Cosmic radiation

Cosmic radiation (also known as cosmic rays) is created in deep space by strong electric and magnetic processes, stripping atoms down to the core and accelerating them to high velocities. Those high energetic particles reach the surface of Mars due to the very thin atmosphere. Like all other ionizing radiation it causes damage to material and health . On Mars, about 1.6% of cosmic rays are absorbed by the thin atmosphere, (pretty good considering the air has 0.6% as much pressure as Earth). Therefore radiation shielding is required for the habitats.

Cosmic rays come in a wide range of energies from lower energy ones from the sun, to more powerful ones from galactic sources, to the most powerful which can only be generated outside our galaxy.

Note that the lowest energy cosmic rays have similar energies to the highest energy particles from the sun, so the two blend into each other. Strategies to mitigate the strongest solar events will help with the weakest cosmic rays. Since cosmic rays come from every direction, being on a planet will automatically halve your cosmic ray dose compared to deep space, since half of the sky is blocked by the planet beneath your feet.

The atmosphere reduces cosmic ray dose. It stops completely lower energy cosmic rays. For the most powerful of cosmic rays, when they hit an air molecule, they shatter into a shower of secondary radiation traveling within 1 degree of the original direction. Much of this secondary radiation can and does reach the Earth's surface.

About 0.390 milliSieverts of radiation per year come from cosmic rays on Earth. (100 milliSieverts of radiation in a year is the smallest amount known to cause an increase of cancer; 400 milliSieverts of radiation in a short time, (under several days), might cause symptoms of radiation poisoning.) In orbit astronauts take ~150 mSV of radiation / year, but this includes radiation from the sun. (I'm trying to find solar and cosmic radiation broken out from each other but no success so far. Rick)

Composition

Cosmic radiation comprises 85% protons, 14% alpha particles, and 1% heavy ions.. Except for the rare heavy elements, this is very much like the composition of the solar wind..

Energies of Cosmic Rays:

Cosmic rays' energy is measured in Electron Volts (eV). This is the energy of one electron accelerating thru a potential difference of one volt.

Most solar cosmic rays are from tens of kilo-electron Volts (keV) to several hundred mega-electron volts (MeV). Rarely (a couple times a decade) during a Coronal Mass Ejection (CME), particles in the tens or hundreds of giga-electron Volts are generated.

Galactic cosmic rays can have arbitrarily low energies, but have maximum energies from 10 GeV to hundreds of tera-electron volts.

Extra galactic cosmic rays have energies of giga or tera electron volts (like 'local' CR) but can have peta-electron volts (PeV) or even up to 50 exa-electron volts (EeV).

Current theory says that 50 EeV particles are the highest we should detect, but occasionally (a handful of times a decade) we detect particles with more energy than this. TWO particles have been detected with over 200 EeV.

It is worth discussing the RANGE of the energies of all of the different things being lumped into the category of Cosmic Rays. Look at the SI prefixes which we are using:

• kilo = 10^3 = 1,000 -- common name is thousand.
• mega = 10^6 = 1,000,000 -- common name is a million.
• giga = 10^9 = 1,000,000,000 -- common name is a billion.
• tera = 10^12 = 1,000,000,000,000 -- common name is a trillion.
• peta = 10^15 = 1,000,000,000,000,000
• exa = 10^18 = 1,000,000,000,000,000,000

Therefore the most powerful cosmic rays are a million times, a million times, a million times more powerful than the lowest energy ones. Lumping these all together as 'cosmic rays' is not helpful. It would be far more useful if the catch bag term "cosmic rays" was split into more distinct categories.

As good rule of thumb, every ten fold increase in energy, results in a thousand fold decrease in the Cosmic Ray flux. (Flux = Particles going thru an area per second.)

https://marspedia.org/images/8/80/CR_Energy_Flux.png

See also:

Energy distribution of cosmic radiation, as measured during the 1977 solar minimum.

Point of origin

Cosmic radiation strikes bodies in the solar system from all directions, as they permeate the galaxy. How they are created is debated, but being accelerating by matter falling into a black hole, from colliding stellar remnants (white dwarfs or neutron stars), or being created by shock waves from supernova are three common explanations of their origins.

