Difference between revisions of "Radiation"

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Natural '''Radiation''' on [[Mars]] is much higher compared with [[Earth]]. The thin [[atmosphere]] provides only a small shielding effect against harmful [[solar radiation]] and [[cosmic radiation]].  Mars also lacks the [[magnetosphere]] that protects Earth.
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[[Image:nuclear_warning_sign.png|right|Nuclear Danger Icon]]
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Natural '''Radiation''' on [[Mars]] is much higher compared with [[Earth]]. The thin [[atmosphere]] provides only a small shielding effect against [[cosmic radiation]].  It provides moderate protection against [[solar radiation]].  Mars also lacks the [[magnetosphere]] that protects Earth.
  
Occasional [[solar flares]] produce particular high doses. Some solar proton events (SPEs) were observed by [[MARIE]] that were not seen by sensors near Earth due to the fact that SPEs are directional. Astronauts on Mars could be warned of SPEs by sensors closer to the Sun and presumably take shelter during these events. This would imply an [[Early warning system (solar radiation)|Early Warning System]] (possibly a network of sensors in orbit around the sun or a single sensor in [[Lagrangian point]] L1) might be needed to ensure all SPEs threatening Mars were detected early enough.
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The average natural radiation level on Mars is 24-30 rads or 240-300 mSv per year<ref name=":0" /><ref name=":5" />. This is about 40-50 times the average on Earth.  
 +
 
 +
1 millisievert [mSv] = 0.1 rad [rd]  
  
 
==Types of Radiation==
 
==Types of Radiation==
 
Radiation comes in a variety of forms:<ref>http://www.nas.nasa.gov/About/Education/SpaceSettlement/designer/needs.html#SHIELDING</ref>
 
Radiation comes in a variety of forms:<ref>http://www.nas.nasa.gov/About/Education/SpaceSettlement/designer/needs.html#SHIELDING</ref>
{| border=1
+
{| border="1"
 
|-
 
|-
!Name  
+
!Name
!Relative Biological<br/> Effectiveness RBE  
+
!Relative Biological<br /> Effectiveness (RBE)
!Source  
+
!Source
 
|-
 
|-
|'''[[X-ray|X-Rays]] and [[gamma ray|Gamma Rays]]'''  
+
|'''[[X-ray|X-Rays]] and [[gamma ray|Gamma Rays]]'''
|1  
+
|1
|[[Radiation belts]], [[solar radiation]], and bremsstrahlung electrons  
+
|[[Radiation belts]], [[solar radiation]], and bremsstrahlung electrons
 
|-
 
|-
|'''[[electron|Electrons]]'''<br/>  
+
|'''[[electron|Electrons]]'''   
1.0 MeV<br/>
+
1.0 MeV<br />
 
0.1 MeV  
 
0.1 MeV  
|<br/>
+
|<br />
1<br/>  
+
1<br />  
 
1.08  
 
1.08  
|Radiation belts  
+
|Radiation belts
 
|-
 
|-
|'''[[proton|Protons]]'''<br/>   
+
|'''[[proton|Protons]]'''<br />   
100 MeV<br/>  
+
100 MeV<br />  
1.5 MeV<br/>   
+
1.5 MeV<br />   
 
0.1 MeV  
 
0.1 MeV  
|<br/>
+
|<br />
1-2<br/>   
+
1-2<br />   
8.5<br/>   
+
8.5<br />   
 
10  
 
10  
|Cosmic rays, inner-radiation belts, and solar [[cosmic rays]]  
+
|[[Cosmic radiation]], inner-radiation belts, and [[solar radiation]]
 
|-
 
|-
|'''[[neutron|Neutrons]]'''<br/>   
+
|'''[[neutron|Neutrons]]'''<br />   
0.05 ev (thermal)<br/>   
+
0.05 ev (thermal)<br />   
1.0 MeV<br/>   
+
1.0 MeV<br />   
 
10 MeV  
 
10 MeV  
|<br/>
+
|<br />
2.8<br/>   
+
2.8<br />   
10.5<br/>   
+
10.5<br />   
 
6.4
 
6.4
|Nuclear interactions in the [[sun]]  
+
|Nuclear interactions in the [[sun]]; on Mars, produced when [[cosmic radiation]] interacts with [[regolith]]
 +
|-
 +
|'''[[alpha particles|Alpha Particles]]'''<br /> 
 +
5.0 MeV<br /> 
 +
1.0 MeV
 +
|<br />
 +
15<br /> 
 +
20
 +
|[[Cosmic radiation]], [[Cosmic rays]]
 +
|-
 +
|[[Heavy Ions|'''Heavy Ions''']]
 +
|Varies widely
 +
|[[Cosmic radiation]]
 
|-
 
|-
|'''[[alpha particles|Alpha Particles]]'''<br/> 
+
| colspan="3" |Table 1: Types of radiation
5.0 MeV<br/> 
 
