Possible natural radiation shielding on Mars

bobunf · 2012-09-21 02:21:36

Researchers at the Rutherford Appleton Laboratory and the York Plasma Institute published findings in the journal Physical Review Letters and at Physics Archive (http://arxiv.org/pdf/1207.2076.pdf) showing retardation and defection of the solar wind ions from miniature magnetospheres on the Moon. The effect is noticeable from the lighter shading around these magnetospheres. The solar wind of charged particles darkens the Moon's surface, but not as much around these magnetospheres after billions of years.

The effect was experimental confirmed suggesting that magnetospheres could be artificially produced to protect humans in space, on the Moon and on Mars. This could have a huge effect on possible radiation shielding issues; a scientific and technological breakthrough with great potential.

Also, there are areas of local fossil magnetic fields on Mars. Could these provide significant radiation shielding as suggested by these observations from the Moon?

SarK0Y · 2012-09-21 12:51:53

Bobunf, how to shield efficiently against charged particles has been known for long Time. But (Gamma/X)-rays & Neutrons are really tough question.

RobertDyck · 2012-09-22 06:35:21

Free neutrons have a half-life of about 10.3 minutes, or 14.9 minutes depending how you measure it. That means they decay to a proton and electron by the time they reach the orbit of Mercury.
Reference: http://hyperphysics.phy-astr.gsu.edu/hb … roton.html

The only neutron radiation near the Moon is generated from the lunar surface by impact of charged particles. That's why Lunar Prospector was able to measure neutron radiation to gain information about lunar soil.

Last edited by RobertDyck (2012-09-22 06:39:11)

Midoshi · 2012-09-22 17:11:30

Mars' crustal fields don't do much in the way of shielding the surface from cosmic radiation. That's because any low energy particles that would be deflected by the crustal field would be absorbed by the atmosphere anyway. The moon is a different story, since it has virtually no atmosphere.

SarK0Y · 2012-09-22 17:23:57

Robert, source of Neutrons could exist on Mars too. Anyway, for effective colonization of outer Space, we need to've such shielding.

bobunf · 2012-09-23 12:06:43

I believe the article I noted above suggests Mars' crustal fields may do something significant in terms of shielding from cosmic radiation, as well as the solar wind. For the reasons noted above, I don’t think neutron radiation is any more of an issue on the surfaces of Mars and other celestial bodies than it is on Earth.

I believe the Martian atmosphere is equivalent to about 2 cm thickness of lead shielding for radiation entering at the zenith. The average attenuation would be much larger. I don't know if that means there is little problem from X-rays and gamma radiation on the surface of Mars, but it assuredly reduces the problem very considerably.

The possibility that properly selecting a location on Mars would largely eliminate radiation dangers has to be a great benefit for any possible long term human presence on Mars.

SarK0Y · 2012-09-23 12:50:25

The possibility that properly selecting a location on Mars would largely eliminate radiation dangers has to be a great benefit for any possible long term human presence on Mars.

Bobunf, it's right strategy 4 any place.

Midoshi · 2012-09-23 20:45:23

bobunf wrote:

I believe the article I noted above suggests Mars' crustal fields may do something significant in terms of shielding from cosmic radiation, as well as the solar wind.

I was unable to find any references to Mars in the paper you linked to. Did you perhaps have another paper in mind?

SpaceNut · 2012-09-23 21:26:11

Mars Magnetic Field Detected

Magnetic Mars

bobunf · 2012-09-23 22:28:03

Midoshi wrote:

I was unable to find any references to Mars in the paper you linked to. Did you perhaps have another paper in mind?

The paper does not indeed have any references to Mars. Sorry for that. I jumped from magnetic fields on the Moon to magnetic fields on Mars feeling there was a direct equivalence. The existence of an atmosphere however does change the radiation environment, I would think for the better.

High energy cosmic rays produce secondary particles, most of which are charged, and could perhaps be deflected by the Martian crustal fields. The paper suggests that local magnetic fields may be much more effective at deflecting charged particles than previously believed. I don't know if that enhanced effect would be sufficient to affect high energy cosmic rays.

I don't think interpretation of anything in the paper, or my knowledge of the subject, which is very, very limited, can go any further than saying that there is some evidence that properly selecting a location on Mars would substantially decrease radiation dangers, which has to be a great benefit for any possible long term human presence on Mars.

I would certainly be interested in any comments on this subject from individuals more knowledgeable than I.

Last edited by bobunf (2012-09-23 22:30:41)

Spaniard · 2012-09-24 05:13:58

RobertDyck wrote:

Free neutrons have a half-life of about 10.3 minutes, or 14.9 minutes depending how you measure it. That means they decay to a proton and electron by the time they reach the orbit of Mercury.
Reference: http://hyperphysics.phy-astr.gsu.edu/hb … roton.html
The only neutron radiation near the Moon is generated from the lunar surface by impact of charged particles. That's why Lunar Prospector was able to measure neutron radiation to gain information about lunar soil.

