If you think that the Moon is only good for reflecting sunlight, you’ve got another think coming.
To human eyes, the Moon is the second brightest visible object, trailing only the Sun.
Moonlight is just reflected light generated from other sources; it’s not self-luminous.
Across the whole electromagnetic spectrum, the Sun always appears much brighter than the Moon.
Until, that is, we launched the Compton gamma-ray observatory, capable of measuring the highest-energy radiation.
The Sun, in gamma-rays, is very quiet, as its emitted radiation tops out at X-ray energies.
The Moon, on the other hand, emits very little light relative to the Sun, but outshines it in gamma-rays.
Across the full electromagnetic spectrum, only in the highest-energy gamma-rays does the Moon outshine the Sun.
This observation alone teaches us that the Moon isn’t generating its gamma-rays by reflecting sunlight.
Unlike the Sun, the Moon’s surface is made of mostly heavier elements, while the Sun is mostly hydrogen and helium.
When cosmic rays (high-energy particles) from throughout the Universe collide with heavy atoms, nuclear recoil causes gamma-ray emission.
With no atmosphere or magnetic field, and a lithosphere rich in heavy elements, cosmic rays produce gamma-rays upon impacting the Moon.
If we had gamma-ray eyes, the Moon would always look “full” from any perspective.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.