1.0 MeV<br/> 
 
Heavy Primaries
 
|<br/>
 
15<br/> 
 
20<br/> 
 
varies widely
 
|Cosmic rays
 
 
|}
 
|}
 +
(RBE is a measure of the damage done to living tissue, relative to gamma rays)
 +
 +
Cosmic radiation comprises 85% protons, 14% alpha particles, and 1% heavy ions.<ref>Schimmerling W. (2011, Feb 5). The Space Radiation Environment: An Introduction. <nowiki>https://three.jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf</nowiki></ref>  Solar radiation includes the same radiation types, but it a higher proportion of protons and its heavy primaries have lower energy levels.  The high-energy heavy primaries in cosmic radiation can penetrate materials that effectively block lower-energy radiation<ref name=":1">Rapp D. (2006). Radiation Effects and Shielding Requirements in Human Missions to the Moon and Mars. Mars 2:46-71. <nowiki>https://doi.org/10.1555/mars.2006.0004</nowiki></ref>.
 +
 +
==Exposure limits==
 +
 +
===Limits for humans===
 +
Exposure to dangerous levels of radiation causes [[radiation sickness]] and cancer.  The average exposure to  radiation on Earth due to natural sources is 6.2 mSv per year<ref name=":3">http://www.ans.org/pi/resources/dosechart/msv.php</ref>. The highest natural exposure is recorded in Ramsar, Iran, where people are exposed up to 260 mSv/y for many generations, with no reported harmful effects<ref name="Ghiassi-Nejad et al 2002">Ghiassi-Nejad et al, <i>Very high background radiation areas of Ramsar, Iran: Preliminary biological studies</i>, Health Physics 82(1):87-93 (February 2002), DOI: 10.1097/00004032-200201000-00011 [https://www.researchgate.net/publication, /11588980_Very_high_background_radiation_areas_of_Ramsar_Iran_Preliminary_biological_studies abstract]</ref>.  This is 13 times the maximum exposure allowed radiation workers each year.  Importantly, this level of radiation is what Mars settlers (living inside shelters of reasonable cost) would expect.  So the 'high' levels of the back ground radiation at Ramsar is good news for Mars settlement.<ref>https://pubmed.ncbi.nlm.nih.gov/17563407/</ref>  A recent study showed that the people living in this city showed increased immunity to gamma ray exposure, tho if this is from evolutionary adaption over many generations, or from the immune system being 'exercised' regularly is not known. <ref>https://pubmed.ncbi.nlm.nih.gov/11769138/</ref>
 +
 +
It should be emphasized, that low level radiation doses spread over a long period of time (long enough that the bodies natural functions have time to repair the damage), are far less dangerous than large doses received in a short amount of time.  (In fact numerous studies show health benefits from extremely low levels of radiation.)  <ref>https://jnm.snmjournals.org/content/59/12/1786</ref>  See also:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6149023/. https://www.sciencedaily.com/releases/2017/09/170913104428.htm.  https://www.ajronline.org/doi/full/10.2214/ajr.179.5.1791137.  https://en.wikipedia.org/wiki/Radiation_hormesis
 +
 +
Some people believe that ANY exposure to radiation, no matter how slight, poses some risk. Small dose - small risk of cancer. High dose - high risk of cancer.  This is the No Minimum Threshold theory of radiation dosage.  This works quite well with fast radiation doses high enough to cause cancers later in life, but the evidence is much weaker for low level does over a long period of time.  No Minimum Threshold is used by regulatory agencies when they wish to be extremely conservative about radiation risks.<ref>https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6043938/</ref> <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref> <ref>https://pubs.rsna.org/doi/10.1148/radiol.2511080671</ref> 
 +
 +
Nevertheless, there are defined legal limits for exposure during work for several professional activities, such as for X-ray assistants, airplane personnel, etc. The International Commission on Radiation Protection recommends that occupational (work-related) radiation exposure be limited to 50 millisieverts (mSv) per year, and limited to 100 mSv over any 5-year period<ref>http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103</ref>. NASA's radiation dose limits for astronauts are established in NASA-STD-3001<ref>NASA. (2015). <i>NASA Space Flight Human-System Standard Volume 1, Revision A: Crew Health.</i> Retrieved from https://standards.nasa.gov/standard/nasa/nasa-std-3001-vol-1</ref>.
 +
 +
There is scientific uncertainty surrounding the health hazard from cosmic and solar radiation, because most past research on the health effects of radiation studied only x-rays and gamma rays<ref name=":1" />. 
  
