Following the water in dry permafrost
By: Catherine Maggiori
Synopsis: In a new article in Astrobiology, authors Mellon et al. discuss the potential habitability of dry permafrost on Mars based on water activity in similar terrestrial analogue samples and models.
Author info: Dr. Catherine Maggiori is an astrobiologist and microbiologist. You can find her at the bench, attempting to set up a Bluesky account, or at the climbing gym.
Article: Humidity Enhancement in Dry Permafrost: The Effects of Temperature Cycles on Habitability
Exploring Mars’ history and habitability has involved numerous mapping missions, probes, and landers that searched for signs of past or present water. Water is an essential requirement for all life as we know it and has driven habitability studies on the Red Planet since at least the early 1990s with the Mars Exploration Program.
This “Water Strategy”, also nicknamed “follow the water”, has steered Martian exploration towards studying and understanding water-related processes on the Red Planet in order to better constrain habitable areas and time periods. There is plenty of evidence that water once flowed on Mars (e.g. river delta features, water ripples, inverted river channels) and may still exist today as subsurface water ice or even a liquid water ocean.
The amount of free water that is available for life in a substance is also known as “water activity” (aw); more specifically, water activity is the ratio of water vapour pressure in a substance to the vapour pressure of pure water. It can range from 0 (absolutely no water present) to 1 (pure water). Some organisms can even use non-liquid water like adsorbed water and water vapour to sustain themselves. The most extreme terrestrial life has a water activity requirement of at least 0.5 - 0.6 for any kind of cell growth or proliferation, indicating that any putative life outside of Earth may share this minimum requirement.
There are lots of Mars analogue environments on Earth (e.g. lava tubes, Atacama Desert, Lake Vida), including dry permafrost, a layer of frozen, relatively ice-free soil. It does, however, contain some water adsorbed on soil particles and/or as water vapour. The Martian surface is overlain with dry permafrost, too; this abundance and potential for water activity could make Martian dry permafrost a potentially habitable environment.
Permafrost on Mars (above) from NASA’s Phoenix lander vs. polygon-wedge permafrost terrain in Svalbard (below). Image from Earth Observatory.
A recent paper in Astrobiology, “Humidity Enhancement in Dry Permafrost: The Effects of Temperature Cycles on Habitability” from Mellon et al., tries to model the prospective habitability of dry permafrost on Mars in terms of water activity in analogue soils. It’s been previously proposed that daily cycles in soil temperature could cause water vapour to accumulate within soil pores and increase the water activity of the surrounding environment. As soil warms up, water desorbs from grain surfaces and the pore space vapour pressure increases, causing heat conduction and vapour diffusion through the soil particles, briefly raising the relative humidity (RH) and water activity. If these cycles can cause the aw to rise above a minimum of 0.6, habitable conditions could occur periodically and support active microbial metabolisms.
Mellon et al. examined changes in RH and water activity in dry permafrost and other soils as temperatures increase. Above freezing, RH and aw are effectively synonymous (RH = aw * 100) and below freezing, they differ as outlined in Fig. 1.
Fig. 1 from Mellon et al. depicting RH vs aw. The dashed line represents the limit of habitability in terms of aw (0.6).
Using 6 different soil types (including one natural dry permafrost from Antarctica), Mellon et al. observed the responses to changing temperatures, with a starting RH of ~55% RH and room temperatures (~20 - 21 °C). Heat was applied for 20 minutes at 50 °C, then turned off and the soil was allowed to cool for 30 minutes.
In general, soils with smaller grain sizes heated more quickly to higher temperatures (likely as a result of more efficient thermal transfer between smaller particles) and the increase in RH after heating was more pronounced closer to the heating source. Fig. 6 shows 4 of the 6 soil types (1 mm beads, Antarctic dry permafrost aka Birch Hill loess, montmorillonite clay, and a bead-clay mix) and a general trend of increasing RH with temperature, followed by a slow decline.
Fig. 6 from Mellon et al. RH vs time for 1 mm beads, Antarctic dry permafrost aka Birch Hill loess, montmorillonite clay, and a bead-clay mix and at distances from the heater of 0.4, 1.4, 2.4, and 3.4 cm, respectively (panels a - d).
These laboratory experiments were performed above 0 °C and indicate that, in general, an increase in soil temperature can raise the soil’s RH and thus water activity into a habitable range (at least briefly). But what about at more accurate Martian environmental conditions (i.e. below 0 °C, 6 mb atm. pressure)?
Based on these experiments, Mellon et al. developed models indicating that for dry permafrost in Martian surface conditions, the rate of heating would be ~40 - 60x less than that observed in their laboratory experiments and result in negligible RH and aw changes. However, if the soil surface is initially shadowed (such as behind a boulder or scarp), the more abrupt heating it receives when it enters direct sunlight could result in a higher soil heating rate and thus reach a water activity value of 0.6. This will be especially prominent when surface temperatures are above -40 °C. The current Martian climate may still be too dry for this periodic surface heating to result in dry permafrost habitability, but it would be more likely to occur at periods of high obliquity, which have occurred in Mars’ past and will occur again.