Cosmic Rays are divided into 4 general classes:

• Solar Cosmic Rays. (SCR) These are not the normal solar wind, but are rare, high-energy particles from Coronal Mass Ejections (CME).
• Galactic Cosmic Rays. (GCR) These are from sources that could conceivably be within our galaxy. ('Normal' high energy events.)
• Extra Galactic Cosmic Rays. (EGCR) These are thought to be generated by active galactic nucleus and quasars. Nothing in our galaxy should make these.
• Extreme Energy Cosmic Rays (Sometimes known as the OMG particles.) We do not have any good explanation of how these are created.

Mitigating Cosmic Rays:

Cosmic rays are blocked by planets, so people on a planet (or in close orbit around one), receive half the cosmic rays that you would in deep space. (The horizon blocks out half the sky, a bit more if you are in a valley.) Cosmic rays (and their Secondary radiation) will penetrate fairly deep thru earth and rock, so only by being deep underground can they be totally eliminated. Experiments on Earth which wish to avoid cosmic rays are done in mines more than 2,000 meters below ground.

That said, people receive cosmic rays everywhere on Earth. People living at high altitudes (which get a higher dose than those living at sea level) show no higher signs of health problems. So trying to eliminate all cosmic rays is impossible and futile. But habitats with 4 meters of water (or a couple meters of soil) between people and the sky would reduce this radiation to levels approaching the higher natural back ground radiation regions on Earth. See Radiation shielding for more details.

Note that small electromagnetic fields do not protect against cosmic rays (tho they do help with low and medium energy solar particles). If people were somehow to give Mars a magnetosphere, (say with superconducting current around the equator), it would help against low energy cosmic rays but not against higher energy ones. (Cosmic rays strike everywhere on Earth, not just near the magnetic poles.)

The only protection from cosmic rays is lots of mass, either a thick atmosphere, or a couple meters of soil. Even then, the protection is not perfect. (Everyone on Earth is bathed by cosmic rays, despite our thick atmosphere and Earth's magnetic field.) See also, albedo neutrons below.

Habitats in a Lava tube have been proposed as an inexpensive way to give many meters of protection from galactic cosmic rays.

Albedo neutrons

Cosmic rays when they hit Mars' soil will set off secondary radiation including free neutrons (called albedo neutrons). Neutrons are not ionizing radiation, but can activate other atoms, which can then beta decay (which is a form of ionizing radiation). This is a radiation source which is very low on Earth, but will add to the background radiation on Mars.

Note that the best way to slow albedo neutrons is mass made out of atoms with a low atomic number. Water (with two hydrogen atoms) is very good. Plastics (with about 2 hydrogen for each carbon atom on the carbon chain) are good for the same reason. Lithium hydroxide (LiOH) has lithium (another low atomic number atom) and a hydrogen so it is effective as well. LiOH is also considerably more dense than water, so it makes the best neutron absorber we currently know of.

Variation because of the solar cycle

The sun has a ~22 year solar cycle with 2 peaks of solar activity each cycle. At the 2 peaks of the cycle (each known as Solar Maximum) the magnetic field is stronger, there are more sunspots, and the sun is slightly hotter. There are more solar flares (coronal mass ejections). At the low point of the cycle the sun is cooler, and less magnetically active. At the peak of the cycle the more powerful magnetic field will deflect the weaker cosmic rays away from the ecliptic and redirect them towards the poles of the sun. Note that the higher power cosmic rays will punch thru regardless, so even when the solar magnetic field is at its strongest, we still get many cosmic rays.

This change in radiation is significant, the radiation increase between the very lowest and strongest solar cycles is ~75% (tho ~34% is more typical). Tho the higher energy cosmic rays penetrate, these are much rarer than the low and medium energy rays, which do get thru. Thus the significant decrease in total energy delivered.

This suggests that we can profitably trade off lower cosmic ray doses for higher solar radiation doses by launching missions during solar maximum.