==Danger==
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===Limits for plants===
Exposure to dangerous levels of radiation causes [[radiation sickness]] and cancer. Any exposure to radiation, no matter how slight, poses some risk. Small dose - small risk of cancer. High dose - high risk of cancer.
+
[[File:Plant radiation.PNG|thumb|Table 2: Need to find source for this table]]
 +
"In general, plants are relatively radiation resistant when growing and extremely resistant as dormant seeds."  Radiation should not interfere with raising plants as food sources, at least not on the time scales of exploration missions.<ref>National Research Council 1996. Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies. Washington, DC: The National Academies Press. <nowiki>https://doi.org/10.17226/5540</nowiki>.</ref>
  
Nevertheless, there are defined legal limits for exposure during work for several professional activities, such as for X-ray assistants, airplane personnel, etc.
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Table 2 shows the limits for plants(source of table required, in space or on mars?).  It shows that in practically all cases plants can survive radiations events that are likely for Mars without any kind of protection.
  
Cool! That's a clever way of loiokng at it!
+
However, a 2021 study in the Netherlands, conducted on two types of plants, has shown that radiation at the Mars surface may reduce yields substantially<ref>TACK, Nynke, WAMELINK, G. W. W., DENKOVA, A. G., ''et al.'' Influence of Martian Radiation-like Conditions on the Growth of Secale cereale and Lepidium sativum. ''Frontiers in Astronomy and Space Sciences'', 2021, p. 127.</ref>. Germination of plants in a protected environment before setting them out in greenhouses might be a potential mitigation measure but further research is needed.
 +
 
 +
==Martian Environment==
 +
 
 +
===Effects of the Martian atmosphere===
 +
Most SPE particles will be stopped by the atmosphere before they reach the surface.  However, interactions with atmospheric particles can produce neutrons; those neutrons can reach the surface, so the health hazard is not eliminated.
 +
 
 +
Cosmic radiation protons are likely to penetrate the atmosphere.  Cosmic ray heavy ions may fragment in the atmosphere, producing lower-mass ions that can still harm astronauts on the surface.
 +
 
 +
Mars' thin atmosphere allows more ultraviolet light to reach the surface, compared to Earth.  However, habitat structural materials and standard space suits should be sufficient to protect humans from UV radiation.<ref name=":4">Beaty DW, Snook K, Carlton A, Eppler D, Farrell B, Heldmann J,...Zeitlin C, on behalf of the Mars Human Precursor Science Steering Group. (2005). An Analysis of the Precursor Measurements of Mars Needed to Reduce the Risk of the First Human Mission to Mars. Available at <nowiki>https://mepag.jpl.nasa.gov/reports/MHP_SSG_(06-02-05).pdf</nowiki></ref>
 +
 
 +
Mars atmosphere effect depends on the considered inclination, as the incoming radiations will cross more matter if it's coming from the horizon rather than from the zenith. For inclination angles greater than ~45°, the atmospheric thickness is in the range from 20-30 g/cm2, and for lower inclination angles, the atmospherie thickness can exceed 100 g/cm2<ref name=":2" />. 
 +
 
 +
===Low gravity effects on atmospheric thickness===
 +
Note that the radiation protection given by the Martian atmosphere is higher than would be expected considering the air pressure.  Pressure can be thought of the weight of the air above you in the atmosphere.  Mars' gravity is 38% of Earths.  So the weight of that air is less than it would be on Earth.  On other words, more air must be above you on Mars to give the same pressure, compared to Earth.  For example, you might think that since Mars' air pressure is 0.6% of Earth's, the radiation protection would also be 0.6%.  However, the mass of air above you on Mars is 1/38% or 2.6 times thicker than that pressure on Earth.  If we were to terraform Mars to have 10% Earth's pressure, the radiation protection by that atmosphere would be 26%. If we were to give Mars an atmosphere of 50% Earth's air pressure, then the Martian atmosphere's radiation protection would be 132% that of Earth.     
 +
 
