Review of the MEPAG Report on Mars Special Regions (2015)

2

The Assessment of the Potential of Terrestrial Lifeforms to Survive and Proliferate on Mars in the Next 500 Years

Two of the three factors appearing in COSPAR’s definition of a Special Region relate to physical variables—that is, the lower limits of water activity and temperature, and the third factor is a timescale. While the bulk of this chapter is concerned with physical variables and the role they play in determining whether or not terrestrial life might proliferate on Mars, it is instructive to consider the timescale first.

The rationale for choosing a timescale is related to our ability, or rather our inability, to predict what the environmental conditions on Mars might be at an arbitrary date in the future. The primary drivers of climatic change on Mars are oscillations in the planet’s orbital parameters such as obliquity and eccentricity. But numerical integrations of dynamical models of the solar system are dominated by non-linear effects at large times, and so predictability is lost. Model calculations quoted in the SR-SAG2 report reveal that changes in key orbital parameters are small over a period of 500 years. Thermal models of Mars referenced in the SR-SAG2 report reveal that the changing orbital parameters are not expected to change the mean martian surface temperature by more than 0.2 K over the next 500 years (Finding 1-1).

FACTORS INFLUENCING THE PHYSICAL AND CHEMICAL LIMITS OF LIFE

The Mars-relevant physical and chemical limits of life (as we know it) summarized and discussed in the SR-SAG2 report focus on the following:

• The presence of chemical compounds that can be used by microbes¹ as a source of carbon, energy, and nutrients;
• The lower temperature limit for cell division;
• The lower temperature limit for metabolic activities;
• The potential decrease of the lower temperature limit in the presence of chaotropic compounds;
• The lower limit of water activity for cell division versus metabolic activities;
• The effects of atmospheric composition and pressure;
• The effects of ultraviolet and ionizing radiation; and
• The combined effects of environmental stressors.

________________

¹ The term “microbe” is used throughout this report as a generic term denoting any prokaryotic or eukaryotic single-cell organisms.

In addition, the potential occurrence of small-scale habitats—those not detectable with existing and planned space instruments, especially in the subsurface—were also addressed in the SR-SAG2 report. In general, the review committee agrees with most of the findings and the conclusions in the SR-SAG2 report. However, the review committee believes that some important aspects of Special Regions were not discussed by SR-SAG2. In particular, the issues of translocation of terrestrial contamination and the behavior of multispecies populations in extreme environments, produce uncertainty in the determination of Special Regions, because such regions might not be isolated from the rest of the planet (translocation), because microbial communities could occupy dispersed, small-scale habitats or might be able to alter (e.g., through the synthesis of extrapolymeric substances and syntrophic consortial interactions) local environmental parameters and syntrophic consortial interactions. These issues, together with the present lack of knowledge about the limits of life on Earth and the uncertainty of the relationship between the large-scale and micro-scale environments at any given place make the definition of Special Regions difficult. The sections below expand on these topics and propose research topics that will help make the definition of Special Regions more effective.

INVESTIGATIONS OF THE LIMITS OF LIFE ON EARTH

The availability of powerful new techniques for the investigation of cellular and molecular processes has resulted in an enormous increase in knowledge about Earth’s biodiversity and, in particular, the ability of only certain organisms to live in extreme environments. However, despite these advances, researchers still have a limited understanding of how microorganisms survive and replicate under extreme conditions. Laboratory studies allow the reproducible exposure of cultivable organisms to standardized, controlled conditions. However, in the past, most such studies were performed with single species, very often with type strains from culture collections and not with isolates obtained directly from extreme environments. Type strains sometimes lose their natural stress resistance because of their repeated cultivation under optimal conditions in the laboratory. In particular, microorganisms living in cold environments—such as psychrophiles and psychrotolerants—are not well understood. One reason for this is that the generally long replication time of this group of organisms requires long-term laboratory investigations extending over several years.

Contrary to laboratory studies with single species, field studies focus on in situ investigation of natural communities. Observations from extreme habitats on Earth provide insight into some of the survival and adaptation mechanisms of communities of organisms at specific periods of time (e.g., diurnal, seasonal, annual) and at different spatial scales. Quite often these investigations cannot be repeated because of the dynamic nature of indigenous microbial communities, their environmental setting, and the interactions that occur between microbes and the environments in which they live. The derivation of generalizations from such studies is challenging. Therefore, the review committee recognizes the need for scientific investigations that deepen our knowledge about the limits of life with a focus on survivability, adaptation, and evolution under martian conditions. The most important conditions are the temperature limits and the bioavailability of water, in particular, the potential utilization of atmospheric water vapor as sole source for water has not been proven, even if some observations suggest it (Azúa-Bustos et al. 2015; Jacobsen et al. 2015). Another important consideration concerns the ability of so-called chaotropic compounds to lower the temperature limit for cell division. As a result, the committee proposes to add text to SR-SAG2’s Finding 3-1 (shown in italics):

Suggestions for future research directions relating to the issues discussed in this section can be found in Appendix A.

