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.

Point of origin

Cosmic radiation strikes bodies in the solar system from all directions, as they permeate the galaxy. They are made of the same particles in the same proportion as those originating from the sun; and are considered part of space weather.. 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 within our galaxy.
• Extra Galactic Cosmic Rays. (EGCR) These are thought to be generated by active galactic nucleus and quasars.
• Extreme Energy Cosmic Rays (Sometimes known as the OMG particles.) We do not have any good explanation of how these are created.

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 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 million * million * million times more powerful than the lowest energy ones. Lumping these all together is not helpful.

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 a meter of water (or a half meter of soil) between people and the sky would reduce this radiation to levels approaching high altitude 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 meter or so of soil or water. Even then, the protection is not perfect. (Everyone on Earth is bathed by cosmic rays, despite our thick atmosphere and Earth's magnetic field.) Habitats in a Lava tube have been proposed as an inexpensive way to give many meters of protection from this radiation.

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! 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, far 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 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. (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 meter of water, or a half meter of 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.

Chronic effects on life

Radiation effects on life have been studied for over 7 decades, cosmic rays (and the Secondary radiation they produce) are well understood. 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 a meter 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).