Microbial O2 production via oxygenic photosynthesis was vital in oxygenating the Earth’s surface environment during the Great Oxygenation Event (GOE) ca. 2.5 to 2.3 billion years ago. However, geochemical, paleontological and genomic data suggest the emergence of oxygenic photosynthesis precedes the GOE by at least 500 million years. This demonstrates that the first appearance of microbial O2 in the environment cannot explain the timing of atmospheric oxygenation. Instead, the GOE was facilitated by Earth’s geodynamic evolution, expanding cyanobacterial habitats and the changing redox state of the mantle, decreasing the abundance of reduced surface rocks, volcanic gases and aqueous solutes. These trends ultimately resulted in magnified O2 production rates and diminished O2 consumption rates. Thus, the GOE can be understood as a misbalance between O2 sources and sinks. One of the most critical O2 sinks on modern Earth is microbial O2 consumption via aerobic respiration, and accumulating evidence suggests its emergence well before the GOE. However, the role of aerobic microorganisms as an O2 sink delaying the GOE remains poorly explored. Here, we review the redox evolution of Earth’s mantle and surface environments, as well as the Archean evolution of aerobic microbial metabolisms. Oxygenic photosynthesis released O2 to the environment, but the secular oxidation of the solid Earth was critical in allowing O2 accumulation. Aerobic respiration expanded in response to the GOE, but our survey suggests it could have been a critical O2 sink even earlier. Hence, aerobic respiration can be seen as geobiological feedback to changes in the Earth system from deep in the mantle up to the surface. However, the timing and rate of O2 consumption by aerobic respiration before the GOE remain poorly constrained. We conclude by highlighting open questions and future research directions to understand the role of the aerobic O2 sink in delaying the GOE.

Ionizing radiation is known to have a destructive effect on biology by causing damage to DNA, cells and the production of reactive oxygen species, among other things. While direct exposure to high-radiation dose is indeed not favorable for biological activity, ionizing radiation can and, in some cases, is known to produce a number of biologically useful products. One such mechanism is the production of biologically useful products via charged particle-induced radiolysis. Energetic charged particles interact with the surfaces of planetary objects such as Mars, Europa and Enceladus without much shielding from their rarefied atmospheres. Depending on the energy of said particles, they can penetrate several meters deep below the surface and initiate a number of chemical reactions along the way. Some of the byproducts are impossible to produce with lower-energy radiation (such as sunlight), opening up new avenues for life to utilize them. The main objective of the manuscript is to explore the concept of a Radiolytic Habitable Zone (RHZ), where the chemistry of galactic cosmic ray-induced radiolysis can be potentially utilized for metabolic activity. We first calculate the energy deposition and the electron production rate using the GEANT4 numerical model, then estimate the current production and possible chemical pathways which could be useful for supporting biological activity on Mars, Europa and Enceladus. The concept of RHZ provides a novel framework for understanding the potential for life in high-radiation environments. By combining energy deposition calculations with the energy requirements of microbial cells, we have defined the RHZ for Mars, Europa and Enceladus. These zones represent the regions where radiolysis-driven energy production is sufficient to sustain microbial metabolism. We find that bacterial cell density is highest in Enceladus, followed by Mars and Europa. We discuss the implications of these mechanisms for the habitability of such objects in the solar system and beyond.

The establishment of the possible presence of life on Mars (past or present) is based on the study of planetary analogues, which allow in situ analysis of the environments in which living organisms adapt to often extreme conditions. Although Mars has been a candidate for hosting life, based on observations made decades ago, it is thanks to the characteristics identified in environments, mainly volcanic, that it has been possible to calibrate instruments and detail the features of the red planet. In this paper, we present a review of the main characteristics of different planetary analogues, particularly deepening the study of Antarctica, to later expose the factors studied in Deception Island that have contributed to considering it as an analogue of Mars from different perspectives. Although geological and geomorphological studies on the analogies of the island already exist, detailed analyses that present the approach of astrobiological analogues are required, thus allowing further research.

Life as we know it requires the presence of liquid water and the availability of nutrients, which are mainly based on the elements C, H, N, O, P and S (CHNOPS) and trace metal micronutrients. We aim to understand the presence of these nutrients within atmospheres that show the presence of water cloud condensates, potentially allowing the existence of aerial biospheres. In this paper, we introduce a framework of nutrient availability levels based on the presence of water condensates and the chemical state of the CHNOPS elements. These nutrient availability levels are applied to a set of atmospheric models based on different planetary surface compositions resulting in a range of atmospheric compositions. The atmospheric model is a bottom-to-top equilibrium chemistry atmospheric model which includes the atmosphere–crust interaction and the element depletion due to the formation of clouds. While the reduced forms of CNS are present at the water cloud base for most atmospheric compositions, P and metals are lacking. This indicates the potential bio-availability of CNS, while P and metals are limiting factors for aerial biospheres.

