If your eyes were sensitive to every form of light, a rainbow wouldn’t end at red and violet. Beyond red you’d see infrared, which we sense as heat, and beyond that the microwaves that cook your food and radio waves that carry music and news to your FM radio. After violet lies ultraviolet (responsible for sunburn), X-rays to peer beneath your skin and finally high-powered gamma rays like those released by highly radioactive materials.
The moon looks white to our eyes because it reflects light of all colors in the rainbow. Combined they make white. The moon like the planets is visible by reflected sunlight. Take away the sun and every moon and planet would go dark. But if our eyes could see the high-energy radiation called gamma rays, the moon would appear brighter than the sun. That’s how NASA’s Fermi Gamma-ray Space Telescope has seen our neighbor in space for the past decade.
Gamma-ray observations are not sensitive enough to clearly see the shape of the Moon’s disk or any surface features — it can only detect a bright glow at the moon’s position in the sky — but images have greatly improved over the years. What makes the moon glow so brightly even without the sun to illuminate it? Fast-moving particles called cosmic rays.
Cosmic rays are mostly protons, the little subatomic nuggets the form the nuclei of atoms. Hydrogen has one proton in its nucleus, oxygen has eight and gold 79. More protons per atom is the reason gold is so much heavier than oxygen. Cosmic rays are mostly single protons accelerated by violent phenomena in the universe like supernova blasts and high-speed jets of particles produced when matter falls into black holes. Like a well-struck ball with a baseball bat, the events propel many of them at near-light speeds across the universe.
Because the particles are electrically charged (a proton has a positive charge, an electron has a negative charge), they’re strongly affected by magnetic fields, which the moon lacks. Unable to screen out the particles, even low-energy cosmic rays can reach the surface. When cosmic rays strike, they interact with the minerals in the powdery lunar soil called regolith and produce gamma-rays. The Moon absorbs most of these gamma rays, but some of them escape into space and are captured by Fermi.
As NASA prepares to return humans to the moon through the Artemis program, these gamma-ray observations are a reminder that astronauts will require protection not only from solar storms but from the same cosmic rays that produce this high-energy gamma radiation.
While the moon’s gamma-ray glow is amazing, the sun still shines brighter in gamma rays that have even higher energies. The sun screens out lower energy cosmic rays because of its magnetic field, but the more powerful ones strike its outer atmosphere and produce gamma rays that reflect back into space.
Because cosmic rays reach the moon from all over the galaxy (and even beyond its confines), the moon lacks its usual cycle of monthly phases, maintaining a full-moon appearance every day. One thing that does change is the moon’s gamma brightness which varies by about 20 percent over the sun’s 11-year activity cycle. Variations in the intensity of the Sun’s magnetic field during the cycle change the rate of cosmic rays reaching the Moon and along with it how many gamma rays are produced.
Don’t you wish you could just flick on a filter and see the universe in ALL of its colors? That’s what astronomers do every day with space-based telescopes like Hubble, Fermi and Chandra — get above the air that obscures so much of the multi-hued rainbow known as the electromagnetic spectrum.