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.