Phil speculates it could be radioactivity from subsurface rocks.
Also, I think this would be a more suitable discussion for Astronomy rather than Q&A, since we don't really know of or have any straight answers to this issue yet.
Last edited by Fiery Phoenix; 2015-Jul-15 at 09:10 PM.
“Of all the sciences cultivated by mankind, Astronomy is acknowledged to be, and undoubtedly is, the most sublime, the most interesting, and the most useful. For, by knowledge derived from this science, not only the bulk of the Earth is discovered, but our very faculties are enlarged with the grandeur of the ideas it conveys, our minds exalted above their low contracted prejudices.” - James Ferguson