 +
===Effects of cosmic rays striking the regolith===
 +
When [[cosmic rays]] strikes regolith, it can cause the impacted atoms to emit their own radiation.  Surrounding regolith particles absorb much of this radiation, with the exception of neutrons.  Neutrons generated in this way are called albedo neutrons.  These neutrons have the potential to add substantially to the radiation dose for astronauts on the surface.<ref name=":4" />
 +
 
 +
Neutrons are well absorbed by water, so blocks of ice or water around habitats would be useful radiation protection.  Lithium hydride is thought to be the most effective neutron absorber ever discovered, and it might be built into the floor of long term habitats. <ref>https://www.tandfonline.com/doi/abs/10.1179/1743284715Y.0000000105?journalCode=ymst20
 +
</ref>  (Lithium hydroxide reacts with water, so it must be kept away from the humid interior of the settlement.)  Alternatively, the regolith or water thickness may be extended to also protect against secondary neutrons.
 +
 
 +
===Dose received by an unprotected human on Mars===
 +
 
 +
====Cosmic radiation====
 +
The equivalent dose rate from cosmic radiation on Earth's surface at sea level is .26 mSv per year<ref name=":3" /> <ref>http://nuclearsafety.gc.ca/eng/resources/radiation/introduction-to-radiation/radiation-doses.cfm</ref> (this makes up ~10% of the annual natural radiation dose, and it increase with altitude).  Based on measurements made by the Curiosity rover, the corresponding figure for the surface of Mars is approximately 230 mSv/year<ref name=":0">Hassler DM, Zeitlin C, Wimmer-Schweingruber RF, Ehresmann B, Rafkin S, Eigenbrode JL,...MSL Science Team. (2014). Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover. Science 343(6169). <nowiki>https://doi.org/10.1126/science.1244797</nowiki></ref>.  More generally, one model estimated that the dose equivalent rate on the surface of Mars ranges from 156.4 mSv/year (at solar maximum) to 273.8 mSv/year (at solar minimum)<ref name=":2" /><ref name=":5">McKenna-Lawlor S, Goncalves P, Keating A, Reitz G, Matthia D. (2012). Overview of Energetic Particle Hazards During Prospective Manned Missions to Mars. Planetary and Space Science. 63: 123-132.</ref>.  A 2005 report by the Mars Human Precursor Science Steering Group estimated that (at solar minimum) the dose from cosmic radiation would be 1.2 +/- 0.5 mSv/day; this includes 0.4 +/- 0.4 mSv/day from albedo neutrons.<ref name=":4" />
 +
 
 +
====Solar Proton Events====
 +
Curiosity also measured the temporary increase in radiation during a single SPE.  The results indicate an increase in equivalent dose rate of approximately 25% over a 1-day interval<ref name=":0" />.  This figure will vary depending on the intensity of a particular SPE.
 +
 
 +
==Effect on material==
 +
Radiation can change the properties of [[plastics]] and metals, making them brittle after a period of time.
  
 
==Protection==
 
==Protection==
[[House]]s should be equipped with a shielding, thick enough to reduce the radiation to a level equal to Earth, that is, almost zero. Best protection may be achieved with houses built in natural [[caves]] or set into cliffs or hillsides.  
+
Long term [[Habitat|habitats]] should be equipped with a [[radiation shielding]], thick enough to reduce the radiation to a level equal to Earth, that is, almost zero. Best protection may be achieved with houses built in natural [[caves]] or set into cliffs or hillsides. (If, however, it is decided that radiation levels equal to the city of Ramsar, Iran, are safe enough, then the thickness of the radiation shielding suggested below can be reduced up to 13 fold.) 
  
[[Space suit]]s must be designed with radiation in mind.  The suit should provide adequate shielding for the occupant.  It may be necessary to design suits with several grades of protection.  Suits designed for short-term use can carry lighter shielding which would reduce weight and improve maneuverability.   
+
Early exploration habitats could have water tanks, or sand bags above where people live.  When radiation goes thru water, every 18 cm reduces the radiation by half.  So a water tank 108 cm thick (6 halvings) will reduce the radiation level by 64 times.  (As a bonus, water is a good neutron absorber.) Packed soil has a halving-distance of 9.1 cm, so 55 cm of hard soil would provide a similar level of protection.  In general, it is far better to use local materials for radiation protection, rather than hauling them from Earth.  See: https://www.imagesco.com/geiger/lead-shielding-guide.html.
 +
 
 +
[[Space suit]]s must be designed with radiation in mind.  The suit should provide adequate shielding for the occupant.  It may be necessary to design suits with several grades of protection.  Suits designed for short-term use can carry lighter shielding which would reduce weight and improve maneuverability.  That said, radiation protection from suits will be much less than inside habitats, so minimizing time on the surface will be the largest protection.  Going outside during solar storms would likely be banned.   
  
