite of Martian meteorites, comprising Chassigny and 11 shergottites. Other Martian meteorites were not included, as they sample rocks that have been altered by fluids at Mars' surface. Our results, obtained by high-resolution stepped combustion and mass spectrometry, show that the magmatic component has a very variable abundance of 1–100 ppm, with δ13C~−20±4‰. This value is close to magmatic carbon determined for Moon and for Vesta (the parent body of the HED basaltic meteorites), but very different from that of Earth.

Numerous studies over the past 30 years have suggested there is a causal connection between the motion of the Sun through the Galaxy and terrestrial mass extinctions or climate change. Proposed mechanisms include comet impacts (via perturbation of the Oort cloud), cosmic rays and supernovae, the effects of which are modulated by the passage of the Sun through the Galactic midplane or spiral arms. Supposed periodicities in the fossil record, impact cratering dates or climate proxies over the Phanerozoic (past 545 Myr) are frequently cited as evidence in support of these hypotheses. This remains a controversial subject, with many refutations and replies having been published. Here I review both the mechanisms and the evidence for and against the relevance of astronomical phenomena to climate change and evolution. This necessarily includes a critical assessment of time series analysis techniques and hypothesis testing. Some of the studies have suffered from flaws in methodology, in particular drawing incorrect conclusions based on ruling out a null hypothesis. I conclude that there is little evidence for intrinsic periodicities in biodiversity, impact cratering or climate on timescales of tens to hundreds of Myr. Although this does not rule out the mechanisms, the numerous assumptions and uncertainties involved in the interpretation of the geological data and in particular in the astronomical mechanisms suggest that Galactic midplane and spiral arm crossings have little impact on biological or climate variation above background level. Non-periodic impacts and terrestrial mechanisms (volcanism, plate tectonics, sea level changes), possibly occurring simultaneously, remain likely causes of many environmental catastrophes. Internal dynamics of the biosphere may also play a role. In contrast, there is little evidence supporting the idea that cosmic rays have a significant influence on climate through cloud formation. It seems likely that more than one mechanism has contributed to biodiversity variations over the past half Gyr.

This paper describes strategies to search for, detect, and identify organic material on the surface and subsurface of Mars. The strategies described include those applied by landed missions in the past and those that will be applied in the future. The value and role of ESA's ExoMars rover and of her key science instrument Mars Organic Molecule Analyzer (MOMA) are critically assessed.

In the next decade, numerous space missions will attempt to detect organic matter on planets and other objects in the Solar System. Recognizing carbon-based life or its remains will be a fundamental goal of future missions. In preparation, studies of organic matter in meteorites are enabling scientists how to discriminate between biogenic and abiogenic materials.

All known life requires phosphorus (P) in the form of inorganic phosphate (PO43− or Pi) and phosphate-containing organic molecules. Pi serves as the backbone of the nucleic acids that constitute genetic material and as the major repository of chemical energy for metabolism in polyphosphate bonds. Arsenic (As) lies directly below P on the periodic table and so the two elements share many chemical properties, although their chemistries are sufficiently dissimilar that As cannot directly replace P in modern biochemistry. Arsenic is toxic because As and P are similar enough that organisms attempt this substitution. We hypothesize that ancient biochemical systems, analogous to but distinct from those known today, could have utilized arsenate in the equivalent biological role as phosphate. Organisms utilizing such ‘weird life’ biochemical pathways may have supported a ‘shadow biosphere’ at the time of the origin and early evolution of life on Earth or on other planets. Such organisms may even persist on Earth today, undetected, in unusual niches.

The adaptability of extremophiles on Earth raises the question of what strategies putative life might have used to adapt to the present conditions on Mars. Here, we hypothesize that organisms might utilize a water–hydrogen peroxide (H2O–H2O2) mixture rather than water as an intracellular liquid. This adaptation would have the particular advantages in the Martian environment of providing a low freezing point, a source of oxygen and hygroscopicity. The findings by the Viking experiments are reinterpreted in light of this hypothesis. Our conclusion is that the hitherto mysterious oxidant in the Martian soil, which evolves oxygen when humidified, might be H2O2 of biological origin. This interpretation has consequences for site selection for future missions to search for life on Mars.

The primary aim of this review is to highlight that sea-ice microbes would be capable of occupying ice-associated biological niches on Europa and Enceladus. These moons are compelling targets for astrobiological exploration because of the inferred presence of subsurface oceans that have persisted over geological timescales. Although potentially hostile to life in general, Europa and Enceladus may still harbour biologically permissive domains associated with the ice, ocean and seafloor environments. However, validating sources of free energy is challenging, as is qualifying possible metabolic processes or ecosystem dynamics. Here, the capacity for biological adaptation exhibited by microorganisms that inhabit sea ice is reviewed. These ecosystems are among the most relevant Earth-based analogues for considering life on ocean worlds because microorganisms must adapt to multiple physicochemical extremes. In future, these organisms will likely play a significant role in defining the constraints on habitability beyond Earth and developing a mechanistic framework that contrasts the limits of Earth's biosphere with extra-terrestrial environments of interest.