No. Half life means that in that time, it would be half neutrons. So, some neutrons always hit.

bobunf · 2012-09-24 10:14:54

If the velocity of neutrons from the sun were 1,000 kps, it would take about 1000 minutes for them to reach the orbit of mercury. That would be about 70 to 100 half lives. Even at 70 half lives, the residual number of neutrons would be about 1/10^21 of the original number.

I think one can pretty well say that there isn’t any neutron radiation coming from the Sun at the orbit of Mars.

RobertDyck · 2012-09-25 10:00:14

Neutrons from fusion travel at 17% the speed of light. These are the highest speed neutrons. Light travels at 2.99792458 x 10^8 m/s or 299,792458 km/s, so 17% of that is 50,964.71786 km/s. Since the percentage is given to only 2 significant digits, round that off to 51,000 km/s. Mercury has an eliptical orbit, it's mean distance from the Sun is 57,910,000 km. The article I linked states two times for the half-live, and I don't know why, so let's use the longer time. That works out to 45,594,000 km per half-life. So the distance to Mercury is slight more than one half-life. Mean distance from the Sun to Mars is 227,940,000 km, that works out to 4.999 half-lives. Again since the speed of a fusion neutron was given to only 2 significant digits, round that off to 5.0. After one half life, only 1/2 the number of neutrons remain. After another half life, that is reduced by 1/2 again. After another half live, that remainder is reduced by half again, etc. So the number that survive the trip to Mercury is 1/(2^5) = 0.03125. Hrmph! That's more than I thought. So how does a neutron spectrometer work when it has neutron noise from the Sun? Anyway, that's a small fraction of what leaves the Sun. And don't expect any galactic cosmic radiation to be neutron; not a single neutron would survive the trip through interstellar space.

Fast neutrons produced by nuclear fission travel at 14,000 km/s. That works out to 4.6 half-lives to reach the orbit of Mercury. So only 0.041 of the neutrons reach Mercury. And 18.2 half-lives to Mars, or 0.00000332 of the fastest fission neutrons reach Mars. That's what I was thinking of.

Last edited by RobertDyck (2012-09-26 00:39:07)

Terraformer · 2012-09-25 13:14:51

I'd be surprised if neutrons were actually emitted by the sun in appreciable quantities. They've got to escape the core first without decaying or being captured, which is very unlikely to happen.

bobunf · 2012-09-25 13:22:41

Isn't 227,940,000 divided by 51,000 = 4.47? That would give closer to 5% of the neutrons arriving at Mars.

Do not the fusion neutrons get slowed by the Sun's gravity? The escape velocity from the Sun is 618 kps, but that's from the surface. The impact of gravity from the core would be significantly higher, maybe about 800 kps?

Of course that decrease in velocity would take place over the whole 227 million kilometer journey to Mars, the effect of gravity decreasing with the inverse square of the distance, i.e., the bigger effect is felt earlier in the journey so that the average velocity would be less than 51,000-800/2.

Is there an effect from friction as the neutron travels through the 700,000 radius of the Sun?

Would the neutrons at Mars be a problem? It didn't seem to be an issue with astronauts at the ISS or going to and from the Moon. The quantity of neutrons from the Sun per square meter at Mars would be less than half the quantity at Earth because of the increasing area of the surface the neutrons are reaching. In other words the exposure would be about 2% of that at Earth.

Sounds to me like neutrons aren't much of an issue by the time one gets all the way to Mars.

Last edited by bobunf (2012-09-25 15:00:26)

RobertDyck · 2012-09-26 00:49:19

bobunf wrote:

Isn't 227,940,000 divided by 51,000 = 4.47? That would give closer to 5% of the neutrons arriving at Mars.

Fusion neutrons travel at 51,000 km/s, but the half-life is 14.9 minutes = 894 seconds. Distance travelled in a half-life is 51,000 * 894 = 45,594,000 km. Mars is 227,940,000 km from the Sun, so 227,940,000 km / 45,594,000 km/half-life = 4.999 half-lives.

Do not the fusion neutrons get slowed by the Sun's gravity?

Good point, yes they would.

Terraformer wrote:

I'd be surprised if neutrons were actually emitted by the sun in appreciable quantities. They've got to escape the core first without decaying or being captured, which is very unlikely to happen.

Yes, but does fusion only occur in the core? I'm sure most does, and you're right those neutrons would get absorbed long before they reached the surface of the Sun.

bobunf · 2012-09-26 02:11:58

Bob wrote, "Isn't 227,940,000 divided by 51,000 = 4.47?" I must have been falling asleep. Sorry.

I think the whole point of the Rutherford research to that it may be possible to find areas on Mars that would require little or no radiation protection; areas that are about as benign as Earth. That would be a huge simplification of any long term stays on Mars.

But it's an empirical question, which will only be determined by actual measurements on the Martian ground.

RobertDyck · 2012-09-26 15:40:38

My understanding is there isn't any neutron radiation in space, at least beyond the orbit of Mercury. For all the reasons already mentioned, but most importantly it's been measured. There is every other form of radiation.

Alpha radiation: this is high speed helium nuclei. This can be stopped by a single sheet of aluminum foil. A spacesuit or the hull of a spacecraft can stop it easily.