 
During severe radiation events, such as [[solar flare|solar flares]], surface [[settlement|settlements]] may use [[storm shelter|storm shelters]] with heavier than normal shielding.
 
During severe radiation events, such as [[solar flare|solar flares]], surface [[settlement|settlements]] may use [[storm shelter|storm shelters]] with heavier than normal shielding.
  
==Open issues==
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"In this work, it is shown that on the Martian surface, almost any amount of aluminum shielding increases exposure levels for humans. The increased exposure levels are attributed to neutron production in the shield and Martian regolith as well as the electromagnetic cascade induced in the Martian atmosphere. This result is significant for optimization of vehicle and shield designs intended for the surface of Mars." <ref name=":2">NASA, Tony C. Slaba, Christopher J. Mertens, and Steve R. Blattnig '' Radiation Shielding Optimization on Mars '', https://spaceradiation.larc.nasa.gov/nasapapers/NASA-TP-2013-217983.pdf, Apr 2013.</ref>
*How much higher is the UV and cosmic radiation?
+
 
*What is the required thickness of a [[regolith]] or [[water]] layer upon a house for radiation shielding?
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"An in-situ shielding strategy will also be of little help unless several hundred g/cm2 of regolith is utilized. Such a strategy would probably require large scale excavation making it an unlikely candidate. Instead, the shielding strategy would rely primarily on material optimization. Options, such as replacing aluminum structures with high hydrogen content carbon composites, could be pursued." <ref name=":2" />  This opinion is open to argument as in-situ resources utilization for any type of settlement should make large amounts of regolith available for construction.  It mainly is true for the very first level of habitats.
*What are the legal limits for radiation exposure for some professional activities?
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*Does regolith emit an own portion of radiation, some kind of secondary radiation due to the long exposure to cosmic and solar radiation?
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The use of g/cm2 can be translated into an equivalent thickness that depends on the material density.  For martian regolith at 2000 kg/m3, a thickness of 1m of regolith is 200 g/cm2.  Water (or ice) is 100 g/cm2.  So the minimum covering for a long term settlement would be 5m or more.  For water, although the radiation absorption is better the density is lower, so about the same thickness would be required for protection. <sup>(to be discussed)</sup>
  
 
==References==
 
==References==
<references/>
+
<references />
  
 
==External links==
 
==External links==
 +
 
*[http://www.ips.gov.au/ IPS:] [http://www.ips.gov.au/Category/Educational/Space%20Weather/Space%20Weather%20Effects/guide-to-space-radiation.pdf A Guide to Space Radiation]
 
*[http://www.ips.gov.au/ IPS:] [http://www.ips.gov.au/Category/Educational/Space%20Weather/Space%20Weather%20Effects/guide-to-space-radiation.pdf A Guide to Space Radiation]
 
*[http://www.niauk.org/radiation-and-safety.html Nuclear Industry Association: Radiation, health and nuclear safety]
 
*[http://www.niauk.org/radiation-and-safety.html Nuclear Industry Association: Radiation, health and nuclear safety]
 +
*[https://hesperia.gsfc.nasa.gov/sspvse/posters/DF_Smart/poster.pdf The frequency distribution of solar proton events: 5 solar cycles and 45 solar cycles]
 +
*[[w:Radiation_hormesis|Low levels of radiation can be stimulatory: Radiation Hormesis]]
  
 
+
[[Category:Radiation Protection]]
[[Category:Hazards]]
 

Latest revision as of 14:38, 17 September 2021

Nuclear Danger Icon

Natural Radiation on Mars is much higher compared with Earth. The thin atmosphere provides only a small shielding effect against cosmic radiation. It provides moderate protection against solar radiation. Mars also lacks the magnetosphere that protects Earth.

The average natural radiation level on Mars is 24-30 rads or 240-300 mSv per year[1][2]. This is about 40-50 times the average on Earth.