The sun for the last several decades has been unusually cool, so the cosmic ray dose is relatively higher than, say, a century ago.

Thinking about comic rays:

Solar cosmic rays are relatively easy to shield against, with the exception of the rare, very high particles from the most powerful of CME. These are 10 thousand to a million times more powerful than normal. For simplicity sake, these once in a decade events can be lumped in with galactic cosmic rays, with the understanding that if they occur during local night, there is complete protection against them by the mass of Mars itself.

Galactic cosmic rays blend in (at low energies) with solar cosmic rays. The low energy ones we will basically ignore. (If we are shielding against solar CR, we will shield against these.) However, these lower energy galactic cosmic rays have an interesting property: the sun's magnetosphere gives us partial protection, so when the sun is most active during a Solar Maximum, we end up getting fewer of these low energy cosmic rays then when the sun is less active. Thus for people in space, the total cosmic ray count is LOWER when he sun is active! (Tho this is mitigated by the higher chance of a very dangerous massive solar storm.) The high energy galactic cosmic rays move so fast that they punch thru magnetic fields. Thus on Earth, we get GCR at all times of the year, near the magnetic poles and at the equator, regardless of the sun's activity. The only protection is to go deep underground. On Earth, we accept the dose of these rays, year in and year out. There is no protecting against them for normal people, so we ignore them.

The Extra-galactic cosmic rays and the OMG particles (both rare) likewise defeat any shielding, and we just have to live with them.

Now the Earth's thicker atmosphere is 62.5 times better at blocking out the high energy cosmic rays than Mars' atmosphere. (Because the mass column of Mars' atmosphere is 1.6% of what it is on Earth.) This means that Mars will have a higher radiation dose from GCR. Realistically, Martian colonists will accept a higher radiation dose, tho this can be mitigated by having a few meters of water or soil between the people and the sky.

Acute effects on equipment

A single strike by a cosmic ray can cause three types of error in electronic equipment:

• Single-event upset
• Single-event latchup
• Single-event burnout

Note that modern integrated circuits (with millions of tiny transistors), are more likely to be effected by cosmic rays than more primitive integrated circuits (with a few tens of thousand large transistors). Native Mars electronic firms may chose to optimize their products for a higher radiation environment. This is one area where lower tech Martian industries may have a local advantage over Earth's. See this report about data faults caused by radiation (all sorts of radiation, not just cosmic rays).

Chronic effects on equipment

None have been noticed after decades of work on the ISS (International Space Station).

Acute effects on life

None have been noticed on the ISS. Note that cosmic rays are a low level of back ground radiation, so they are not an 'acute' radiation dose. (Sudden bursts of high radiation are much more dangerous than low levels over a long time.) Six astronauts or cosmonauts who have spent a few long missions on the ISS or Mir space stations have received radiation doses greater (in some cases double), than the radiation dose from a 2.5 year long Mars mission. None of them have shown any health issues from radiation.

See Long duration space flight.

Chronic effects on life

Radiation effects on life have been studied for over 7 decades, cosmic rays (and their Secondary radiation) are well understood. Low level radiation exposure (tho above high back ground levels) have an aging effect on mammals. Diseases related to cellular senescence (especially cancer) become more common.

The chronic effects on life are based on what the long term level of radiation is. The average radiation level on Earth is 2.4 mSv (240 mrem) per year (note this includes ALL radiation, not just cosmic rays). However, the Iranian city of Ramsar has natural levels of radiation 30 to 50 times higher than average on Earth, and show no statistical increase of cancers or early death than elsewhere. This suggests that total radiation levels of up to 100 mSv per year, would be a reasonable target for Martian habitats. This value is 5 times higher than the (very conservative) levels of radiation allowed for workers in nuclear plants.

Since habitats have been designed, with 2 meters of dirt as radiation protection, which approach this value, a well constructed habitat would be unlikely to have a noticeable chronic effect on life. Poorly constructed habitats with little or no thought given to radiation protection would be expected to have a statistically higher chance of cancer (the most common form of disease caused by radiation), and a noticeable decrease in average lifespans.