If you could see gamma rays [ http://antwrp.gsfc.…] - photons with a million or more times the energy of visible light - the Moon would appear brighter than the Sun! The startling notion is demonstrated by this image of the Moon from the Energetic Gamma Ray Experiment Telescope (EGRET [ http://cossc.gsfc.n…]) in orbit on NASA's Compton Gamma Ray Observatory [ http://cossc.gsfc.n…] from April 1991 to June 2000. Then, the most sensitive instrument of its kind, even EGRET could not see the quiet Sun which is extremely faint at gamma-ray energies. So why [ http://www.aas.org/…S025002.html ] is the Moon bright? High energy charged particles, known as cosmic rays [ http://imagine.gsfc…cosmic_rays.html ], constantly bombard the unprotected lunar surface generating gamma-ray photons. EGRET's gamma-ray vision [ http://cossc.gsfc.n…index.html ] was not sharp enough to resolve a lunar disk or any surface features, but its sensitivity reveals the induced gamma-ray [ http://antwrp.gsfc.…] moonglow. So far unique [ http://glast.gsfc.n…], the image was generated from eight exposures made during 1991-1994 and covers a roughly 40 degree wide field of view with gamma-ray intensity represented in false color.