Beta radiation: high speed electrons. This is even weaker, can be stopped by a single sheet of paper, or plastic film, or the outer layer of human skin. If sufficiently intense, beta radiation can cause skin cancer, but nothing more because it can't get through skin. However beta radiation in space is not nearly intense enough to do that. So this is also not a problem.

Gamma radiation: high energy photons, higher frequency than X-rays. These are dangerous and do exist in space. They are an issue.

X-rays: also exist, and also dangerous. They have less energy than gamma, so can be blocked by metal and don't do as much damage.

proton: the Sun spews a lot of this. In fact most radiation from the Sun is proton. It's dangerous, and can penetrate deeply. However, a magnetic field or electro-static charge can deflect it. A pile of regolith (Mars soil) will also block it, although it takes about 2 metres to be effective.

heavy ion galactic cosmic radiation: this is the most nasty stuff of all. It's nuclei of heavy elements, stuff like iron. Galactic cosmic radiation comes in from outside our solar system. It has significant speed to start with, but the Sun's magnetic field pulls in it and accelerates like a particle accelerator. By the time it reaches the inner solar system, it's flying at extreme speed. The worst part of this is when it hits something, such as radiation shielding, the particles can split. Then you get multiple particles, each smaller and lighter, and each travelling slower, but still heavier than helium and moving dangerously fast. This is like breaking a bullet into a shotgun blast. Each piece passes through a different path through the human body causing damage to every cell it passes through. So light shielding can actually make it worse. Liquid hydrogen can slow heavy ions without breaking them, each collision with a liquid hydrogen molecule slows it a little, if the tank is thick enough the heavy ion particle will be slow enough that the tank wall completely stops it. But this requires a very big tank of liquid hydrogen. The second best shielding is water, which is after all mostly hydrogen (H2O). The next best is a plastic that's mostly hydrogen with a fairly light element, such a polyethylene or polypropylene, which are polymerized H2C. But it still takes several feet of the stuff for effective shielding. The good news is the atmosphere of Mars is the best shielding of all. According to reports from the MARIE instrument team at JSC, an instrument on Mars Odyssey, heavy ion radiation that reaches the surface of Mars should be about 90% blocked at a high altitude location such as Meridiani Planum, where Opportunity landed. At a location 2km below the datum such as Mawrth Vallis, one of the alternate locations considered for Curiosity, the atmosphere blocks about 99% of heavy ion radiation. So the conclusion is get out of space, get your ass to the surface.

Medium ion galactic cosmic radiation: This comes from the same source, but the particles are smaller. About half gets through the atmosphere of Mars. Splitting is not so much an issue, so regolith (Mars soil) can shield. Again you need 2 metres.

Light ion galactic cosmic radiation: Again the same source, particles are smaller yet but larger than helium. About 80% gets through the atmosphere of Mars, but particle splitting is not an issue at all. So regolith is perfect shielding. Again, 2 metres depth.

UV: this is ultraviolet light. Mars does have an ozone layer, but it so thin that it does not effectively block UV. Total intensity of UV is lower than reaches Earth, so UV-A is actually less, but without an ozone layer UV-B and UV-C get through. UV-B causes skin cancer, UV-C can sterilize microbes and cause radiation burns. However, back before Apollo NASA developed a window coating that blocks this. Thin layers of gold, nickel and silver oxide (only silver is an oxide), laid down with vacuum deposition. The result is "sun glasses" type effect for helmet visors and space craft/station windows. About 80% of visible light gets through, but 98% of UV-B and UV-C is blocked when applied to polycarbonate (helmet visor plastic), or 99% blocked when applied to glass. This is called spectrally selective coating, and has been commercialized. It's sold for building windows under the names Heat Mirror or Low-e. It also reflects IR, and be adjusted to reflect more radiant heat from short wave length IR from very hot things like the Sun, and reflect less long wavelength IR from warm things like the floor, walls or furniture. The net effect keeps heat out, every popular in the southern 'states. Or turn it around and the effect is reversed, reflect more long wave and less short wave, net effect traps heat in. Very popular in Canada. This coating has already been applied to plastic film, so can be applied to an inflated greenhouse. Since this is metal, it blocks X-rays as well as UV.

For a science mission, the most dangerous phase is getting to Mars. Once you're on Mars, we can deal with radiation.

Marie was in Mars orbit. Surface radiation is an estimate based on well established calculations, using military nuclear studies. All this will be verified by Curiosity.

Last edited by RobertDyck (2012-09-26 15:43:35)

SarK0Y · 2012-09-26 19:57:09

Anyway, we have no rights to exclude Neutrons radiation because they can be byproduct of high-energy particle interactions

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#1 2012-09-21 02:21:36

Possible natural radiation shielding on Mars

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Re: Possible natural radiation shielding on Mars

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#8 2012-09-23 20:45:23

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#10 2012-09-23 22:28:03

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#13 2012-09-25 10:00:14

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#14 2012-09-25 13:14:51

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#15 2012-09-25 13:22:41

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#16 2012-09-26 00:49:19

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#17 2012-09-26 02:11:58

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