Findings of the NASA MEPAG Mars Special Regions Science Analysis Group

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Executive Summary

INTRODUCTION AND APPROACH

Current Planetary Protection policy designates a categorization IVc for spacecraft potentially entering into a “special region” of Mars that requires specific constraints on spacecraft development and operations.

NASA requested that MEPAG charter a Special Regions Science Analysis Group (SR-SAG) to develop a quantitative clarification of the definition of “special region” that can be used to distinguish between regions that are “special” and “non-special” and a preliminary analysis of specific environments that should be considered “special” and “non-special.”

The SR-SAG used the following general approach: Clarify the terms in the existing COSPAR definition; establish temporal and spatial boundary conditions for the analysis; identify applicable threshold conditions for propagation; evaluate the distribution of the identified threshold conditions on Mars; analyze on a case-by-case basis those purported geological environments on Mars that could potentially exceed the biological threshold conditions. Furthermore, describe conceptually the possibility for spacecraft-induced conditions that could exceed the threshold levels for propagation.

The following represent the results of the SR-SAG study in which “special regions” are more practically defined, a comprehensive distillation of our current understanding of the limits of terrestrial life and relevant martian conditions, and an analytical approach is presented to consider special regions with current and future improvements in our understanding. The specific findings of the SAG reported in the executive summary are in bold.

DEFINITION

The existing definition of “special region” (from COSPAR 2002 & 2005, NASA, 2005) is “… a region within which terrestrial organisms are likely to propagate, or a region which is interpreted to have a high potential for the existence of extant Martian life forms. Given current understanding, this applies to regions where liquid water is present or may occur. ” The SR-SAG determined that in order to proceed with identifying special regions, some words needed clarification. The word propagate is taken to mean reproduction (not just growth or dispersal). Also, the focus on the word “likely” is taken to apply to the probability of specific geological conditions during a certain time period and not to probability of growth or terrestrial organisms. While the report does concentrate on the salient parameters of forward contamination and martian environmental conditions, it does not address the second clause of the definition concerning probability of martian life, as there is no information.

The study limited itself to special regions that may exist on Mars to environmental conditions that may exist within the next 100 years, a period reasonably within our predictive capabilities and within which astronauts are expected to be on the surface of Mars. The SAG also considered only the upper five meters of the red planet as the maximum depth to which current spacecraft could access as a consequence of failure during entry, descent and landing. Environments deeper than five meters were considered important as possible habitats for life but were also considered in need of specific information about the expected nature of the environment to be accessed and the operational approach taken by the robotic platform, and therefore should be approached on a case-by-case basis.

LIMITS TO MICROBIAL LIFE

The approach of the study group was to find any terrestrial representative that demonstrated the ability to reproduce under the worst environmental conditions. Although many factors may limit microbial growth and reproduction, the known overriding environmental constraints on Mars are low temperature and aridity, and a surface that is bathed in ultraviolet and galactic cosmic radiation.

Life on Earth has been able to survive extremely low temperatures, but for this study, the figure of merit is the ability to reproduce. An extensive review of the literature on low temperature metabolic/reproductive studies reveals an exponential decrease in microbial metabolism, enabling long-term survival maintenance or perhaps growth. However, experiments and polar environments themselves have failed to show microbial reproduction at temperatures below -15°C. For this reason, with margin added, a temperature threshold of -20°C is proposed for use when considering special regions.

Although many terrestrial microorganisms can survive extreme desiccation, they all share the absolute requirement for liquid water to grow and reproduce. Various measures are used to quantify the availability of liquid water to biological systems, but the one that was used to integrate biology and geology for this analysis was water activity (aw). Pure water has a water activity of 1.0, and water activity decreases with increasing solute concentration and with decreasing relative humidity. Some example water activities are: sea water aw = 0.98, saturated NaCl = 0.75, ice at -40°C = 0.67. For this application, water activity has the advantage in that it is a quantity that can be derived and measured, and applied across multiple length scales in equilibrium. The lowest known water activity that allows microbial growth is for a yeast in an 83% (W/V) sucrose solution where aw = 0.62. Based on current knowledge, terrestrial organisms are not known to be able to reproduce at a water activity below 0.62; with margin, an activity threshold of 0.5 is proposed for use when considering special regions.

WATER ON MARS

Water on Mars in best analyzed in two broad classifications: the portions of Mars that are at or close to thermodynamic equilibrium and those that are in long-term disequilibrium.

In considering martian equilibrium conditions, the repeatability of thermal inertia results from data set to data set suggests that numerical thermodynamic models are generally accurate to better than a few degrees during most seasons and are even more accurate on an annual average. Comparison between Mars Odyssey Gamma Ray Spectrometer (GRS) measurements and theoretical models of ice stability based on these same thermodynamic numerical models demonstrates excellent agreement between theory and observation. A critically important value of models is that they have predictive value down to spatial scales much finer than that achievable by observational data, and so, although there are macroscopic processes that can produce distinct departures from equilibrium, the scale tends to be local to regional, not microscopic.

