Nitrogen Incorporation in Potassic and Micro- and Meso-Porous Minerals: Potential Biogeochemical Records and Targets for Mars Sampling.

We measured the N concentrations and isotopic compositions of 44 samples of terrestrial potassic and micro- and meso-porous minerals and a small number of whole-rocks to determine the extent to which N is incorporated and stored during weathering and low-temperature hydrothermal alteration in Mars surface/near-surface environments. The selection of these minerals and other materials was partly guided by the study of altered volcanic glass from Antarctica and Iceland, in which the incorporation of N as NH4+ in phyllosilicates is indicated by correlated concentrations of N and the LILEs (i.e., K, Ba, Rb, Cs), with scatter likely related to the presence of exchanged, occluded/trapped, or encapsulated organic/inorganic N occurring within structural cavities (e.g., in zeolites). The phyllosilicates, zeolites, and sulfates analyzed in this study contain between 0 and 99,120 ppm N and have δ15Nair values of -34‰ to +65‰. Most of these minerals, and the few siliceous hydrothermal deposits that were analyzed, have δ15N consistent with the incorporation of biologically processed N during low-temperature hydrothermal or weathering processes. Secondary ion mass spectrometry on altered hyaloclastites demonstrates the residency of N in smectites and zeolites, and silica. We suggest that geological materials known on Earth to incorporate and store N and known to be abundant at, or near, the surface of Mars should be considered targets for upcoming Mars sample return with the intent to identify any signs of ancient or modern life.

Temporal variation of planetary iron as a driver of evolution.

Iron is an irreplaceable component of proteins and enzyme systems required for life. This need for iron is a well-characterized evolutionary mechanism for genetic selection. However, there is limited consideration of how iron bioavailability, initially determined by planetary accretion but fluctuating considerably at global scale over geological time frames, has shaped the biosphere. We describe influences of iron on planetary habitability from formation events >4 Gya and initiation of biochemistry from geochemistry through oxygenation of the atmosphere to current host-pathogen dynamics. By determining the iron and transition element distribution within the terrestrial planets, planetary core formation is a constraint on both the crustal composition and the longevity of surface water, hence a planet's habitability. As such, stellar compositions, combined with metallic core-mass fraction, may be an observable characteristic of exoplanets that relates to their ability to support life. On Earth, the stepwise rise of atmospheric oxygen effectively removed gigatons of soluble ferrous iron from habitats, generating evolutionary pressures. Phagocytic, infectious, and symbiotic behaviors, dating from around the Great Oxygenation Event, refocused iron acquisition onto biotic sources, while eukaryotic multicellularity allows iron recycling within an organism. These developments allow life to more efficiently utilize a scarce but vital nutrient. Initiation of terrestrial life benefitted from the biochemical properties of abundant mantle/crustal iron, but the subsequent loss of iron bioavailability may have been an equally important driver of compensatory diversity. This latter concept may have relevance for the predicted future increase in iron deficiency across the food chain caused by elevated atmospheric CO2.

Habitability Models for Astrobiology.

Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency among them, being different in function to those used by ecologists. Habitability models are not only used to determine whether environments are habitable, but they also are used to characterize what key factors are responsible for the gradual transition from low to high habitability states. Here we review and compare some of the different models used by ecologists and astrobiologists and suggest how they could be integrated into new habitability standards. Such standards will help improve the comparison and characterization of potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science, and the synergy between ecologists and astrobiologists is necessary to expand our understanding of the habitability of Earth, the Solar System, and extrasolar planets.

Warm early Mars surface enabled by high-altitude water ice clouds.

Despite receiving just 30% of the Earth's present-day insolation, Mars had water lakes and rivers early in the planet's history, due to an unknown warming mechanism. A possible explanation for the >102-y-long lake-forming climates is warming by water ice clouds. However, this suggested cloud greenhouse explanation has proved difficult to replicate and has been argued to require unrealistically optically thick clouds at high altitudes. Here, we use a global climate model (GCM) to show that a cloud greenhouse can warm a Mars-like planet to global average annual-mean temperature ([Formula: see text]) ∼265 K, which is warm enough for low-latitude lakes, and stay warm for centuries or longer, but only if the planet has spatially patchy surface water sources. Warm, stable climates involve surface ice (and low clouds) only at locations much colder than the average surface temperature. At locations horizontally distant from these surface cold traps, clouds are found only at high altitudes, which maximizes warming. Radiatively significant clouds persist because ice particles sublimate as they fall, moistening the subcloud layer so that modest updrafts can sustain relatively large amounts of cloud. The resulting climates are arid (area-averaged surface relative humidity ∼25%). In a warm, arid climate, lakes could be fed by groundwater upwelling, or by melting of ice following a cold-to-warm transition. Our results are consistent with the warm and arid climate favored by interpretation of geologic data, and support the cloud greenhouse hypothesis.