1 millisievert [mSv] = 0.1 rad [rd]

Types of Radiation

Radiation comes in a variety of forms:[3]

Name Relative Biological
Effectiveness (RBE)
Source
X-Rays and Gamma Rays 1 Radiation belts, solar radiation, and bremsstrahlung electrons
Electrons

1.0 MeV
0.1 MeV


1
1.08

Radiation belts
Protons

100 MeV
1.5 MeV
0.1 MeV


1-2
8.5
10

Cosmic radiation, inner-radiation belts, and solar radiation
Neutrons

0.05 ev (thermal)
1.0 MeV
10 MeV


2.8
10.5
6.4

Nuclear interactions in the sun; on Mars, produced when cosmic radiation interacts with regolith
Alpha Particles

5.0 MeV
1.0 MeV


15
20

Cosmic radiation, Cosmic rays
Heavy Ions Varies widely Cosmic radiation
Table 1: Types of radiation

(RBE is a measure of the damage done to living tissue, relative to gamma rays)

Cosmic radiation comprises 85% protons, 14% alpha particles, and 1% heavy ions.[4]  Solar radiation includes the same radiation types, but it a higher proportion of protons and its heavy primaries have lower energy levels.  The high-energy heavy primaries in cosmic radiation can penetrate materials that effectively block lower-energy radiation[5].

Exposure limits

Limits for humans

Exposure to dangerous levels of radiation causes radiation sickness and cancer. The average exposure to radiation on Earth due to natural sources is 6.2 mSv per year[6]. The highest natural exposure is recorded in Ramsar, Iran, where people are exposed up to 260 mSv/y for many generations, with no reported harmful effects[7]. This is 13 times the maximum exposure allowed radiation workers each year. Importantly, this level of radiation is what Mars settlers (living inside shelters of reasonable cost) would expect. So the 'high' levels of the back ground radiation at Ramsar is good news for Mars settlement.[8] A recent study showed that the people living in this city showed increased immunity to gamma ray exposure, tho if this is from evolutionary adaption over many generations, or from the immune system being 'exercised' regularly is not known. [9]

It should be emphasized, that low level radiation doses spread over a long period of time (long enough that the bodies natural functions have time to repair the damage), are far less dangerous than large doses received in a short amount of time. (In fact numerous studies show health benefits from extremely low levels of radiation.) [10] See also: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6149023/. https://www.sciencedaily.com/releases/2017/09/170913104428.htm. https://www.ajronline.org/doi/full/10.2214/ajr.179.5.1791137. https://en.wikipedia.org/wiki/Radiation_hormesis

Some people believe that ANY exposure to radiation, no matter how slight, poses some risk. Small dose - small risk of cancer. High dose - high risk of cancer. This is the No Minimum Threshold theory of radiation dosage. This works quite well with fast radiation doses high enough to cause cancers later in life, but the evidence is much weaker for low level does over a long period of time. No Minimum Threshold is used by regulatory agencies when they wish to be extremely conservative about radiation risks.[11] [12] [13]

Nevertheless, there are defined legal limits for exposure during work for several professional activities, such as for X-ray assistants, airplane personnel, etc. The International Commission on Radiation Protection recommends that occupational (work-related) radiation exposure be limited to 50 millisieverts (mSv) per year, and limited to 100 mSv over any 5-year period[14]. NASA's radiation dose limits for astronauts are established in NASA-STD-3001[15].

There is scientific uncertainty surrounding the health hazard from cosmic and solar radiation, because most past research on the health effects of radiation studied only x-rays and gamma rays[5]

Limits for plants

Table 2: Need to find source for this table

"In general, plants are relatively radiation resistant when growing and extremely resistant as dormant seeds."  Radiation should not interfere with raising plants as food sources, at least not on the time scales of exploration missions.[16]

Table 2 shows the limits for plants(source of table required, in space or on mars?). It shows that in practically all cases plants can survive radiations events that are likely for Mars without any kind of protection.

However, a 2021 study in the Netherlands, conducted on two types of plants, has shown that radiation at the Mars surface may reduce yields substantially[17]. Germination of plants in a protected environment before setting them out in greenhouses might be a potential mitigation measure but further research is needed.

Martian Environment

Effects of the Martian atmosphere

Most SPE particles will be stopped by the atmosphere before they reach the surface.  However, interactions with atmospheric particles can produce neutrons; those neutrons can reach the surface, so the health hazard is not eliminated.

Cosmic radiation protons are likely to penetrate the atmosphere.  Cosmic ray heavy ions may fragment in the atmosphere, producing lower-mass ions that can still harm astronauts on the surface.

