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.
As seen in X-rays against the cosmic background, the Moon’s illuminated (bright) and non-illuminated portions (dark) are clearly visible in this early X-ray image taken by ROSAT. The X-rays, like almost all wavelengths of light, arise mostly from reflected emission from the Sun. (DARA, ESA, MPE, NASA, J.H.M.M. SCHMITT)
Moonlight is just reflected light generated from other sources; it’s not self-luminous.
The size, wavelength and temperature/energy scales that correspond to various parts of the electromagnetic spectrum. You have to go to higher energies, and shorter wavelengths, to probe the smallest scales. Although the Moon reflects sunlight, the most energetic photons from the Sun normally top out at X-ray energies. (NASA AND WIKIMEDIA COMMONS USER INDUCTIVELOAD)
Across the whole electromagnetic spectrum, the Sun always appears much brighter than the Moon.
This 1991 photo shows the Compton Gamma-Ray Observatory being deployed in space during April 7, 1991 from the Space Shuttle Atlantis. This observatory was humanity’s first space-based gamma-ray satellite, and was part of NASA’s original great observatories program which included Hubble, Compton, Chandra and Spitzer. (NASA/KEN CAMERON)
Until, that is, we launched the
Compton gamma-ray observatory, capable of measuring the highest-energy radiation.
A diagram of the EGRET instrument, which was used for observing the highest-energy photons aboard the Compton Gamma-Ray Observatory. The EGRET instrument is the only one capable of measuring photons with energies between about 20 MeV up to around 30 GeV: higher energy photons than the Sun typically emits. (NASA CGRO SCIENCE SUPPORT CENTER, NRA, APPENDIX G)
The Sun, in gamma-rays, is very quiet, as its emitted radiation tops out at X-ray energies.
The Sun’s light across the electromagnetic spectrum is due to nuclear fusion, which primarily converts hydrogen into helium. The nuclear reactions produce neutrinos and radiation that extends from the radio all the way up into the X-ray, but gamma-rays are only produced rarely: during flaring events. (NASA/SOLAR DYNAMICS OBSERVATORY)
The Moon, on the other hand, emits very little light relative to the Sun, but outshines it in gamma-rays.
Between 1991 and 1994, the Moon passed into the Compton Gamma-Ray Observatory’s field-of-view multiple times, where the instrument was capable of observing it. Compton detected high-energy gamma-rays from the Moon with its EGRET instrument, and the energy spectrum of the lunar gamma radiation are consistent with a model of gamma ray production by cosmic ray interactions with the lunar surface. The Moon is brighter than the (non-flaring) Sun in these high energies. (D. J. THOMPSON, D. L. BERTSCH (NASA/GSFC), D. J. MORRIS (UNH), R. MUKHERJEE (NASA/GSFC/USRA))
Across the full electromagnetic spectrum, only in the highest-energy gamma-rays does the Moon outshine the Sun.
A thin crescent moon, just one day after the new moon, sets in the west. The remaining disk is still illuminated by the light reflected from Earth that’s then incident upon the lunar surface. The fact that the Moon always appears full in Gamma-Rays, even when just a thin crescent is illuminated by the Sun, teaches us that it isn’t reflected sunlight that’s causing these lunar Gamma-Rays. (NEAL SIMPSON OF FLICKR)
This observation alone teaches us that the Moon isn’t generating its gamma-rays by reflecting sunlight.
Using data from NASA’s Lunar Reconnaissance Orbiter (LRO) and its narrow angle camera (LROC), we can now construct 3D models of the surface of the Moon and simulate any potential landing sites for missions. Our current understanding teaches us that the Moon’s surface is made of many heavier elements, is surrounded by practically no atmosphere at all, and has a negligible magnetic field. This combination of factors basically creates ‘the perfect storm’ for generating gamma-rays from high-energy nuclear recoils. (NASA / SVS / LROC)
Unlike the Sun, the Moon’s surface is made of mostly heavier elements, while the Sun is mostly hydrogen and helium.
The only time the Sun produces gamma-rays is during flaring events, when accelerated, high-energy protons can collide with heavier nuclei, producing an excited-state nucleus that emits gamma-rays. During quiet conditions, these fast protons will only interact with hydrogen or helium nuclei, which do not produce these gamma-rays. On the Moon’s surface, however, heavy nuclei abound, and the creation of excited-state nuclei that then emit gamma-rays is ubiquitous. (NASA/GSFC/GORDON D. HOLMAN)
When cosmic rays (high-energy particles) from throughout the Universe collide with heavy atoms, nuclear recoil causes gamma-ray emission.
Cosmic rays produced by high-energy astrophysics sources can reach any object in the Solar System, and appear to permeate our local region of space omnidirectionally. When they collide with Earth, they strike atoms in the atmosphere, creating particle and radiation showers at the surface. When they strike the heavy elements present on the Moon’s surface, they can induce a nuclear recoil/reaction that winds up producing the high-energy gamma-rays we observe. (ASPERA COLLABORATION / ASTROPARTICLE ERANET)
With no atmosphere or magnetic field, and a lithosphere rich in heavy elements, cosmic rays produce gamma-rays upon impacting the Moon.
Although the Sun doesn’t typically generate either gamma-rays or cosmic-rays that account for what we see on the Moon, its complex magnetic field undergoes cyclical changes on an 11-year timescale. These changes can alter the gamma-ray flux from the Moon, over time, by up to about 20%. (© NASA’S GODDARD SPACE FLIGHT CENTER/BRIDGMAN)
If we had gamma-ray eyes, the Moon would always look “full” from any perspective.
With 7 panels of ever-increasing observing time, from 2 months up through 128 months, we can see how a gamma-ray image of the Moon becomes sharper and sharper over time. This image was taken by NASA’s flagship gamma-ray observatory, Fermi, in energies of 31 MeV or higher. In these high-energy gamma-rays, the Moon indeed outshines the Sun. (NASA/DOE/FERMI LAT COLLABORATION)
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.