In situ elemental imaging of planetary surface regolith at a spatial resolution of 100s to 1000s of microns can provide evidence of the provenance of rocks or sediments and their habitability, and can identify post-depositional diagenetic alteration affecting preservation. We use high-resolution elemental maps and XRF spectra from MapX, a flight prototype in situ X-ray imaging instrument, to demonstrate this technology in rock types relevant to astrobiology. Examples are given for various petrologies and depositional/diagenetic environments, including ultramafic/mafic rocks, serpentinites, hydrothermal carbonates, evaporites, stromatolitic cherts and diagenetic concretions.

Although astrobiology studies how life functions and evolves, ecology is still largely overlooked in astrobiology research. Here we present an argument for astroecology, a merger of ecology and astrobiology, a self-aware scientific endeavour. Ecology is rarely mentioned in influential documents like the NASA Astrobiology Strategy (2015), and terms such as ‘niche’ can end up being used in a less precise fashion. As ecology deals with sequential levels of organization, we suggest astrobiologically-relevant problems for each of these levels. Organismal ecology provides ecological niche modelling, which can aid in evaluating the probability that Earth-like life would survive in extraterrestrial environments. Population ecology provides a gamut of models on the consequences of dispersal, and if lithopanspermia can be validated as a form of space dispersal for life, then metabiospheres and similar astrobiological models could be developed to understand such complex structure and dynamics. From community ecology, the discussion of habitability should include the concept of true vacant habitats (a misnomer, perhaps better called ‘will-dwells’) and contributions from the blossoming field of microbial ecology. Understanding ecosystems by focusing on abiotic properties is also key to extrapolating from analogue environments on Earth to extraterrestrial ones. Energy sources and their distribution are relevant for ecological gradients, such as the biodiversity latitudinal gradient – would tropics be species-rich in other inhabited planets? Finally, biosphere ecology deals with integration and feedback between living and non-living systems, which can generate stabilized near-optimal planetary conditions (Gaia); but would this work for other inhabited planets? Are there ‘strong’ (like Earth) and ‘weak’ (perhaps like Mars) biospheres? We hope to show ecology can contribute relevant ideas to the interdisciplinary field of astrobiology, helping conceptualize further levels of integration. We encourage new partnerships and for astrobiologists to take ecology into account when studying the origin, evolution and distribution of life in the universe.

Free-floating planets (FFPs) can result from dynamical scattering processes happening in the first few million years of a planetary system's life. Several models predict the possibility, for these isolated planetary-mass objects, to retain exomoons after their ejection. The tidal heating mechanism and the presence of an atmosphere with a relatively high optical thickness may support the formation and maintenance of oceans of liquid water on the surface of these satellites. In order to study the timescales over which liquid water can be maintained, we perform dynamical simulations of the ejection process and infer the resulting statistics of the population of surviving exomoons around FFPs. The subsequent tidal evolution of the moons’ orbital parameters is a pivotal step to determine when the orbits will circularize, with a consequential decay of the tidal heating. We find that close-in ($a \lesssim 25$ RJ) Earth-mass moons with carbon dioxide-dominated atmospheres could retain liquid water on their surfaces for long timescales, depending on the mass of the atmospheric envelope and the surface pressure assumed. Massive atmospheres are needed to trap the heat produced by tidal friction that makes these moons habitable. For Earth-like pressure conditions (p0 = 1 bar), satellites could sustain liquid water on their surfaces up to 52 Myr. For higher surface pressures (10 and 100 bar), moons could be habitable up to 276 Myr and 1.6 Gyr, respectively. Close-in satellites experience habitable conditions for long timescales, and during the ejection of the FFP remain bound with the escaping planet, being less affected by the close encounter.

