Science Priorities for Mars Sample Return

VI.J.Planetary Protection

In this area, we need to be especially careful to distinguish our discussion of MSR in general from planning for the first MSR.

In the past few years a new category of planetary protection has been defined and which is of particular interest to astrobiologists and the search for modern life. These are “special regions.” Special regions were defined in order to provide extra protection to Martian environments where terrestrial microbes could propagate (COSPAR, 2002; 2005). In simple terms, special regions have been interpreted to be environments with recent liquid water, and this has been quantitatively refined as locations where two environmental conditions are simultaneously met: the temperature is greater than -20°C, and the activity of water is greater than 0.5 (MEPAG SR-SAG, 2006; COSPAR, in prep.). Such environments, if they could be identified, would be of very attractive targets for MSR because of the potential for extant indigenous Martian life.

Accessing and sampling special regions is permitted by planetary protection policy. Doing so would require the sterilization of the spacecraft components that penetrate the special region. For missions that access a special region by means of roving, reaching, or drilling, the subsystems that penetrate the special region would need to be sterilized. For missions that land within a special region, or missions where a special region is present within the landing error ellipse, the entire landed system would need to be sterilized because of the potential of a spacecraft failure and break-up during EDL. In either case, MSR development would be significantly more complex in an engineering sense, thus increasing mission cost and risk. 

At the time of this writing, no Martian environment, either surface or subsurface, has been identified that is known to meet the technical thresholds for a special region. However, there are several kinds of environments that exist on Mars for which it there is ambiguity as to whether they meet the threshold conditions for “special region” or not (e.g. mid-latitude gullies, pasted-on terrain). In the latter case, since they cannot be shown to be “not-special”, and for the purpose of planetary protection, which requires conservative approaches, they are treated as if they are “special”. In addition, MEPAG SR-SAG (2006) pointed out several kinds of geologic environments that if discovered by future researchers could qualify as special (active volcanoes, hydrothermal vents, large very young large craters).

For the purpose of planning MSR, it is important to recognize the difference between the attitude of planetary protection towards an environment with ambiguity (if there is a question, be conservative and treat as if special), and that of the science community (we need a high degree of confidence that the samples we want would be at the site selected—MSR is not the right choice of a mission to be probing sites that have major uncertainty).

Discussion—planning for the first MSR. The reality is that we cannot at present credibly propose that the objective of the first MSR be to find the extant Martian life and return it to Earth—we don’t know where to go or what to sample. We have no information on the environmental habitability factors for unknown Martian life forms. Our best guess is that the most favorable environment would be one with liquid water (which would qualify as a special region), but we have not yet made that discovery. For these reasons, proposed Scientific Objective #2 of this report is phrased “assess the evidence for pre-biotic processes, past life, and/or extant life”. The specific strategy for how to achieve this is left up to future landing site selection committees and other such planning teams. Note that since pre-biotic chemistry and the possibility of past life can be evaluated without the need to go to a special region, special region access is not required to achieve this objective. The objective could be alternatively be achieved through the study of ancient environments and their geologic products (e.g., sedimentary, igneous, and hydrothermal rocks), for which knowledge of their distribution is much more confidently known.

However, retaining the option to sample a special region with the first MSR would be valuable scientifically in the sense that it may allow us to respond to the discovery of liquid water within the next 5-10 years. Thus, for the first MSR mission, this question boils down to balancing the probability of making such a discovery against the increased cost of mission development (the development cost would be incurred whether the mission is eventually sent to a special region or not). This trade is best evaluated by a joint science-engineering team that could specifically evaluate both the cost and the benefit.

ND-SAG additionally observes that MEPAG SR-SAG (2006) presented very convincing arguments that for thermodynamic reasons, the most prospective part of Mars for near-surface modern liquid water is the latitude band 30-60° (both north and south latitude), and that it is almost impossible for liquid water to exist equatorward of 30° latitude. If MSR adopts equatorial landing site restrictions for EDL reasons, there would be little reason to believe that special region access capability could actually be used.

N

• 
  ND-SAG finds that a scientifically compelling first MSR mission could be designed without including the capability to access and sample a special region.
  
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  It would be desirable scientifically to retain the option of responding to a post-2007 discovery that changes our understanding of Martian special regions. However, the same could also be said of many other possible ways to enhance MSR--deciding which would be a prudent investment would entail cost-benefit comparisons of the various options, and consideration of budget availability.
  
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  Based on our understanding of Mars as of 2007, unless MSR had the ability to land poleward of 30 degrees latitude, access very rough terrain, or achieve a significant subsurface penetration (e.g. >5 m), MSR would unlikely to be able to make use of incremental special regions capability.
  

1. 
   Destroy or alter organic material within the sample, including components such as amino acids, polycyclic aromatic hydrocarbons, and paraffins, needed to evaluate hypotheses involving prebiotic chemistry, past life, and modern life.
   
2. 
   Destroy or alter many hydrous minerals such as clays, sulfates, and hydroxides that are essential to interpreting the aqueous history of Mars.
   
3. 
   For samples that are not encapsulated, components released from one sample could react with other samples, causing the samples to no longer be representative of the Martian environment.
   
4. 
   Adversely affect studies on possible (unknown) oxidant phases in regolith samples.