Protecting Mars special regions with potential for life to propagate

A special region on Mars is a region classified by COSPAR for the purposes of Planetary protection where terrestrial organisms are likely to propagate, or interpreted to have a high potential for existence of extamt Martian life forms. . Based on current understanding, this includes any region with a high enough temperature for Earth organisms to propagate (above -18°C), and with water in a form accessible to them (water activity higher than 0.6), both requirements to be satisfied simultaneously.

Other environmental factors such as the perchlorates and other chemistry, ionizing radiation, UV radiation, and low atmospheric pressure are not used to restrict special regions, because some Earth microbes tolerate them. The presence of multiple environmental factors simultaneously is also not used to restrict special regions because of the existence of polyextremophiles that can withstand multiple simultaneous extreme conditions.

In principle native Martian life could have additional capabilities, and so, be able to propagate at lower temperatures or with lower water activity (one suggestion is a mixture of water and hydrogen peroxide as internal solvent in the cells ). However, since these capabilities are unknown, they are not used to determine special regions.

The requirements also apply to spacecraft induced special regions. Missions need to study these in the planning phase, for instance the potential to create them through impact or sources of thermal energy foreign to Mars. If a hard landing risks biological contamination of a special region, it has to be sterilized sufficiently to prevent this (COSPAR category IVc). .

There are currently no confirmed special regions. However there are many uncertain regions such as the recurring slope lineae. These are treated as special regions for the purposes of planetary protection, until more is known.

Catherine Conley, NASA's Planetary protection officer at the time, explains why the spacecraft we send to Mars are sterilized, 33 seconds into this video

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 * Third video on overview page of the NASA Office of Planetary Protection.

"“So we have to do all of our search for life activities, we have to look for the Mars organisms, without the background, without the noise of having released Earth organisms into the Mars environment”"

See also
 * Possible present day habitats for life on Mars (Incuding potential Mars special regions)

Limits of water activity 0.6 and temperature -18°C
The term "Special region" is understood to apply to any region on Mars where liquid and not too salty water can occur in a temperature range suitable for terrestrial life. The water activity for terrestrial life has to be above 0.6. As an example, honey in dry air has a water activity level of 0.6, and no terrestrial microbes can grow in honey.

The temperature also has to be above -18°C and both of these have to be satisfied simultaneously. Water that is too salty but warm enough or too cold with enough water activity does not count as a special region.

If a margin is added, as in previous reports, a special region would be defined as one with water activity higher than 0.5 and temperature above -23°C

The report remarks that the water activity limit of 0.6 is well determined with no terrestrial life known that can reproduce below that. They do note that some lichens and some cyanobacteria can use the humidity of the air alone but say they haven't found definite evidence that any terrestrial organism can use ambient humidity alone for cell reproduction. For more recent experiments see Lichens, cyanobacteria and molds growing in humidity of simulated Martian atmosphere.

Microscale habitats are a particular challenge. The microenvironments can include vapour aerosols in the atmosphere or within soil cavities, porous rocks etc, vapor-phase water or liquid coming off ice, deliquescing salts, aqueous films on rock or soil grains, thermal springs, and condensation of dew

The lower temperature limit of Earth life is not so well understood, because of practical difficulties measuring extremely low rates of metabolism and cell division. However they were asked to check only for reproduction of Earth life on Mars 500 years into the future. They give the example of cryptoendolithic microbial communities in the Antarctic Dry Valleys which successfully invade sandstone over time periods of 1,000 to 10,000 years. The -18°C limit they give is sufficient to protect Mars from Earth life over a 500 year future timeframe.

The Mars surface is abundant in chaotropic agents that disrupt hydrogen bonding, such as  MgCl2,CaCl2, FeCl3, FeCl2, FeCl, LiCl, and perchlorate salts. These can disrupt biological processes at higher temperatures. However, at temperatures below 10°C, they are beneficial for numerous species of microbes, reducing the lowest temperature for them to propagate by up to 10°C to 20°C. They found no research into whether this effect also occurs at -18°C or below. They note the possibility that this could depress the lower temperature limit, but experiments haven't been conducted on long enough timescales, with a potential doubling time of between several months and years

Perchlorates, ionizing radiation, UV, low pressure and multiple stressors not used to limit potential special regions
COSPAR considered many other environmental factors on Mars in addition to the temperature range and water activity. None of these were used to limit potential special regions.


