Cosmic Rays and Chemicals

Did you know that, seen in high energy gamma rays, the Moon is actually brighter than the Sun? Remarkably, the photons that make up this image carried over 20 million electron volts of energy each. That already sounds like a lot, but to put it into perspective, a single photon of red light only carries 2 electron volts. The image was taken by the Compton gamma ray observatory which, between 1991 and 2000, gave us our first proper view of the gamma ray Universe; simultaneously, we were given a remarkable insight into the high energy processes that occur constantly on our nearest celestial neighbour’s surface. Every second, the Moon is being bombarded by millions of cosmic rays. These cosmic rays interact with atoms, causing reactions which release energies much higher than stellar fusion ever could…

A few of the responses to my last entry gave me the impression I should explain a little more about the Moon and some of the unusual forces that are acting there. This is all about the little known process of cosmic ray spallation. Now, I’ve written about cosmic rays before. They’re high energy particles. A soup of protons, stray electrons and atomic nuclei (mostly helium nuclei) is constantly being spat out by the fusion reactions going on in the Sun,* and these bombard anything in their path. If they should happen to collide with the nucleus of an unsuspecting atom, a cosmic ray contains enough energy to simply shatter it!

While we’re quite well protected from the brunt of this assault down here on Earth, by a nice thick atmosphere and a magnetic field, some amount of cosmic rays still get through. Believe me. They make a mess out of the data that astronomers like myself try and collect. The Moon, however, receives no such protection. For 4 billion years now** the Moon has been bombarded constantly by these cosmic rays. They’ve had plenty of time to fragment those elements on the lunar surface into lighter ones. That fragmentation is known as cosmic ray spallation, and it’s the only means by which certain light elements can be effectively created. Lithium (Li), Beryllium (Be) and Boron (B) are three elements which cannot be created by stars. Stellar fusion creates most of the elements that make up everything around (and including) us, but Li, Be and B are destroyed by stars. Instead they’re fused into carbon or broken up into helium. As a result, these really are three of the rarest elements in the Universe.

Another thing found on the lunar surface is Helium-3, also known as 3He. 3He is useful because it’s one of the best fuels for nuclear fusion. The biggest supply of helium, and thus 3He, in the Solar system is the Sun itself. Unfortunately, the Sun is a star, and so it’s completely inaccessible to us. However, the Sun is constantly streaming particles away from itself. Amongst the cosmic rays are fusion byproducts, or unfused nuclei such as 3He. The Moon mops these up like a sponge, and has been doing so for billions of years. This handy little map shows the expected abundances of 3He on the lunar surface (red areas have higher concentrations). If the map looks vaguely familiar, it’s because the basaltic minerals that make up the Moon’s mares are expected to contain more 3He. Should humanity ever attempt to start a fusion economy, those dark patches on the lunar surface are simply peppered with the best supply of 3He for several million miles.

*And if those are scary, there are events out in the Universe like supernovae, black hole accretion, quasars, and as yet unidentified phenomena which all produce cosmic rays with energies so high that they dwarf anything produced by the Sun!

**And don’t forget that the Universe is only 13.7 billion years old. That’s a substantial fraction of the total age of the Universe.

Images:
Gamma Ray Moon – NASA/Compton
Lunar 3He Map – Lunar Networks Blog

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
This entry was posted in Imported from Livejournal, physics and tagged , , , . Bookmark the permalink.

11 Responses to Cosmic Rays and Chemicals

  1. Pingback: Black hole nucleosynthesis | Supernova Condensate

  2. havoc_theory says:

    Neutron flux is a major concern due to the safety hazards to the personnel and material degradation because of the induced radioactivity and the rise of lattice defects. He3 is said to produce no neutrons at all, but it’s oversimplification because there will be D-D reaction going in parallel.
    You see, Coulomb repulsion contributes to the minimal temperature, density and confinement time required to achieve the breakeven. E.g. see Lawson criterion. The increase of temperature or density inevitably means the increase of the energy loses and it is very nonlinear (sigma*T^4 and such).

  3. maxdwolf says:

    Re: Mining Helium
    Ah, whoops. I was clearly mixing it up w. one of the dozens of board games I’ve seen up there. I’m guessing there are retrofits for playing this software on modern machines out there somewhere. I’ll put that search on my to do list.
    Of course, one of the big differences is that no small tribe could just go out on its own. Also it can’t be broken down to such small increments as most terrestrial colonization has been. Yes, the Europeans did some big leaps on sailing ships. But everywhere they went they found people who already had done it. The exception that really impresses me are those people that spread throughout the Pacific sans compass or timepiece.

  4. invaderxan says:

    Re: Mining Helium
    Heh… It’s a retro video game about the arduous journey to the West coast of North America that was faced by early settlers.
    http://www.virtualapple.org/oregontraildisk.html
    Colonisation has always been difficult business, I guess…

  5. maxdwolf says:

    Re: Mining Helium
    I have some friends up north that hold a regular game night that might. I’ll see if I can get in a game if ever I’m up there again.

  6. invaderxan says:

    Re: Mining Helium
    Ever play Oregon Trail? It seems fairly analogous to me… ;)

  7. invaderxan says:

    Oh, believe me I know. Unfortunately, while nuclear reactions can do what the alchemists once dreamed of and actually transmute one metal into another, on a large scale the process is… not easy. Not easy and not cheap. I don’t think I’m actually exaggerating when I say, it would likely be cheaper to mine the moon than produce these things on Earth.

  8. invaderxan says:

    Actually, neutron flux isn’t relevant to the “frenzy.” I doubt neutrons would be desirable in a fusion reactor. While useful in fission reactors, neutrons don’t participate much in the kind of fusion reactions which would be viable for energy production. Coulomb repulsion is a valid point, but at the kind of kinetic energies involved, I don’t believe it’s a major consideration. 3He fusion forms part of the proton-proton chain of reactions in stars. If coulomb repulsion was a major hindrance, it wouldn’t form one of the most basic stellar fusion reactions.
    The reason why 3He is considered the best fuel for nuclear fusion is simply due to its output. The fusion of two 3He nuclei gives out approximately 12.9 MeV of energy — amongst the highest of all the feasible fusion reactions. The only fusion reactions which put out more energy utilise 6Li (also found on the Moon). Per unit mass, this gives 3He arguably the highest energy density of any known potential fuel.

  9. maxdwolf says:

    Mining Helium
    I cannot address most of the objection put forward. But I would like to point out that we have technologies on the table that may well replace chemical rockets within the next two or three generations. Also, after the initial venture I’m thinking there would be shipments up of only those resources that could not be synthesized, mined, or recycled at the mining colony.
    Still very high costs, but perhaps worth it in the future depending on needs and returns.

  10. Is there any analogous process that could be used to easily produce Li/B/Be on Earth? I mean, Li and B are just SO FREAKING IMPORTANT to organic chemists, and Be is so useful for x-ray optics…

  11. havoc_theory says:

    I do not understand this widespread He-3 frenzy.
    First, neutron flux is not the main problem with fusion. Actually, it could be desirable property for the hybrid reactors.
    Second, He-3 has two protons and it doubles its charge and effectively quadruplies Coulomb repulsion compared to hydrogen isotopes. And we didn’t achieve industrial break-even for hydrogen.
    Third, I’ve seen figures like 10 ton/yr of He-3 would suffice to fuel entire Earth economy. Quick estimate gives that would mean mining some one billion tons of regolith yearly (anybody check?), which resembles the annual mining of iron ore worldwide. Now imagine putting the industry of this scale on the Moon, given everything we have are chemical rockets.

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