Why is the moon so bright?
Published: August 6, 2015
The moon is actually quite dim, compared to other astronomical bodies. The moon only seems bright in the night sky because it is so close to the earth and because the trees, houses, and fields around you are so dark at night. In fact, the moon is one of the least reflective objects in the solar system. The DSCOVER spacecraft captured this single photograph of the moon and the earth. Both the earth and the moon are illuminated by the same amount of sunlight coming from the same angle in this photo. As you can see in this photo, the earth is much brighter than the moon.
In general, we can see objects because they direct light into our eyes (or into cameras which record information that is later used by display screens to direct light into our eyes). There are two main ways that an object can direct light into our eyes. Either the object creates new light or it reflects light that already existed. Objects that create light tend to also reflect ambient light, so that they tend to be the brightest objects around. Examples include campfires, light bulbs, candle flames, and computer screens. In terms of astronomical bodies, stars are the main objects that create significant amounts of visible light, and therefore are some of the brightest objects in the universe. In contrast, planets and moons do not generate their own visible light*. If a planet somehow became large enough to initiate nuclear fusion and begin glowing, it would no longer be a planet. It would be a star.
Since planets and moons do not emit light, the only reason we can see them is because they reflect light from some other source. The strongest source of light in our solar system is the sun, so usually we see planets and moons because they are reflecting sunlight. The amount of sunlight incident on a moon or planet that gets reflected depends on the materials in its surface and atmosphere as well as its surface roughness. Snow, rough ice, and clouds are highly reflective. Most types of rock are not. Therefore, a planet that is covered with clouds, such as Earth or Venus, is generally brighter than a rocky moon or planet that has no atmosphere.
There are two main types of reflectivity: specular reflectivity and diffuse reflectivity. Specular reflectivity measures how much of the incoming light gets reflected by the object in the direction given by the mirror angle. In contrast, diffuse reflectivity measures how much light gets reflected in all directions. A mirror has high specular reflectivity and low diffuse reflectivity. In contrast, sand has low specular reflectivity and high diffuse reflectivity. In everyday life, we experience specular reflectivity as the perception of mirror images and glare spots on the surface of objects. We experience diffuse reflectivity as a somewhat uniform brightness and color that exists on the surface of the object and is roughly the same no matter what our viewing angle is. Many objects display significant amounts of both specular reflectivity and diffuse reflectivity. For instance, a red polished sports car looks red from all angles because of its diffuse reflectivity, while at the same time displays bright spots of glare because of its specular reflectivity. In general, roughening a surface tends to increase its diffuse reflectivity and decrease its specular reflectivity. This is true because a rough surface has many little reflecting planes all oriented differently which scatter light in many different directions. In fact, the easiest way to turn a strong specular reflector into a strong diffuse reflector is to roughen it up. For instance, take a smooth sheet of ice and scratch it up. You turn a surface that is bright only in the mirror direction of the light source into a surface that bright in all directions.
When it comes to planets and moons, the surface roughness is quite high. For this reason, their overall brightness is best described by their diffuse reflectivity. There are several ways to define and measure the diffuse reflectivity. In the context of planets and moons, the common and perhaps most useful way is to define it in terms of "bond albedo". The bond albedo is the average amount of total light scattered by the body in any direction, relative to the total amount of light that is incident. A bond albedo of 0% represents a perfectly black object and a bond albedo of 100% represents an object that scatters all of the light. The earth has a bond albedo of 31%. In contrast, the moon has a bond albedo of 12%. To bring this closer to home, the moon has the same bond albedo as old asphalt, such as is found in roads and parking lots. The bond albedo of major objects in our solar system are listed below as reported in the textbook Fundamental Planetary Science: Physics, Chemistry, and Habitability by Jack K. Lissauer and Imke de Pater.
As this table makes clear, the moon is one of the dimmest objects in our solar system. If Triton, one of Neptune's moons, were to become the moon of the earth, then it would be about seven times brighter in the night sky than our current moon. Triton is bright because almost all of its surface is covered by several layers of rough ice. In contrast, earth's moon is so dark because it contains very little ice, snow, water, clouds, and atmosphere. The moon consists mostly of rock dust and dark rocks that are similar in composition to rocks on earth. The albedo values in the table above are averages since the albedo varies through time. For example, the number of clouds covering the earth varies from season to season. Therefore, the albedo of the earth varies a few percent throughout the year.
The perceived brightness of a planet or moon (i.e. what we see with our eyes), depends on three things: (1) the object's albedo, (2) the total amount of light that is hitting the object in the first place, and (3) the distance between the object and the eye or camera that is viewing it. Planets and moons that are closer to the sun receive much more sunlight and therefore generally have a higher perceived brightness. Also, planets and moons that are closer to the earth have more of their reflected light reach the earth and therefore generally have a higher perceived brightness as seen from earth. The moon indeed looks brighter than Venus to a human standing on earth's surface, but that's just because the moon is so close to earth.
*Note that many planets and moons can create small amounts of light through localized phenomena. Examples of such phenomena include lightning, glowing lava, and atmospheric aurora. While such phenomena can lead to stunning photos when captured by nearby spacecraft, they generate such little light that they do not contribute significantly to the brightness of the planet or moon when viewed from a distance.