 * The chemical environment of the Mars surface, including the perchlorates, are considered favourable for life, in particular, perchlorates can be an oxidant for hydrogen and carbon monoxide oxidizing organisms
 * The low pressures of the Mars atmosphere can be tolerated by some terrestrial bacteria
 * The UV flux is blocked by less than 1 mm of regolith or other organisms
 * From the MSL RAD measurements, ionizing radiation levels from cosmic radiation are so low as to be negligible. The intermittent solar storms increase the dose only for a few days and the Martian surface provides enough shielding so that the total dose from solar storms is less than double that from cosmic radiation/ Over 500 years the Mars surface would receive a cummulative dose of less than 50 Gy, far less than the dose where 90% of even a radiation senstiive bacterium such as e-coli would die (LD90 of ~200 - 400 Gy). These  facts  are  not  used  to  distinguish  Special Regions on Mars.
 * On Mars multiple stressors are present simultaneously, but polyextromphiles can often cope with multiple simultaneous stresses either using the same mechanism for them all or multiple mechanisms

Extant life with additional capabilities are not used to deliminate uncertain or special regions
The report briefly discusses the phrase:

"‘‘any region which is interpreted to have a high potential for the existence of extant martian life forms  is  also  defined  as  a  Special  Region’’"

There has been speculation that Martian life might be able to tolerate conditions that are outside of the range of terrestrial life, for instance, able to reproduce at much lower temperatures, and perhaps also in conditions with less water activity than is possible for terrestrial life.

The report doesn't give examples, but one such is Joop Houtkooper and Dirk Schulze Makuch's proposal in 2007 that life on Mars may be using a mixture of water and biogenically created hydrogen peroxide as its internal solvent. He gave this as a possible form of life to explain some puzzling aspects of the Viking lander biological experiments. On cooling, the eutectic of 61.2% (by weight) mix of water and hydrogen peroxide has a freezing point of −56.5 °C, and also tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment. . It would prefer regions with lower temperatures, and would avoid liquid water. Conditions at the poles would be optimal, but it could also survive in the equatorial regions visited by Viking

However the capabilities of any Martian life are currently unknown. The review committees on the Martian special regions have decided that since we lack any data on their capabilities, this requirement can't be used to determine any additional special regions. So they use the capabilities of Earth life exclusively

No known special regions
So far there are no known special regions. Most of them would require verification of existence of microhabitats that are impossible to see directly from orbit. However, there are many uncertain regions where it is possible that as we advance our knowledge of Mars, some of them turn out to be special regions. Amongst the top candidates are the Warm Seasonal flows on Mars (Recurrent Slope Lineae).

Another top candidate is based on the droplet like features that formed on the landing legs of the Phoenix lander. It landed in what is thought to be an ancient ocean bed near the north pole, the first and so far the only spacecraft to land successfully in polar regions. In December 2013, Nilton Renno and his team using the Michigan Mars Environmental Chamber were able to simulate the conditions at its landing site and the droplets. They formed salty brines within minutes when salt overlaid ice, with the salt, especially perchlorates, acting as an "antifreeze". The team concluded that suitable conditions for brine droplets may be widespread in the polar regions. Nilton Renno talks about their results in this video

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For more on these and other candidates, see
 * Possible present day habitats for life on Mars (Incuding potential Mars special regions).

Spacecraft induced special regions in nominal landings or impact scenarios
The requirements also apply to regions that may be made into a special region by a spacecraft, for instance through impact melting ice, or sources of thermal energy foreign to Mars. If a hard landing risks biological contamination of a special region, it has to be sterilized sufficiently to prevent this (COSPAR category IVc).

Missions need to study their potential to create Spacecraft-Induced special regions during the planning phase and take action to make sure they are not inadvertently created. Spacecraft also need to avoid special regions if not sterilized sufficiently to prevent contaminating them. The risk of spacecraft induced special regions needs to be evaluated separately for each mission, taking account of the spacecraft and the landing ellipse.

Special regions could be caused during a nominal landing through the warming effects of rocket exhausts, and temporary special regions could also be created when the hot aeroshell, heatshield, backshell or skycrane compnents land on icy ground in a nominal landing. The spacecraft is normally well insulated, so thermal sources within do not leak much radiation. However, special regions can also be caused in normal use of the lander or rover, by vibration or drilling. Examples include the vibrating sieve on the Viking sampler, the Rock Abrasion Tool (RAT) on the Mars Exploration Rover, the drills on Curiosity or ExoMars, and wheel slip or scuff if the rover gets temporarily "stuck". The lifetime of the special region would depend on the volume melted and whether or not it is heated enough for boiling to be initiated, which can happen at low temperatures with the low Martian atmospheric pressures (over much of the surface, pure water boils at 0°C). Once boiling starts, it may self deplete rapidly and so self-destruct.

