In recent decades, much research has been conducted relating to the potential habitability of Mars. One angle this can take is trying to understand if certain hardy organisms could survive if transplanted from Earth to Mars. Research can focus on what vulnerabilities ultimately limit the survivability of a given organism or what challenges and environmental stressors the Martian environment may present to a colonising organism. This article introduces these topics and focuses mostly on lichens as promising test subjects when considering Martian habitability. Primary research is presented as well, which explores how samples of lichen cope when exposed to Mars analogue conditions in a Small Mars Chamber at The Open University. An evaluation of the samples is given, which concludes that some of the lichen, particularly the crustose form, display resistance to environmental stressors which are present on Mars.
Challenges to Life on Mars:
A fascination with life on Mars has permeated our cultures and steered related scientific research for at least the last century. However, around the turn of the new millennium, there was a shift in the expectation of the possibility of life on Mars. As the extremities of the Martian climate were explored and the prospect of finding life there dimmed, the question expanded – if it were not probable to find life on Mars, was it possible to put it there?
The Martian surface presents inhospitable conditions that only some extreme organisms could potentially tolerate. And although there are a few organisms that, in theory, could survive on Mars temporarily, long-term survival remains a challenge. Organisms that seem to have promising characteristics, such as desiccation resistance or radiation resistance, have been targeted for astrobiological research. These include, but are not limited to, certain forms of lichen (other potential candidates come in the form of tough halophytes and cyanobacteria, for example, Nostoc commune). Lichens were identified as potential candidates for survival beyond Earth’s atmosphere as early as the 1950s, when the lichen Chlorella pyrenoidosa was first sent into space alongside the space-faring dogs, Belka and Strelka, during an early Soviet experiment. Among the difficulties that life would face on Mars are high levels of radiation, due to the atmosphere’s thinness and low magnetism. The upper Martian atmosphere is also missing any substitute for Earth’s ozone layer or Van Allen belt, upon which life on Earth depends heavily to avoid irradiation. Supposing that a test organism was able to deal with high doses of UV and other radiation (for example, by possessing high concentrations of accessory pigments), there would still be the limitation that liquid water cannot exist on the surface of Mars. This is due to the low atmospheric pressure giving cause for the sublimation of any uninsulated ice. This would be very problematic for humans. Any organism that hoped to resist a dry death on Mars must have stalwart desiccation resistance. This could mean the ability to retain water despite the Martian climate or to survive despite losing water to it.
Mars’ dryness dates back to the end of the Noachian period (3.7 billion years ago), before which, it is theorized that there was an abundance of water on at least the northern hemisphere. In the most recent Martian geological period, the Amazonian, there may have been fleeting periods of dampness. However, the resulting moisture would have been too short-lived for life to evolve on such a timescale. These periods of dampness are thought to come about when Mars tilts into a position where one of its poles better faces the sun. Subsequently, polar ice would melt or sublimate and the new atmospheric moisture would condense around the mid-latitude regions, dampening the regolith.
Another problem that any colonizer from Earth would have with survival on Mars, would be the gaseous composition of the atmosphere itself. Aside from having essentially no water vapour, the atmosphere is primarily composed of CO2 (carbon dioxide) and lacks the stable and unreactive N2 (nitrogen) component to which life on Earth is accustomed. One main advantage that N2 offers to life on Earth is that it is an inert, unreactive gas that builds atmospheric pressure up to 101 Kilopascals (Kpa).
Further challenges to life on Mars include the composition of the regolith and the challenges involved with transporting living samples from Earth to Mars. Regarding the regolith, a very complex set of issues arises concerning the availability of nutrients and the water retention of certain regolith components. Regarding the issue of transporting living samples to Mars, the research is more positive. In large organisms such as humans there are complex systems that can be interrupted or damaged by takeoff, spaceflight and landing. In contrast, organisms like lichens are not at the same risk from transportation from one planet to another. For instance, they are not perturbed by weightlessness and are able to live well in zero gravity. Furthermore, plants of various kinds have been grown aboard the International Space Station (ISS) for years, mostly with success. Another aspect of the journey to Mars that lichens should be untroubled by is the landing. A series of experiments researching the panspermia hypothesis (the theory that life on Earth originated elsewhere in the universe and was transported here by extraterrestrial material, such as meteorites) have shown that simple, small organisms are often not fatally damaged by G-forces. What can kill them is instantaneous heat damage at the point of impact. This means that if protected properly, there may be very little risk to organisms such as lichens during transit to Mars.