Planetary geodynamics may have an important influence over planetary habitability and the boundaries of the circumstellar habitable zone (CHZ) in space and time. To investigate this we use a minimal parameterized model of the co-evolution of the geosphere and atmosphere of Earth-like planets around F, G, K and M main sequence stars. We found the CHZ for the present Solar System located between 0.92 and 1.09 au for a 1.0 M$_{\oplus }$ Earth-like planet, extendible to 1.36 au for a 4.0 M$_{\oplus }$ planet. In the literature, the CHZ varies considerably in width and border location, but the outer edges tend to be more spread out than the inner edges, showing a higher difficulty in determining the outer edge. Planetary mass has a considerable effect on planetary geodynamics, with low-mass planets cooling down faster and being less capable of maintaining a rich carbon dioxide atmosphere for several billions of years. Age plays a particularly important role in the width of the CHZ as the CHZ contracts in both directions: from the inner edge (as stellar luminosity increases with time), and from the outer edge (as planetary heat flux and seafloor spreading rate decrease with time). This strongly affects long-lived habitability as the 5 Gyr continuous CHZ may be very narrow or even non-existent for low-mass planets (<0.5 M$_{\oplus }$) and fast-evolving high-mass stars (>1.1 M$_{\odot }$). Because of this, the mean age of habitable terrestrial planets in our Galaxy today may be younger than Earth's age. Our results suggest that the best targets for future surveys of biosphere signatures may be planets between 0.5 and 4.0 M$_{\oplus }$, in systems younger than the Solar System. These planets may present the widest and long-lived CHZ.

This article analyses how the first circumnavigation of the world, from 1519 to 1522, introduced South America as a key space in the formation of the ‘global’, thus producing a historical point of inflection. We examine the commercial and political plans and networks that began to function as a result of this new connectivity, which turned the American continent into a major global axis. The analysis focuses on the way in which this voyage gave new prominence to an unexplored region of the world, namely the southernmost tip of America, thus changing the notion of habitability that had prevailed for centuries in Europe. These changes questioned the authority of ‘ancient’ Greek thinkers and strengthened a European historical narrative that appropriated the discovered territories and distinguished the extreme southern part of America from other southern regions, as symbolized through figures such as the Patagonian giants. I consider these changes based on evidence from Spanish sources.

The English homelessness scheme has been lauded as being one of the most progressive in the world for offering an individually legally enforceable right to housing to those people who meet the statutory criteria. Its definition of homelessness is also liberal by comparison with many other countries within Europe and beyond, extending significantly beyond the stereotypical rooflessness experienced by rough sleepers. Nevertheless, the scheme is highly selective and targeted, and assesses homelessness through a test of relative need, rather than enshrining a minimally acceptable standard of housing. It thereby creates a category of the marginally housed whose housing needs are assessed as insufficiently poor to be officially categorised as homeless, yet who are living in severely inadequate housing. To reduce the uncertainty and contingency of the current test, the paper proposes the adoption of a new test of habitability.

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 term habitable is used to describe planets that can harbour life. Debate exists as to specific conditions that allow for habitability but the use of the term as a planetary variable has become ubiquitous. This paper poses a meta-level question: What type of variable is habitability? Is it akin to temperature, in that it is something that characterizes a planet, or is something that flows through a planet, akin to heat? That is, is habitability a state or a process variable? Forth coming observations can be used to discriminate between these end-member hypotheses. Each has different implications for the factors that lead to differences between planets (e.g. the differences between Earth and Venus). Observational tests can proceed independent of any new modelling of planetary habitability. However, the viability of habitability as a process can influence future modelling. We discuss a specific modelling framework based on anticipating observations that can discriminate between different views of habitability.

Earth-Like is an interactive website and twitter bot that allows users to explore changes in the average global surface temperature of an Earth-like planet due to variations in the surface oceans and emerged land coverage, rate of volcanism (degassing) and the level of the received solar radiation. The temperature is calculated using a simple carbon–silicate cycle model to change the level of CO2 in the atmosphere based on the chosen parameters. The model can achieve a temperature range exceeding −100°C to 100°C by varying all three parameters, including freeze-thaw cycles for a planet with our present-day volcanism rate and emerged land fraction situated at the outer edge of the habitable zone. To increase engagement, the planet is visualized by using a neural network to render an animated globe, based on the calculated average surface temperature and chosen values for land fraction and volcanism. The website and bot can be found at earthlike.world and on twitter as @earthlikeworld. Initial feedback via a user survey suggested that Earth-Like is effective at demonstrating that minor changes in planetary properties can strongly impact the surface environment. The goal of the project is to increase understanding of the challenges we face in finding another habitable planet due to the likely diversity of conditions on rocky worlds within our Galaxy.