The analysis must also look at the outcomes of an off nominal "Breakup and Burnup" scenario. Ice could be heated up by direct transfer from a potentially hot structure from atmosphereic heating as well as the kinetic energy of the impact converted to heat. Any radioisotope heating unit and the RTG's would generate heat energy for several decades. If it lands within a dry capping layer of regolith then it's likely to be separated from the wet layer it causes, but if fully buried in ice or icy soil it could cause pooling and persistence of water near to the RTG The report looked at distributions of ice and concluded that ice in the tropics is buried too deep to be a consideration

However the 2014/5 review corrected this due to evidence of ice present at depths of less than one r in pole-facing slopes

The report also considered the droplets that formed on the Phoenix legs as well as a reported stickiness of its excavated soil samples that was reduced on exposure to the atmosphere, both of which suggested deliquescence, and concluded that these also need to be taken into consideration

The 2014 report provided a map of regions of Mars where there may be ice below the surface as well as potential RSL's. The 2015 review however said that such maps are most useful if accompanied by cautionary remarks on their limitations, as they are subject to change with new discoveries and because of the potential for microhabitats. See below.

Revisions of the definition of a special region
The definition of a special region has been revised several times. In the 2006 study it was implicit that a special region must be defined by warm enough temperatures combined with sufficient water activity. If the Mars surface is mapped using those requirements alone and requiring them to overlap, the map would be blank.

The only place where habitats could occur in the 2006 report were deep below the surface, or perhaps places like the gully systems where water could be exposed to the surface dynamically from the subsurface.

However the 2014 study finds that though both are not present simultaneously, often they are present on the surface within a 24 hour period of each other due to the extreme day - night cycles on Mars. That then makes it possible that terrestrial biology could bridge the gap (e.g. retain the water through to warmer temperatures in the same day). Referring to the Viking and Curiosity (MSL) landing sites they say (page 941 of )

"... where in some  seasons  the  temperature  required for microbial replication was regularly reached during the driest part of the day, whereas at night, when the temperature was too low for replication, the relative humidity at the site was above 0.6 and nearly always close to 1.0. The non-overlap of the required values for a Special Region is reflected in Finding 4-12, but the fact that both could be reached within the same 24 h period, regularly, suggests that there may be a way for organisms to connect the favorable aspects of those periods across a bridge of biotic adaptation."

They go on to say that they didn't have any evidence yet that terrestrial organisms could bridge that gap, and they had some evidence that suggested it might be unbridgeable. However where the atmospheric pressure is above the triple point of water, precipitation that reaches the ground could melt and provide a temporary habitat. Also some materials such as clays, and the organisms themselves could retain more water than the soil itself.

Before the 2014 report was published, both NASA and ESA took steps to have it reviewed independently. This 2015 review concurred with 29 findings of the original report. They did not support one of them and proposed modifications to 15 others. The 2015 review comittee in their report said that they believe that some important aspects were not covered in the previous report. For instance:

For this the review says that research so far is not sufficient to answer the question and that this could be confirmed or rejected in terrestrial Mars simulation chambers
 * The possibility of terrestrial contamination translocation (blown in the Martian dust)
 * The ability of multi-species microbial communities to alter dispersed small-scale habitats. They could synthesize substances and adjust environmental parameters to make them more habitable.

The 2015 review remarks that this makes the definition of a special region more problematic. There are many examples of small-scale and even microscale communities on Earth including biofilms that may be only a few cells thick. Microbes can propagate in these biofilms despite adverse and extreme surrounding conditions. To identify such potential habitats requires a better understanding of the temperature and water activity of potential microenvironments on Mars, for instance in the interior of craters, or microenvironments underneath rocks. These may provide favourable conditions for establishing life on Mars even when the landscape-scale temperature and humidity conditions would not permit it.

Caution on use of maps
The committees for the 2014/5 revisions caution that maps of regions with higher or lower probability to host special regions should be accompanied by cautionary remarks on their limitations. Any such map can only represent the current state of konwledge, which is incomlete and subject to change as new information is obtained.

Instead each mission needs to be considered on a case by case basis using all the available data. They give the example of Schiaparelli where, for example, all HiRISE images of the landing site were inspected for the possible presence of RSL's.