Protecting Mars special regions with potential for life to propagate: Difference between revisions
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A ''special region'' on Mars
(A special region is considered to be "... 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." (Reference: COSPAR 2002 & 2005, NASA, 2005))"''}}</ref>. . 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) {{refn|name=special_region_def|(see section 7.1.1. Recommended organism-based parameters defin-ing the limits of life, and the requirements for Mars Special Regions, page 940 of<ref name=MarsSpecialRegions2014/>)
{{quote|Conditions on the surface of Mars
(1) the temperature (T) is 255 K (-18°C) or above (Finding 3-1) and<br>
(2) water activity (aw) is above 0.60 (Finding 3-4).
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}}, both requirements to be satisfied simultaneously.
Other environmental factors such as the perchlorates and other chemistry {{refn|name=perchlorates}}, ionizing radiation{{refn|name=ionizing_radiation}}
}}., UV radiation{{refn|name=UV}} }}, and low atmospheric pressure{{refn|name=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{{refn|name=multifactor}}.
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<ref name=peroxide_life>{{cite journal | title = A Possible Biogenic Origin for Hydrogen Peroxide on Mars | journal = International Journal of Astrobiology | volume = 6 | issue = 2 | pages = 147 | date = 2007-05-22 | first = Joop M. | last = Houtkooper |author2=Dirk Schulze-Makuch | doi = 10.1017/S1473550407003746 | arxiv = physics/0610093 |bibcode = 2007IJAsB...6..147H }}</ref> ). 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<ref name=MarsSpecialRegions2014/>. If a [[hard landing]] risks biological contamination of a special region, it has to be sterilized sufficiently to prevent this (COSPAR category IVc)<ref name=MarsSpecialRegions2014/>.{{refn|name=spacecraft_impact
{{quote|Thus, during the planning phases, missions will study their own potential to create Spacecraft-Induced SpecialRegions by the presence of a lander itself or by non-nominaloperations during the descent phase and will take action toensure that Special Regions are not inadvertently created.Robotic spacecraft will need to avoid Special Regions if they are not clean enough to avoid contaminating those regions. Although current requirements are the same as those met by the Viking missions of the mid-1970s, no spacecraft sent to Mars since that time has been clean enough to enter a Special Region}}}}<ref name=MarsSpecialRegions2014/>.▼
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{{refn|name=spacecraft_impact|Section 8. Summary of <ref name=MarsSpecialRegions2014/>
▲{{quote|Thus, during the planning phases, missions will study their own potential to create Spacecraft-Induced SpecialRegions by the presence of a lander itself or by non-nominaloperations during the descent phase and will take action toensure that Special Regions are not inadvertently created.Robotic spacecraft will need to avoid Special Regions if they are not clean enough to avoid contaminating those regions. Although current requirements are the same as those met by the Viking missions of the mid-1970s, no spacecraft sent to Mars since that time has been clean enough to enter a Special Region}}}}. The risk of spacecraft induced special regions needs to be evaluated separately for each mission, taking account of the spacecraft and the landing ellipse<ref name=MarsSpecialRegions2014/>.
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<ref name=appel/>.
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* The UV flux is blocked by less than 1 mm of regolith or other organisms {{refn|name=UV|(see 3.5. Ultraviolet radiation on the surface of Mars page 901 of<ref name=MarsSpecialRegions2014/>).
{{quote|'''Finding 3-7''': The martian UV radiation environment is rapidly lethal to unshielded microbes but can be attenuated by global dust storms and shielded completelyby<1 mm of regolith or by 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
{{quote|'''Finding 3-8''': From MSL RAD measurements, ionizing radiation from GCRs at Mars is so low as to be negligible. Intermittent SPEs can increase the atmospheric ionization down to ground level and increase the total dose, but these events are sporadic and last at most a few (2–5)days. These facts are not used to distinguish Special Regions on Mars.}}
{{quote|Over a 500-year time frame, the martian surface could be estimated to receive a cumulative ionizing radiation dose of less than 50 Gy, much lower than the LD90 (lethal dose where 90% of subjects would die) for even a radiation-sensitive bacterium such as E. coli (LD90 of ~200–400 Gy)}}
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===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
{{quote|... 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.}}}}
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}}
* The ability of multi-species microbial communities to alter dispersed small-scale habitats.
▲The cells are embedded in a matrix of externally produced substances (such as polysaccharides, proteins, lipids and DNA) and adjust environmental parameters to make them more habitable{{refn|In chapter 2 of <ref name=MarsSpecialRegionsReview2015/>:
{{quote|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}}
}}. There are many examples of small-scale and even microscale communities on Earth including biofilms
* microscale habitats that can't be detected from orbit.
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}}
The 2014 report looked at distributions of ice and concluded that ice in the tropics is buried too deep to be a consideration{{refn|<ref name=MarsSpecialRegions2014/>
{{quote|SR-SAG2 Finding 5-3: Depths to buried ice deposits in the tropics and mid-latitudes are considered to be >5 m.}}}}
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{{quote|Revised Finding 5-3: In general, depths to buried ice deposits in the tropics are considered to be >5 m. However, there is evidence that water ice is present at depths of <1 m on pole-facing slopes in the tropics and mid latitudes. Thus, a local detailed analysis for a particular area is necessary to determine if it could be a Special Region.}}
}}
* Utility of maps
The 2014 report<ref name=MarsSpecialRegions2014/> 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<ref name=MarsSpecialRegionsReview2015/>.
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