Where ice is in vapor-diffusive exchange with the atmosphere, the equilibrium temperature (the frostpoint), is at about -75°C on contemporary Mars. Ice is not stable with respect to sublimation in places where diurnal or seasonal temperature fluctuations significantly exceed -75°C. Thus Mars’ ample supply of near-surface water is stubbornly sequestered in solid form at temperatures below the frost point, either on the polar caps or in vast high latitude, subsurface deposits. While the surface of Mars at many low-latitude locations may exceed 0°C in the peak of the day, the temperature 10-20 cm below those surfaces remains perpetually below -40°C. Were liquid to hypothetically form at a higher surface temperature, it would be transported in a matter of minutes or hours to the relatively cold region just below the surface, and eventually to a permanent polar or subpolar reservoir by evaporation and condensation. Thus, persistent liquid water at or near the martian surface requires a significant departure from the general planetary setting in the form of either long-term disequilibria (such as geothermal sources) or from short-term disequilibria (an impactor).

The equilibrium water activity of martian regolith can be calculated as a function of temperature, using a mean absolute humidity of 0.8 microbar and assuming equilibrium with the atmosphere. In warm regolith, aw is literally orders of magnitude too small to support life. Water activity approaches unity at the frostpoint, but at extremely low temperatures. If however, there is a significant barrier to equilibration with the atmosphere, there is a possibility of much higher absolute humidity, and, therefore, significantly higher aw at warmer temperatures. Desert crusts have been proposed as a potential mechanism to provide a diffusion barrier, and were considered in this study. Although crusts on Mars have been observed at the past landing sites, and other crust types are hypothetically possible elsewhere, experience with desert crusts on Earth shows that the effect of a semi-permeable crust is to retard, not prevent, the achievement of equilibrium.

Where the surface and shallow subsurface of Mars are at or close to thermodynamic equilibrium with the atmosphere (using time-averaged, rather than instantaneous, equilibrium), temperature and water activity in the martian shallow subsurface are considerably below the threshold conditions for propagation of terrestrial life. The effects of thin films and solute freezing point depression are included within the water activity.

While an extensive literature speculates on mechanisms to form liquid water on Mars at different times in the past and under different climate conditions, common to all of them is the explicit understanding that present-day equilibrium conditions do not support the persistence of liquid water at the surface. Uncertainty exists about whether previous conditions were persistent or episodic, with some attributing conditions to be punctuated, due to impact effects, while others envisioning longer term stable early climates. More recently, orbital forcing has been recognized as a factor driving climate change, with 50 kyr being the shortest climate cycle affecting latitudinal precipitation.

The SAG considered possible environments in long-term disequilibrium, where water and temperature were in equilibrium under conditions at an earlier time, but for which conditions have changed, and do not hold for the present. Geological deposits might survive for 104 – 107 years by virtue of giving up their water very slowly. The SAG examined several potential sites for long-term disequilibrium, either theoretical or actually observed, such as gullies, mid-latitude features of purported snow/ice deposits, remnant glacial deposits, craters, volcanoes, slope streaks, recent outflow channels, possible hydrothermal vents, low-latitude ground ice, and polar caps.

• Some—although, certainly, not all—gullies and gully-forming regions might be sites at which liquid water comes to the surface within the next 100 years. At present, there are no known criteria by which a prediction can be made as to which—if any—of the tens of thousands of gullies on Mars could become active—and whether the fluid involved is indeed water—during this century.
• Because some of the ‘pasted-on’-type mantle has a spatial, and possibly a genetic, relationship to gullies (which in turn are erosional features possibly related to water), the ‘pasted-on’ mantle may be a special region. The mid-latitude mantle, however, is thought to be desiccated, with low potential for the possibility of transient liquid water in modern times. Because the mid-latitude mantle and some kinds of gullies may have a genetic relationship, the mantle is interpreted to have a significant potential for modern liquid water.
• No craters with the combination of size and youthfulness to retain enough heat to exceed the temperature threshold for propagation have been identified on Mars to date.
• We do not have evidence for volcanic rocks on Mars of an age young enough to retain enough heat to qualify as a modern special region or suggest a place of modern volcanic or hydrothermal activity.
• Despite a deliberate and systematic search spanning several years, no evidence has been found for the existence of thermal anomalies capable of producing near-surface liquid water.
• The martian polar caps are too cold to be naturally occurring special regions in the present orientation of the planet.

The SR-SAG proposes that martian regions may be categorized as non-special if the temperature will remain below -20°C or the water activity will remain below 0.5 for a period of 100 years after spacecraft arrival. All other regions on Mars are designated as either special or uncertain. An uncertain region is treated as special until it is shown to be otherwise. The SAG found no regions to be special, but found uncertainty with the gully and possibly-related ‘pasted-on’ mantle regions. In this context, the SAG has listed Mars environments that may be “special” and classified those that have observed features probably associated with water, those that have a non-zero probability of being associated with water, and those areas that, if found, would have a high probability of being associated with water.

A map has been developed that provides a generalized guidelines for the distribution of areas of concern that may be treated as special regions.

It should be noted that even in a region determined to be “non-special,” it is possible a spacecraft may create an environment that meets the definition of “special” or “uncertain.” It is possible for spacecraft to induce conditions that could exceed for some time the threshold conditions for biological propagation, even when the ambient conditions were ‘not special’ before the spacecraft arrived. Whether a special region is induced or not depends on the configuration of the spacecraft, where it is sent, and what it does. This possibility is best evaluated on a case-by-case basis. In summary, within the upper five meters most of Mars is either too cold or too dry to support the propagation of terrestrial life. However, there are regions that are in disequilibrium, naturally or induced, that could be classified as “special” or enough uncertainty exists to be unable to declare the regions as “non-special.”