LIFE IN EXTREME ENVIRONMENTS AND IN MULTISPECIES COMMUNITIES

The SR-SAG2 report identified the ability of microorganisms to withstand multiple stressors as an important area of research. Extreme ecosystems on Earth are often subjected to a multitude of conditions considered to push the limits of microbial life. For instance, surfaces of ice shelves in both the Arctic and Antarctic harbor conditions that combine multiple physiological stresses imposed on microorganisms, such as low temperatures, high levels of ultraviolet radiation, and several-fold annual variations in salinity (Mueller et al. 2005). Permafrost environments are subjected to long-term exposure to sub-zero temperatures, background radiation, limited liquid water availability, and frequent very-low-nutrient conditions.

In nature, microorganisms typically live and proliferate as members of communities rather than as single cells or populations. A widespread growth form of life in natural habitats occurs as multispecies biofilms where the cells are embedded in a self-produced extracellular matrix consisting of polysaccharides and proteins, which includes other macromolecules such as lipids and DNA. These so-called extrapolymeric substances (EPS) provide protection against different environmental stressors (e.g., desiccation, radiation, harmful chemical agents, and predators). Biofilms are highly organized structures that enable microbial communication via signaling molecules, disperse cells and EPS, distribute nutrients and release metabolites, and facilitate horizontal gene transfer.

The majority of known microbial communities on Earth are able to produce EPS, and the protection provided by this matrix enlarges their physical and chemical limits for metabolic processes and replication. EPS also enhances their tolerance to simultaneously occurring multiple stressors and enables the occupation of otherwise uninhabitable ecological niches in the microscale and macroscale. The presence of EPS within a microbial community has implications for several aspects of the SR-SAG2 report, including the physical and chemical limits for life, the dimension of habitable niches versus the actual resolution capability of today’s instruments in Mars orbit, colonization of brines, and tolerance to multiple stressors. In extreme cold and salty habitats (e.g., brines of sea ice and cryopegs in permafrost), EPS has been found to be an excellent cryoprotectant (Goordial et al. 2013). For instance, production of EPS by the marine psychrophilic bacterium Colwellia psychrerythraea increases in response to low temperatures, to high pressure, and to salinity (Marx et al. 2009). Another example is the EPS produced by hypolithic microbial communities that develop on the undersides of translucent rocks in the Dry Valleys of Antarctica, which is thought to facilitate the water-holding capacity of cells and promote microbial survival, growth, and succession (Makhalanyane et al. 2013; de los Ríos et al. 2014).

The production of EPS enhances the resistance of cells to a wide variety of environmental stresses, when compared to their resistance in planktonic growth mode, and enables microbial communities to thrive in nearly any undisturbed environment that receives sufficient water and nutrients. Given the wide distribution and advantages that communities of organisms have when they live as biofilms enmeshed in copious amounts of EPS, it is likely that any microbial stowaways that could survive the trip to Mars would need to develop biofilms to be able to establish themselves in clement microenvironments in Special Regions so that they could grow and replicate. This consideration raises a fundamental question about the probability of a successful colonization by microbial contaminants from Earth in martian habitats, one recently formulated in an essay by Siefert et al. (2012). Studies have been conducted to determine the bioburden found on spacecraft and their assembly facilities (e.g., Satomi et al. 2006; Rettberg et al. 2006; Moissl-Eichinger et al. 2012, 2015). But, to date, there have been no experimental attempts to determine whether the number and type of cells that remain on spacecraft after sterilization and/or after launch and travel through space (e.g., even in low Earth orbit) are sufficient to establish a population and/or community of microorganisms within a Mars Special Region.

Suggestions for future research directions relating to the issues discussed in this section can be found in Appendix A.

