15 research outputs found
Computer modeling and experimental work on the astrobiological implications of the martian subsurface ionising radiation environment
Any microbial life extant in the top meters of the martian subsurface is likely to be held dormant
for long periods of time by the current permafrost conditions. In this potential habitable zone,
a major environmental hazard is the ionising radiation field generated by the flux of exogenous
energetic particles: solar energetic protons and galactic cosmic rays. The research reported
here constitutes the first multidisciplinary approach to assessing the astrobiological impact of
this radiation on Mars.
A sophisticated computer model has been constructed de novo to characterise this complex
subsurface ionising radiation field and explore the influence of variation in crucial parameters
such as atmospheric density, surface composition, and primary radiation spectra. Microbiological
work has been conducted to isolate novel cold-tolerant bacterial strains from the Dry
Valleys environment of Antarctica, an analogue site to the martian surface, and determine
their phylogenetic diversity and survival under high-dose gamma-ray exposure frozen at -79!C,
a temperature characteristic of the martian mid-latitude permafrost.
Original results are presented pertinent to microbial survival time, persistence of organic biomarkers,
and calibration of the optically stimulated luminescence dating technique, as a function of
depth. The model predicts a population of radiation resistant cells to survive in martian permafrost
soil for 450,000 years at 2 m depth, the proposed drill length of the ExoMars rover.
The Antarctic culturing studies identified representatives of four bacterial genera. The novel
isolate Brevundimonas sp. MV.7 is found to show 99% 16S sequence similarity to cells discovered
in NASA spacecraft assembly clean rooms, with the experimental irradiation determining
this strain to suffer 10-6 population inactivation after a radiation dose of 7.5 kGy in martian
permafrost conditions. Integrating the modelling and experimental irradiation, this research
finds a contaminant population of such cells deposited just beneath the martian surface would
survive the ambient cosmic radiation field for 117,000 years
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Constraints on a potential aerial biosphere on Venus: II. Ultraviolet radiation
Despite the harsh conditions in the atmosphere of Venus, the possibility for an aerial habitable zone exists. A thermal habitable zone is predicted to exist at an altitude range of 62 to 48âŻkm, above which temperatures drop below the lower thermal limit of cell growth and below which temperatures exceed the evaporation temperature. Many biocidal factors must be considered for the complete definition of an aerial habitable zone; in this study we consider the constraint specifically from the perspective of biocidal solar ultraviolet (UV) intensity in the atmosphere. We simulated the penetration of solar ultraviolet and visible light through the atmosphere using a radiative transfer model, to determine the spectral environment (and thus the UV biocidal effect) as a function of altitude in the atmosphere of Venus. At the top of the thermal aerial habitable zone (62âŻkm) the incoming solar irradiance creates a severely challenging UV environment, with extremophiles such as Deinococcus radiodurans expected to be able to endure these UV conditions for approximately 80âŻs. At an altitude of around 59âŻkm the biologically-weighted UV irradiance drops below that calculated for the Archean Earth, and continues to fall with decreasing altitude until at 54âŻkm it is less than that found currently at the surface of Earth. Crucially, longer wavelength photosynthetically active light continues to penetrate to these altitudes and below, resulting in a solar radiation environment in the venusian atmosphere below around 54âŻkm that screens biologically-damaging UV radiation yet permits the process of photosynthesis. Whilst not claiming to suggest the existence of an aerial habitable zone in general, by considering thermal conditions, ionising radiation and the UV flux environment of the venusian cloud deck alone, we define a potential habitable zone that extends from 59âŻkm to 48âŻkm. This region should form the focus of future remote and in situ astrobiological investigations of Venus
Ionization of the Venusian atmosphere from solar and galactic cosmic rays
The atmospheres of the terrestrial planets are exposed to solar and galactic cosmic rays, the most energetic of which are capable of affecting deep atmospheric layers through extensive nuclear and electromagnetic particle cascades. In the Venusian atmosphere, cosmic rays are expected to be the dominant ionization source below âŒ100 km altitude. While previous studies have considered the effect of cosmic ray ionization using approximate transport methods, we have for the first time performed full 3D Monte Carlo modelling of cosmic ray interaction with the Venusian atmosphere, including the contribution of high-Z cosmic ray ions (Z=1-28). Our predictions are similar to those of previous studies at the ionization peak near 63 km altitude, but are significantly different to these both above and below this altitude. The rate of atmospheric ionization is a fundamental atmospheric property and the results of this study have wide-reaching applications in topics including atmospheric electrical processes, cloud microphysics and atmospheric chemistry
Transitory Microbial Habitat in the Hyperarid Atacama Desert
Traces of life are nearly ubiquitous on Earth. However, a central unresolved question is whether these traces always indicate an active microbial community or whether, in extreme environments, such as hyperarid deserts, they instead reflect just dormant or dead cells. Although microbial biomass and diversity decrease with increasing aridity in the Atacama Desert, we provide multiple lines of evidence for the presence of an at times metabolically active, microbial community in one of the driest places on Earth. We base this observation on four major lines of evidence: a physico-chemical characterization of the soil habitability after an exceptional rain event, identified biomolecules indicative of potentially active cells [e.g., presence of ATP, phospholipid fatty acids (PLFAs), metabolites, and enzymatic activity], measurements of in situ replication rates of genomes of uncultivated bacteria reconstructed from selected samples, and microbial community patterns specific to soil parameters and depths. We infer that the microbial populations have undergone selection and adaptation in response to their specific soil microenvironment and in particular to the degree of aridity. Collectively, our results highlight that even the hyperarid Atacama Desert can provide a habitable environment for microorganisms that allows them to become metabolically active following an episodic increase in moisture and that once it decreases, so does the activity of the microbiota. These results have implications for the prospect of life on other planets such as Mars, which has transitioned from an earlier wetter environment to today's extreme hyperaridity. [Abstract copyright: Copyright © 2018 the Author(s). Published by PNAS.
