Christopher P. McKay

Our concept of where life can thrive on Earth has advanced over the past 70 years to include extreme ionizing radiation, high temperatures, the deep subsurface, hydrothermal vents on the deep ocean floor, extreme arid deserts, and the ice-covered lakes and high mountain valleys of Antarctica. This expanding understanding of the biosphere has coincided with the development of space exploration programs, and it has informed those programs with regard to the search for life on other water worlds in our Solar System—especially Mars, Europa, and Enceladus. Titan presents a reverse of this approach. The interesting organic solids and fluids on that world have no analog in Earth habitability but have inspired suggestions of possible biological systems unlike any on Earth. If realized, the discovery of life on Titan would stretch the concept of habitability just as it stretches the concept of life as we know it. Habitability studies on exoplanets may follow both of these paths: we will look for habitability on exoplanets based on observed habitats on Earth, and we will also use observations of exoplanets as grist for contemplation of lifestyles different from anything we know on Earth.

Main conclusions: Contrary to previous assumptions, dew and frost cannot be regarded as water sources for lithobionts in the McMurdo Dry Valleys (MDV) of Antarctica, which therefore solely rely on snowmelt water for growth. Therefore, and contrary to previous claims, MDV can be regarded as a good analogue for life on Mars. Rock-dwelling chlorolichens and cyanobacteria provide most of the total biomass of the ice-free zone of Antarctica, i.e., the McMurdo Dry Valleys (MDV), and yet, the water sources of MDV are not clear. In addition to snowmelt water that provides water to the lichen-dominated cryptoendolithic communities, many scholars also advocate the use of dew or frost as important water sources for chasmoendolithic cyanobacteria. The implications of these suggestions are large, especially due to the fact that the MDV serves as an important analogue for life on Mars. Based on 5-year long analysis of the four growing months (November, December, January, February) in three stations, we show that the likelihood of both sources to take place is low. Rock temperatures as measured by one of the stations (Marble Point) allowed us to also perform a detailed analysis. Rock temperatures never reached the dewpoint temperature, excluding the formation of dew. As for frost, the likelihood is extremely low during the growing season and an optimistic evaluation yields a possible occurrence of frost for 0.8 h per year. We, therefore, conclude that neither dew nor frost may serve as a meaningful water source for the chasmoendolithic cyanobacteria, which, therefore, rely on snowmelt water only. The endolithic communities of the MDV may be justifiably regarded as the best analogue for life on Mars.

Dry permafrost underlain by ice-cemented permafrost has been reported in several locations in Antarctica. Initially thought to be relic ice, it is now understood that this subsurface ice is in equilibrium with the surface conditions, although it is not in equilibrium with the atmosphere. We use year-round data from University Valley in the Dry Valleys and Elephant Head in the Ellsworth Mountains to investigate the seasonal variations in water vapour flux that control the depth to the ice table under dry permafrost. Our analysis shows that the mean annual water vapour density of the soil surface exceeds the atmospheric value by a factor of up to ~2 due to summer snow. The attenuation and phase shift of the annual temperature cycle with depth result in colder temperatures at the ice table than at the surface of the soil in summer. We conclude that this temperature gradient, combined with the summer snow, provides the flux of water to the ice table necessary to maintain the ice. In University Valley, reducing the snow days by 40% moves the stability depth of the ice table from 42 to 66 cm. Increasing the snow days by 60% shifts the ice table to 17 cm. These variations can explain the observed gradient in the depth to the ice table in University Valley.

Many of the recently discovered Earth-like exoplanets are hosted by M and F stars, stars that emit intense UVC, especially during a flare. We studied whether such planets are nevertheless habitable by irradiating a desert lichen, Clavascidium lacinulatum, with 254-nm 55 W/m ² UVC nonstop for 3 months in the laboratory. Only 50% of its algal photobiont cells were inactivated. To put this in perspective, we used the same setup to challenge the photobiont cells but grown in pure culture, and Deinococcus radiodurans , the most radiation-resistant bacterium on Earth. Entire monolayers of hundreds of cells were inactivated in just 60 s. Further studies indicated that the cortex of the lichen was rendered UVC-opaque by deposits of phenolic secondary metabolites in its interstices. The lichen was injured only because, while most photochemical reactive oxygen species were quenched, photochemical ozone was not. We conclude that UVC-intense exoplanets are not necessarily uninhabitable to photosynthetic organisms.

The search for life in the solar system often focuses on water and on environments where habitable conditions exist, persistently or occasionally. In this search, dry permafrost (ice-free frozen soil) has received minimal attention. It was previously proposed that within martian dry permafrost the water activity ( a w , an essential property for habitability) could be enhanced by diurnal thermal cycles and water desorption from soil grains, but the details remain unexplored. We examined a w in dry soil (which contained only vapor and adsorbed water) through experiments and numerical simulations and contrasted the results with a habitability threshold for terrestrial organisms ( a w     >     0.6 ). We found that heating cycles in a soil raised a w . As water vapor desorbs from warming soil grains, it diffuses toward cooler adjacent soil, where a fraction of this incoming vapor enhances the local a w . In laboratory tests with loess and clay soils, we observed a w to increase by 0.06–0.12. Extrapolating from laboratory to permafrost conditions by using numerical simulations, we found that some Antarctic soils can be boosted periodically into a habitable range. In contrast, the current martian climate is too dry or cold for this a w -enhancement process to impact habitability. However, high-obliquity periods on Mars are analogous to the Antarctic case.

The notion of liquid water beneath the ice layer at the south polar layered deposits (SPLD) of Mars is an interesting possibility given the implications for astrobiology and possible human habitation. A body of liquid water located at a depth of 1.5 km has been inferred from radar data in the South Polar Cap. However, the high temperatures that would facilitate the existence of liquid water or brine at that depth are not consistent with estimations of heat flow that are based on the lithosphere’s flexure. Attempts to reconcile both issues have been inconclusive or otherwise unsuccessful. Here, we analyze the possible role(s) of subsurface ammonia and/or methanol in maintaining water in a liquid state at subsurface temperatures that are compatible with the lithosphere strength. Our results indicate that the presence of these compounds at the base of the SPLD can reconcile the existence of liquid water with previous estimations of surface heat flow.

We report on the pore size distribution and water retention of Beacon sandstone from Antarctica that harbours a cryptoendolithic community, predominantly lichens, just below the surface. We measured the pore size distribution, drying time and equilibrium relative humidity of sandstone samples that were colonized by lichens. The incremental pore volume distribution peaks at ~10 μm radius, but ~20% of the pore volume occurs for a radius < 0.02 μm. Water from snowmelt fills ~20% of the total pore volume. It takes ~4–5 days of evaporation to lose 90% of the water. As the rock loses water, the equilibrium relative humidity remains at 99% until an appreciable amount (80–90%) of the pore water is lost, after which the equilibrium relative humidity starts to decrease. The relative humidity remains at > 80% (68 h) long after the water content falls to < 98% (19 h) - the point at which liquid water is estimated no longer to be present. Lichens can remain active in air with high relative humidity (> 80%). Thus, the pore size distribution of the sandstone may play a role in explaining why lichens dominate these sandstones.