1. Recent Research (Xanthoria elegans):
One informative publication relating to astrobiological research on lichen involved the long-term Lichen and Fungi Experiment (LIFE). The experiment was conducted on cultures of the algal photobiont of the lichen Xanthoria elegans. The researchers sought to characterize the extent of the ultrastructural damage suffered as a result of long term space exposure (1.5 years) by means of transmission electron microscopy and measuring post-flight photosynthetic activity. Another aim of the experiment was to subject Xanthoria elegans to Martian analogue conditions. This experiment covered a lot of ground, as it tested the survivability of Xanthoria elegans in two environments it could face if a sample were ever to be sent to Mars, both Low Earth Orbit and Mars itself. The details and exact parameters of the experiment are too extensive to list here. The results of the Mars analogue tests concluded that between 50-80% and 60-90% of the photobiont (photosynthetic cells) and mycobiont (fungal cells) respectively were viable after exposure.
Further reading into this and experiments like it revealed a common theme. Most of the experiments involving the survivability of lichens on Mars appear to be primarily concerned with re-cultivation after exposure and not measuring photosynthetic rate or other biological functions while under Martian analogue conditions. This gap could be due to the general understanding that the Martian atmosphere is too hostile for any lifeform from Earth achieving a photosynthetic rate that would be measurable under those conditions. This could be compounded by the fact that to quantify metabolic rates, while any experimental apparatus was also under Martian analogue conditions represents a technical challenge.
2. Recent Research (The Regolith):
A most basic prerequisite to life is H2O, especially liquid water. As mentioned previously, at one point in time much of the Martian surface was exposed to liquid water. Evidence for this includes still visible structures that appear to have needed water (liquid and/or ice) to occur. For example, inverted fluvial channels and eskers. This idea of water having an important geomorphological effect on Mars was also brought up in a public lecture by Butcher (2017) and a paper by (Balme, et al., 2017). Looking at similar environments and structures on Earth, it is possible to narrow down the types of test organisms with the best chances of survival in corresponding areas on Mars. One example is the lichen Pleopsidium chlorophanum, which was selected for experimentation due to its low-water habitat, here on Earth.
As referred to in the previous section of this report, the regolith on Mars is a survival limitation in itself. Although most of the basic prerequisite compounds for life are there in one form or another, they are often in places or forms whereby organisms would find it impossible to utilize properly. However, one promising sign, pointing to the possible survivability of Earth-originating organisms on Mars, is that there are salts in the regolith which could hold water in their crystals. Experiments that have been set up to investigate this aspect generally examine the sorption rates of different soil components. The soil components are selected for their respective suitability as Martian regolith analogues. For example, the smectite Ca-montmorillonite (a silica mineral, common in the relatively dry environment of Texas) is thought to be typical of the type of smectite that would have been present in the early Noachian period of the Martian timeline. In such experiments, organisms that have a hygroscopic property (the tendency to absorb water from the atmosphere), as well as features that display resistance to the Martian atmosphere, are used to investigate if a water exchange can be set up between the organism and the surrounding halites (the mineral form of a salt) and/or smectites. One such organism that has been used in these experiments is the cyanobacterium Nostoc commune. It was chosen because it is hygroscopic even under low temperatures and its extracellular polysaccharide (EPS) biofilm provides an extra defence against the harsh Martian environment. This gives Nostoc commune its characteristic radiation and desiccation resistance. If experiments like this one turn out promising results, the possibility of sustaining life beneath or within the Martian regolith will become increasingly realistic.
3. Primary Experimentation:
To support conclusions drawn from the wider reading for this project, a short and simple experiment was conducted to demonstrate the survivability of lichen in Mars analogue conditions. The experiment took place in one of the Small Mars Chambers at The Open University in Milton Keynes. The objective was to expose the samples to Mars analogue conditions for 4 hours to characterise the effects of short-term exposure and compare between the different organisms in the samples. The conditions chosen were a pressure of 6.00 millibar (mbr) and a temperature of -45°C. For this experiment, it was not possible to recreate the radiation of the surface of Mars due to technical limitations and safety concerns. The exposure conditions were chosen because of their similarity to conditions in certain locations on Mars. The samples were all growing on an iron substrate (a rail clamp from a disused railway) and included many patches of crustose lichen, a colony of Cladonia (1cm x 1.5cm x 1.5cm) and one of an unidentified wet moss (4cm x 2cm x 1cm). The range of different lichens and the moss would mean that the effects of the Mars analogue conditions could be demonstrated on organisms of differing levels of resistance. The fact that the samples were growing on an iron substrate was apt as it emulated the iron compound rich regolith of Mars. Under the chosen conditions, it was predicted that the crustose lichen would fare the best, with minor or no damage, the Cladonia would survive, but be slightly compromised by its more delicate structures and the wet moss would fare the worst due to its dependence on much more water than the lichens.
Figure 1: (left) Crustose lichen on iron substrate and Figure 2: (right) Cladonia and moss growing on iron substrate
The samples were placed in the chamber and the chamber was evacuated until the target pressure was achieved, this took less than 30 seconds. Over 40 minutes the target temperature of -45°C was achieved by mounting the Small Mars Chamber into a Climatic Test Chamber. The samples were then left in this condition for 4 hours.