Dynamical and Biological Panspermia Constraints Within Multi-planet Exosystems.

As discoveries of multiple planets in the habitable zone of their parent star mount, developing analytical techniques to quantify extrasolar intra-system panspermia will become increasingly important. Here, we provide user-friendly prescriptions that describe the asteroid impact characteristics which would be necessary to transport life both inwards and outwards within these systems within a single framework. Our focus is on projectile generation and delivery and our expressions are algebraic, eliminating the need for the solution of differential equations. We derive a probability distribution function for life-bearing debris to reach a planetary orbit, and describe the survival of micro-organisms during planetary ejection, their journey through interplanetary space, and atmospheric entry.

Ablation of Venusian oxygen ions by unshocked solar wind.

As an Earth-like planet Venus probably had a primordial dipole field for several million years after formation of the planet. Since this dipole field eventually vanished the ionosphere of Venus has been exposed to the solar wind. The solar wind is shocked near Venus, and then scavenges the ionospheric particles through the magnetosheath and the magnetotail. The escape rate of oxygen ions (O+) estimated from spacecraft observations over the past several decades has manifested its importance for the evolution of planetary habitability, considering the accumulated effect over the history of Venus. However, all the previous observations were made in the shocked solar wind and/or inside the wake, though some simulations showed that unshocked solar wind can also ablate O+ ions. Here we report Venus Express observations of O+ ions in the unshocked solar wind during the solar minimum. The observations suggest that these O+ ions are accelerated by the unshocked solar wind through pickup processes. The estimated O+ escape rate, 2.1 × 1024 ions/s, is comparable to those measured in the shocked solar wind and the wake. This escape rate could result in about 2 cm global water loss over 4.5 billion years. Our results suggest that the atmospheric loss at unmagnetized planets is significantly underestimated by previous observations, and thus we can emphasize the importance of an Earth-like dipole for planetary habitability.

Terrestrial aftermath of the Moon-forming impact.

Much of the Earth's mantle was melted in the Moon-forming impact. Gases that were not partially soluble in the melt, such as water and CO2, formed a thick, deep atmosphere surrounding the post-impact Earth. This atmosphere was opaque to thermal radiation, allowing heat to escape to space only at the runaway greenhouse threshold of approximately 100 W m(-2). The duration of this runaway greenhouse stage was limited to approximately 10 Myr by the internal energy and tidal heating, ending with a partially crystalline uppermost mantle and a solid deep mantle. At this point, the crust was able to cool efficiently and solidified at the surface. After the condensation of the water ocean, approximately 100 bar of CO2 remained in the atmosphere, creating a solar-heated greenhouse, while the surface cooled to approximately 500 K. Almost all this CO2 had to be sequestered by subduction into the mantle by 3.8 Ga, when the geological record indicates the presence of life and hence a habitable environment. The deep CO2 sequestration into the mantle could be explained by a rapid subduction of the old oceanic crust, such that the top of the crust would remain cold and retain its CO2. Kinematically, these episodes would be required to have both fast subduction (and hence seafloor spreading) and old crust. Hadean oceanic crust that formed from hot mantle would have been thicker than modern crust, and therefore only old crust underlain by cool mantle lithosphere could subduct. Once subduction started, the basaltic crust would turn into dense eclogite, increasing the rate of subduction. The rapid subduction would stop when the young partially frozen crust from the rapidly spreading ridge entered the subduction zone.

Quantum Tunnelling to the Origin and Evolution of Life.

Quantum tunnelling is a phenomenon which becomes relevant at the nanoscale and below. It is a paradox from the classical point of view as it enables elementary particles and atoms to permeate an energetic barrier without the need for sufficient energy to overcome it. Tunnelling might seem to be an exotic process only important for special physical effects and applications such as the Tunnel Diode, Scanning Tunnelling Microscopy (electron tunnelling) or Near-field Optical Microscopy operating in photon tunnelling mode. However, this review demonstrates that tunnelling can do far more, being of vital importance for life: physical and chemical processes which are crucial in theories about the origin and evolution of life can be traced directly back to the effects of quantum tunnelling. These processes include the chemical evolution in stellar interiors and within the cold interstellar medium, prebiotic chemistry in the atmosphere and subsurface of planetary bodies, planetary habitability via insolation and geothermal heat as well as the function of biomolecular nanomachines. This review shows that quantum tunnelling has many highly important implications to the field of molecular and biological evolution, prebiotic chemistry and astrobiology.