DETECTABILITY OF POTENTIAL SMALL-SCALE MICROBIAL HABITATS

The definition of Mars Special Regions is based on temperature and humidity conditions that are measured on spatial scales that do not reflect these conditions within microscale niches that can be potential habitats for microbial communities. Physical and chemical conditions in microenvironments can be substantially different from those of larger scales. Although the SR-SAG2 report considered the microenvironment (Finding 3-10), the implications of the lack of knowledge about microscale conditions was only briefly considered.

There are many examples of small-scale and microscale environments on Earth (see e.g., Lindsay and Brasier 2006) that can host microbial communities, including biofilms, which may only be a few cell layers thick. The biofilm mode of growth, as noted previously, can provide affordable conditions for microbial propagation despite adverse and extreme conditions in the surroundings. On Earth, the heterogeneity of microbial colonization in extreme environments has become more obvious in recent years (e.g., Azúa-Bustos et al. 2015). To identify Special Regions across the full range of spatial scales relevant to microorganisms, a better understanding of the temperature and water activity of potential microenvironments on Mars is necessary. For instance, the interior of the crater Lyot in the northern mid-latitude has been described as an optimal microenvironment with pressure and temperature conditions that could lead to the formation of liquid water solutions during periods of high obliquity (Dickson and Head 2009). Craters, and even microenvironments underneath and on the underside of rocks, could potentially provide favorable conditions for the establishment of life on Mars, potentially leading to the recognition of Special Regions where landscape-scale temperature and humidity conditions would not enable it.

The review committee agrees with Finding 3-10 of the SR-SAG2 report but stresses the significance of the microenvironment and the role it might play on the definition of a Special Region in areas that (macroscopically speaking) would not be considered as such. This issue will be expanded on in Chapter 5.

Suggestions for future research directions relating to the issues discussed in this section can be found in Appendix A.

TRANSLOCATION OF TERRESTRIAL CONTAMINATION

Microbial cells and spores are fairly ubiquitous in Earth’s atmosphere (Burrows et al. 2009; Després et al. 2012) and have been found in a diverse variety of other environments, including the deep ocean (Nunoura et al. 2015); in 3.6-km-deep groundwater accessed via South African gold mines (Moser et al. 2003); in sub-sea floor sediments (Schrenk et al. 2010); at almost 4 km depth in ice sheets above subglacial Lake Vostok (Priscu et al. 1999); in the outer reaches of the stratosphere (Pearce et al. 2009), and 70 km above Earth’s surface (Imshenetsky et al. 1978). Atmospheric transport can move microbial cells and spores over long distances, as is known from investigations of foreign microbes delivered to North America from Africa via Saharan dust (Chuvochina et al. 2011; Barberàn et al. 2014) and Asia (Smith et al. 2012).

A potential problem with designating Special Regions on Mars is that viable microorganisms that survive the trip to Mars could be transported into a distant Special Region by atmospheric processes, landslides, avalanches (although this risk is considered minimal), meteorite impact ejecta, and lander impact ejecta. In addition to dilution effects, the flux of ultraviolet radiation within the martian atmosphere would be deleterious to most airborne microbes and spores. However, dust could attenuate this radiation and enhance microbial viability. In addition, for microbes growing not as single cells but as tetrades or larger cell chains, clusters, or aggregates, the inner cells are protected against ultraviolet radiation. Examples are methanogenic archaea like Methanosarcina, halophilic archaea like Halococcus, or cyanobacteria like Gloeocapsa. This is certainly something that could be studied and confirmed or rejected in terrestrial Mars simulation chambers where such transport processes for microbes (e.g., by dust storms) are investigated. The SR-SAG2 report does not adequately discuss the transport of material in the martian atmosphere. The issue is especially worthy of consideration because if survival is possible during atmospheric transport, the designation of Special Regions becomes more difficult, or even irrelevant. Experiments conducted in facilities such as the Mars Surface Wind Tunnel at NASA’s Ames Research Center or the low-pressure recirculating wind tunnels in the Mars Simulation Laboratory at Aarhus University² may shed light on this issue.

Suggestions for future research directions relating to the issues discussed in this section can be found in Appendix A.

In summary, the SR-SAG2 report’s assessment of the potential for terrestrial life to survive and proliferate on Mars is comprehensive. Of the 14 findings related to this topic (2-2, 2-3, and 3-1 through 3-12), the review committee finds no objection to 13 of them (see Appendix B) and proposes that a small caveat be added to Finding 3-1.

________________

² For details of the Ames and Aarhus facilities see, respectively http://www.nasa.gov/centers/ames/business/planetary_aeolian_facilities.html and http://marslab.au.dk/windtunnel-facilities/wind-tunnel/.