After the 4 hours of exposure, the samples could be returned to room temperature. This function of the Climatic Test Chamber was quicker, at approximately 10 minutes. After this, the Small Mars Chamber could be repressurised, which took a further 10 minutes. The warming and repressurising were done in this way to avoid the samples being disturbed by air rushing in during a rapid repressurisation or from condensation from any frozen moisture. Any extra disturbance to the samples at this stage would make it more difficult to examine the direct effects of the exposure to Mars analogue conditions.
Once the samples were fully retrieved, they could be examined in greater detail, under a microscope and compared with pre-exposure observations. Results from this experiment could also be evaluated in the context of results of experiments found during secondary research for this project.
Figure 3 (above, left): Cladonia and moss samples, pre-exposure, Figure 4 (above, middle): Cladonia sample, post-exposure and Figure 5 (above, right): Dehydrated moss sample, post-exposure.
Figure 6 (above, left): Micrograph of Cladonia apothecium, pre-exposure. Figure 7 (above, right): Micrograph of Cladonia apothecium, post-exposure.
Figure 8 (above, left): Cladonia thallus, pre-exposure. Figure 9 (above, right): Cladonia thallus, post-exposure.
Figure 10 (above, left): Crustose lichen, pre-exposure. Figure 11 (above, right): Crustose lichen, post-exposure.
Figure 12 (above, left): Moss sporophyte, pre-exposure. Figure 13 (above, right): Moss sporophyte, post-exposure.
When comparing Figures 3 and 4 the structural resistance of the Cladonia can be seen. Between Figures 3 and 4, the only visible perturbations to the structure are some dehydration and discolouration. In figure 5 we can see that the moss sample dried out more substantially and greater discolouration can be seen. Given a longer exposure time, it would seem reasonable that the Cladonia would out-perform the moss even further. In Figures 6 and 7 the discolouration of the Cladonia structure can be more clearly seen. Prior to exposure, the apothecia of the Cladonia were a healthy green, contrasting the image of a comparable Apothecium post-exposure (exactly the same apothecium could not be used for observation both pre- and post-exposure, as it had to be removed for mounting on the microscope slide). The discolouration may be caused by the freezing of the sample disturbing the surface of the structure. Figures 8 and 9 show comparable levels of damage to parts of the main thallus of the Cladonia to that of its apothecia. It might have been expected that the apothecia would fare worse, being the more delicate structures, but this was not evident in these Figures. It is possible that given longer exposure time, differentials in coping could become visible across different parts of the Cladonia. Figures 10 and 11 confirm what was suggested in much of the reading for this project, that crustose lichens possess the best morphology to cope with exposure to Mars analogue conditions. Between the two Figures, there is no apparent disturbance to the crustose lichen. Figures 12 and 13 show that the more watery sporophytes of the moss sample were not able to cope with the exposure, to the point that some of the structures were destroyed.
Most striking, was that the crustose and Cladonia samples remained relatively unperturbed despite exposure and that the predictions of which organisms would fare best were shown to be accurate. These results also lined up with what was found in the wider reading for this project. The ability of the lichen samples to survive, post-exposure, despite being completely dehydrated and frozen supports the idea that they could be best suited to survive despite exposure to some of the conditions found on Mars. One limitation that the experiment for this article had, compared to more advanced experiments, was an inability to add a radioactive component. An important part of simulating Martian conditions is replicating the surface radiation. By omitting radiation for practical reasons, there can be some doubt about the quality of the results of this article’s experiment, as it is probable that radiation would interact with other environmental stressors to produce different results. The Martian environment presents a series of hostile conditions and creates multiple difficulties for any colonial organisms. However, by understanding the limitations of potential pioneer organisms, colonisation of Mars becomes a more realistic prospect. Some organisms mentioned in this article, such as the cyanobacterium Nostoc commune and the lichen Xanthoria elegans are examples of organisms which could theoretically survive in certain conditions on Mars, although this is far from being tested on Mars. Supporting this understanding are the results of experiments which show the resistance of some lichen to Mars analogue conditions, as seen in the experimentation for this project.
It can be concluded that candidate organisms for survival on Mars would have to resist desiccation, irradiation and depressurisation. Those requirements cast a favourable light on some lichen due to their suitability to low water environments, their pigmentation and general toughness. Lichen will also be mostly unaffected by other alien conditions on Mars, as they would not be perturbed by the diminished gravity and abundant CO2 in the atmosphere and low temperatures. In the future, it is likely that automated or manned missions to Mars will begin to test the survivability of organisms in the Martian environment itself.
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About the Author
Callum Fraser, from Milton Keynes, recently graduated from St. Paul\’s Catholic School with A-levels in Maths, Biology, Chemistry, Creative Writing and an A* Extended Project Qualification (EPQ). He has since accepted an offer to study Biology at Bangor University, with the long-term dream of diverting into Astrobiology, working for a space agency and one day going to Mars.