What is the Oxygen Isotope Composition of Venus? The Scientific Case for Sample Return from Earthâs âSisterâ Planet
Venus is Earthâs closest planetary neighbour and both bodies are of similar size and mass. As a consequence, Venus is often described as Earthâs sister planet. But the two worlds have followed very different evolutionary paths, with Earth having benign surface conditions, whereas Venus has a surface temperature of 464 °C and a surface pressure of 92 bar. These inhospitable surface conditions may partially explain why there has been such a dearth of space missions to Venus in recent years.The oxygen isotope composition of Venus is currently unknown. However, this single measurement (Î17O) would have first order implications for our understanding of how large terrestrial planets are built. Recent isotopic studies indicate that the Solar System is bimodal in composition, divided into a carbonaceous chondrite (CC) group and a non-carbonaceous (NC) group. The CC group probably originated in the outer Solar System and the NC group in the inner Solar System. Venus comprises 41% by mass of the inner Solar System compared to 50% for Earth and only 5% for Mars. Models for building large terrestrial planets, such as Earth and Venus, would be significantly improved by a determination of the Î17O composition of a returned sample from Venus. This measurement would help constrain the extent of early inner Solar System isotopic homogenisation and help to identify whether the feeding zones of the terrestrial planets were narrow or wide.Determining the Î17O composition of Venus would also have significant implications for our understanding of how the Moon formed. Recent lunar formation models invoke a high energy impact between the proto-Earth and an inner Solar System-derived impactor body, Theia. The close isotopic similarity between the Earth and Moon is explained by these models as being a consequence of high-temperature, post-impact mixing. However, if Earth and Venus proved to be isotopic clones with respect to Î17O, this would favour the classic, lower energy, giant impact scenario.We review the surface geology of Venus with the aim of identifying potential terrains that could be targeted by a robotic sample return mission. While the potentially ancient tessera terrains would be of great scientific interest, the need to minimise the influence of venusian weathering favours the sampling of young basaltic plains. In terms of a nominal sample mass, 10 g would be sufficient to undertake a full range of geochemical, isotopic and dating studies. However, it is important that additional material is collected as a legacy sample. As a consequence, a returned sample mass of at least 100 g should be recovered.Two scenarios for robotic sample return missions from Venus are presented, based on previous mission proposals. The most cost effective approach involves a âGrab and Goâ strategy, either using a lander and separate orbiter, or possibly just a stand-alone lander. Sample return could also be achieved as part of a more ambitious, extended mission to study the venusian atmosphere. In both scenarios it is critical to obtain a surface atmospheric sample to define the extent of atmosphere-lithosphere oxygen isotopic disequilibrium. Surface sampling would be carried out by multiple techniques (drill, scoop, âvacuum-cleanerâ device) to ensure success. Surface operations would take no longer than one hour.Analysis of returned samples would provide a firm basis for assessing similarities and differences between the evolution of Venus, Earth, Mars and smaller bodies such as Vesta. The Solar System provides an important case study in how two almost identical bodies, Earth and Venus, could have had such a divergent evolution. Finally, Venus, with its runaway greenhouse atmosphere, may provide data relevant to the understanding of similar less extreme processes on Earth. Venus is Earthâs planetary twin and deserves to be better studied and understood. In a wider context, analysis of returned samples from Venus would provide data relevant to the study of exoplanetary systems