Panspermia is the idea that life migrates naturally through space. Although an old idea, there has been much recent theoretical and experimental work developing the idea in recent years. In this review, this progress is considered and placed in context. Ideas concerning Panspermia now include mathematical treatments of the likelihood of transfer of life from Mars to Earth, the possibility of life transferring between the natural satellites of an outer planet such as Jupiter, and mathematical treatments and models of life migrating out of a Solar System. Not all predictions of the likelihood of successful Panspermia are positive, and some are contradictory. At present, Panspermia can neither be proved nor disproved. Nevertheless, Panspermia is an intellectual idea which holds strong attraction. However, at the heart of Panspermia is a still un-resolved mystery: in order to migrate, life has to start somewhere, and we still cannot tackle that moment of origin.

The future biosphere on Earth (as with its past) will be made up predominantly of unicellular micro-organisms. Unicellular life was probably present for at least 2.5 Gyr before multicellular life appeared and will likely be the only form of life capable of surviving on the planet in the far future, when the ageing Sun causes environmental conditions to become more hostile to more complex forms of life. Therefore, it is statistically more likely that habitable Earth-like exoplanets we discover will be at a stage in their habitable lifetime more conducive to supporting unicellular, rather than multicellular life. The end stage of habitability on Earth is the focus of this work. A simple, latitude-based climate model incorporating eccentricity and obliquity variations is used as a guide to the temperature evolution of the Earth over the next 3 Gyr. This allows inferences to be made about potential refuges for life, particularly in mountains and cold-trap (ice) caves and what forms of life could live in these environments. Results suggest that in high latitude regions, unicellular life could persist for up to 2.8 Gyr from present. This begins to answer the question of how the habitability of Earth will evolve at local scales alongside the Sun's main sequence evolution and, by extension, how the habitability of Earth-like planets would evolve over time with their own host stars.

Within the next few years, the first Earth-mass planets will be discovered around other stars. Some of those worlds will certainly lie within the classical ‘habitable zone’ of their parent stars, and we will quickly move from knowing of no exoEarths to knowing many. For the first time, we will be in a position to carry out a detailed search for the first evidence of life beyond our Solar System. However, such observations will be hugely taxing and time consuming to perform, and it is almost certain that far more potentially habitable worlds will be known than it is possible to study. It is therefore important to catalogue and consider the various effects that make a promising planet more or less suitable for the development of life. In this work, we review the various planetary, dynamical and stellar influences that could influence the habitability of exoEarths. The various influences must be taken in concert when we attempt to decide where to focus our first detailed search for life. While there is no guarantee that any given planet will be inhabited, it is vitally important to ensure that we focus our time and effort on those planets most likely to yield a positive result.

The issue of the variation of the surface ultraviolet (UV) environment on Mars was investigated with particular emphasis being placed on the interpretation of data in a biological context. A UV model has been developed to yield the surface UV irradiance at any time and place over the Martian year. Seasonal and diurnal variations were calculated and dose rates evaluated. Biological interpretation of UV doses is performed through the calculation of DNA damage effects upon phage T7 and Uracil, used as examples for biological dosimeters. A solar UV ‘hotspot’ was revealed towards perihelion in the southern hemisphere, with a significant damaging effect upon these species. Diurnal profiles of UV irradiance are also seen to vary markedly between aphelion and perihelion. The effect of UV dose is also discussed in terms of the chemical environment of the Martian regolith, since UV irradiance can reach high enough levels so as to have a significant effect upon the soil chemistry. We show, by assuming that H2O is the main source of hydrogen in the Martian atmosphere, that the stoichiometrically desirable ratio of 2:1 for atmospheric H and O loss rates to space are not maintained and at present the ratio is about 20:1. A large planetary oxygen surface sink is therefore necessary, in contrast with escape to space. This surface oxygen sink has important implications for the oxidation potential and the toxicology of the Martian soil. UV-induced adsorption of {\rm O}_{2}^{-} super-radicals plays an important role in the oxidative environment of the Martian surface, and the biologically damaging areas found in this study are also shown to be regions of high subsurface oxidation. Furthermore, we briefly cover the astrobiological implications for landing sites that are planned for future Mars missions.

Amino acids and nucleic acid bases are very important for the living organisms. Thus, their protection from decomposition, selection, pre-concentration and formation of biopolymers are important issues for understanding the origin of life on the Earth. Minerals could have played all of these roles. This paper discusses several aspects involving the adsorption of amino acids and nucleic acid bases onto minerals under conditions that could have been found on the prebiotic Earth; in particular, we recommend the use of minerals, amino acids, nucleic acid bases and seawater ions in prebiotic chemistry experiments. Several experiments involving amino acids, nucleic acid bases, minerals and seawater ions are also suggested, including: (a) using well-characterized minerals and the standardization of the mineral synthesis methods; (b) using primary chondrite minerals (olivine, pyroxene, etc.) and clays modified with metals (Cu, Fe, Ni, Mo, Zn, etc.); (c) determination of the possible products of decomposition due to interactions of amino acids and nucleic acid bases with minerals; (d) using minerals with more organophilic characteristics; (e) using seawaters with different concentrations of ions (i.e. Na+, Ca2+, Mg2+, SO42− and Cl−); (f) using non-protein amino acids (AIB, α-ABA, β-ABA, γ-ABA and β-Ala and g) using nucleic acid bases other than adenine, thymine, uracil and cytosine. These experiments could be useful to clarify the role played by minerals in the origin of life on the Earth.