Mars' thin atmosphere allows more ultraviolet light to reach the surface, compared to Earth.  However, habitat structural materials and standard space suits should be sufficient to protect humans from UV radiation.[18]

Mars atmosphere effect depends on the considered inclination, as the incoming radiations will cross more matter if it's coming from the horizon rather than from the zenith. For inclination angles greater than ~45°, the atmospheric thickness is in the range from 20-30 g/cm2, and for lower inclination angles, the atmospherie thickness can exceed 100 g/cm2[19].

Low gravity effects on atmospheric thickness

Note that the radiation protection given by the Martian atmosphere is higher than would be expected considering the air pressure. Pressure can be thought of the weight of the air above you in the atmosphere. Mars' gravity is 38% of Earths. So the weight of that air is less than it would be on Earth. On other words, more air must be above you on Mars to give the same pressure, compared to Earth. For example, you might think that since Mars' air pressure is 0.6% of Earth's, the radiation protection would also be 0.6%. However, the mass of air above you on Mars is 1/38% or 2.6 times thicker than that pressure on Earth. If we were to terraform Mars to have 10% Earth's pressure, the radiation protection by that atmosphere would be 26%. If we were to give Mars an atmosphere of 50% Earth's air pressure, then the Martian atmosphere's radiation protection would be 132% that of Earth.

Effects of cosmic rays striking the regolith

When cosmic rays strikes regolith, it can cause the impacted atoms to emit their own radiation.  Surrounding regolith particles absorb much of this radiation, with the exception of neutrons.  Neutrons generated in this way are called albedo neutrons.  These neutrons have the potential to add substantially to the radiation dose for astronauts on the surface.[18]

Neutrons are well absorbed by water, so blocks of ice or water around habitats would be useful radiation protection. Lithium hydride is thought to be the most effective neutron absorber ever discovered, and it might be built into the floor of long term habitats. [20] (Lithium hydroxide reacts with water, so it must be kept away from the humid interior of the settlement.) Alternatively, the regolith or water thickness may be extended to also protect against secondary neutrons.

Dose received by an unprotected human on Mars

Cosmic radiation

The equivalent dose rate from cosmic radiation on Earth's surface at sea level is .26 mSv per year[6] [21] (this makes up ~10% of the annual natural radiation dose, and it increase with altitude). Based on measurements made by the Curiosity rover, the corresponding figure for the surface of Mars is approximately 230 mSv/year[1]. More generally, one model estimated that the dose equivalent rate on the surface of Mars ranges from 156.4 mSv/year (at solar maximum) to 273.8 mSv/year (at solar minimum)[19][2]. A 2005 report by the Mars Human Precursor Science Steering Group estimated that (at solar minimum) the dose from cosmic radiation would be 1.2 +/- 0.5 mSv/day; this includes 0.4 +/- 0.4 mSv/day from albedo neutrons.[18]

Solar Proton Events

Curiosity also measured the temporary increase in radiation during a single SPE.  The results indicate an increase in equivalent dose rate of approximately 25% over a 1-day interval[1].  This figure will vary depending on the intensity of a particular SPE.

Effect on material

Radiation can change the properties of plastics and metals, making them brittle after a period of time.

Protection

Long term habitats should be equipped with a radiation shielding, thick enough to reduce the radiation to a level equal to Earth, that is, almost zero. Best protection may be achieved with houses built in natural caves or set into cliffs or hillsides. (If, however, it is decided that radiation levels equal to the city of Ramsar, Iran, are safe enough, then the thickness of the radiation shielding suggested below can be reduced up to 13 fold.)

Early exploration habitats could have water tanks, or sand bags above where people live. When radiation goes thru water, every 18 cm reduces the radiation by half. So a water tank 108 cm thick (6 halvings) will reduce the radiation level by 64 times. (As a bonus, water is a good neutron absorber.) Packed soil has a halving-distance of 9.1 cm, so 55 cm of hard soil would provide a similar level of protection. In general, it is far better to use local materials for radiation protection, rather than hauling them from Earth. See: https://www.imagesco.com/geiger/lead-shielding-guide.html.

Space suits must be designed with radiation in mind. The suit should provide adequate shielding for the occupant. It may be necessary to design suits with several grades of protection. Suits designed for short-term use can carry lighter shielding which would reduce weight and improve maneuverability. That said, radiation protection from suits will be much less than inside habitats, so minimizing time on the surface will be the largest protection. Going outside during solar storms would likely be banned.

During severe radiation events, such as solar flares, surface settlements may use storm shelters with heavier than normal shielding.

"In this work, it is shown that on the Martian surface, almost any amount of aluminum shielding increases exposure levels for humans. The increased exposure levels are attributed to neutron production in the shield and Martian regolith as well as the electromagnetic cascade induced in the Martian atmosphere. This result is significant for optimization of vehicle and shield designs intended for the surface of Mars." [19]

"An in-situ shielding strategy will also be of little help unless several hundred g/cm2 of regolith is utilized. Such a strategy would probably require large scale excavation making it an unlikely candidate. Instead, the shielding strategy would rely primarily on material optimization. Options, such as replacing aluminum structures with high hydrogen content carbon composites, could be pursued." [19] This opinion is open to argument as in-situ resources utilization for any type of settlement should make large amounts of regolith available for construction. It mainly is true for the very first level of habitats.

The use of g/cm2 can be translated into an equivalent thickness that depends on the material density. For martian regolith at 2000 kg/m3, a thickness of 1m of regolith is 200 g/cm2. Water (or ice) is 100 g/cm2. So the minimum covering for a long term settlement would be 5m or more. For water, although the radiation absorption is better the density is lower, so about the same thickness would be required for protection. (to be discussed)

References

  1. 1.0 1.1 1.2 Hassler DM, Zeitlin C, Wimmer-Schweingruber RF, Ehresmann B, Rafkin S, Eigenbrode JL,...MSL Science Team. (2014). Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover. Science 343(6169). https://doi.org/10.1126/science.1244797
  2. 2.0 2.1 McKenna-Lawlor S, Goncalves P, Keating A, Reitz G, Matthia D. (2012). Overview of Energetic Particle Hazards During Prospective Manned Missions to Mars. Planetary and Space Science. 63: 123-132.
  3. http://www.nas.nasa.gov/About/Education/SpaceSettlement/designer/needs.html#SHIELDING
  4. Schimmerling W. (2011, Feb 5). The Space Radiation Environment: An Introduction. https://three.jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf
  5. 5.0 5.1 Rapp D. (2006). Radiation Effects and Shielding Requirements in Human Missions to the Moon and Mars. Mars 2:46-71. https://doi.org/10.1555/mars.2006.0004
  6. 6.0 6.1 http://www.ans.org/pi/resources/dosechart/msv.php
  7. Ghiassi-Nejad et al, Very high background radiation areas of Ramsar, Iran: Preliminary biological studies, Health Physics 82(1):87-93 (February 2002), DOI: 10.1097/00004032-200201000-00011 /11588980_Very_high_background_radiation_areas_of_Ramsar_Iran_Preliminary_biological_studies abstract
  8. https://pubmed.ncbi.nlm.nih.gov/17563407/
  9. https://pubmed.ncbi.nlm.nih.gov/11769138/
  10. https://jnm.snmjournals.org/content/59/12/1786
  11. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6043938/
  12. https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation
  13. https://pubs.rsna.org/doi/10.1148/radiol.2511080671
  14. http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103
  15. NASA. (2015). NASA Space Flight Human-System Standard Volume 1, Revision A: Crew Health. Retrieved from https://standards.nasa.gov/standard/nasa/nasa-std-3001-vol-1
  16. National Research Council 1996. Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies. Washington, DC: The National Academies Press. https://doi.org/10.17226/5540.
  17. TACK, Nynke, WAMELINK, G. W. W., DENKOVA, A. G., et al. Influence of Martian Radiation-like Conditions on the Growth of Secale cereale and Lepidium sativum. Frontiers in Astronomy and Space Sciences, 2021, p. 127.
  18. 18.0 18.1 18.2 Beaty DW, Snook K, Carlton A, Eppler D, Farrell B, Heldmann J,...Zeitlin C, on behalf of the Mars Human Precursor Science Steering Group. (2005). An Analysis of the Precursor Measurements of Mars Needed to Reduce the Risk of the First Human Mission to Mars. Available at https://mepag.jpl.nasa.gov/reports/MHP_SSG_(06-02-05).pdf
  19. 19.0 19.1 19.2 19.3 NASA, Tony C. Slaba, Christopher J. Mertens, and Steve R. Blattnig Radiation Shielding Optimization on Mars , https://spaceradiation.larc.nasa.gov/nasapapers/NASA-TP-2013-217983.pdf, Apr 2013.
  20. https://www.tandfonline.com/doi/abs/10.1179/1743284715Y.0000000105?journalCode=ymst20
  21. http://nuclearsafety.gc.ca/eng/resources/radiation/introduction-to-radiation/radiation-doses.cfm

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