What are Asteroids made of?

Asteroid Colors

Color is one of the important qualities that we use to identify the nature of objects in the natural world around us: the sky is blue, plants are green, snow is white and rocks are brown. Even in the city, colors tell us about what things are made of: cement is gray, marble is white, bricks are red, metals are silvery-gray, and blacktop is, well, black. Colors of objects also tell us things about time and processes, too: green leaves turn red and then brown when they die, iron turns from silvery-gray to red as it rusts, and fresh blacktop turns grayish black as traffic wears it wears it down.

Colors in nature

Figure 1. Colors in Nature. Spring leaves, fall leaves, red bricks, white marble, rusty nail, and fresh blacktop.

The color an object appears to the eye depends on the color of the light shining on it (everything looks blue in a blue light), and the colors of the spectrum the object reflects. Green leaves appear green when you shine a white light on it because it reflects light in the green part of the spectrum while it absorbs light in the red an blue part of the spectrum. Similarly, bricks appear red because they absorb green and blue light and reflect red light.

If we describe or measure the color of an object carefully, we can tell the difference between different classes of objects. For example, the green of oak tree leaves is a little different from the green of pine needles or the green of rye grass. Young leaves are often a lighter shade of green than full-grown leaves. Unfortunately, the many different kinds of objects that exist show literally millions of different colors. How can we identify all those different colors? We can, and do, use names to define particular colors, like yellow-green or reddish brown. But for millions of colors you need millions of names, and that can become a barrier to communication. For example, if a friend of yours told you about a flower with cerulean-colored blossoms, would you know what she meant? It can also get a little weird. Just visit your local paint store and read some of the names paint companies give to their hundreds of different paint colors, like “Morning Mist.” How would you know what color it was without a paint chart? How would you make such a color?

There is another way to define different colors, however, and that is to use numbers, specifically to use ratios or percentages of specific, well-defined colors. Paint companies actually use this approach: no matter how strange the name, each color is defined by a specific percentage of each of the three colors cyan, magenta, and yellow to be mixed in a gallon of white paint. Similarly, colors on your computer screen are defined as definite mixtures of the colors red, green, and blue. For example, let’s define a color we’ll call “asteroid blue” as a mix of 82 parts blue, 48 parts green, and 25 parts red, where the parts are measured (fractions of) a scale from 1 (dark) to 255 (bright). Anyone who wants to make asteroid blue simply mixes together the right fractions of blue, green and red.

Color recipe

Figure 2. The recipe for making “asteroid blue.”

The colors of asteroids are defined by ratios of different specific, well-defined colors. One set of colors used to identify asteroid colors is: Ultraviolet, Blue, Visual (green), Red and Infrared, or UBVRI for short. Astronomers measure the brightness of an asteroid in each of the five colors and then form ratios that they plot on graphs. Figure 3 is a graph comparing two of the possible color ratios that can be formed from the five colors: B/V and U/B. The measured color ratios of different asteroids plot in clusters in different parts of the graph. Astronomers have defined different classes of asteroid types based on their color ratios. Five of the more important classes are shown in the graph. Descriptions of each class are found in Table 1.

Color diagram for asteroids

Figure 3. U-B vs. B-V Color Diagram for Asteroids. The letters in the legend refer to different classes of asteroids in Table 1. Click the image for a larger version.

If you wanted to find out the class of an asteroid you were investigating, all you need to do is measure the UBVRI colors of the asteroid, form the U/B and B/V ratios, and plot them in Figure 3. Your determination of class may not be exact, because some of the groups of points overlap somewhat, but you would have a much better idea of the type of asteroid you are investigating.

Once you know the class of an asteroid, you can relate it to similar classes of meteorites (Table 1 — see “The Meteorite Connection” for information about meteorite types), and through the meteorites to terrestrial rocks and geologic processes. Once you know something about the composition of an asteroid and the processes that have operated on its surface and interior, you can begin to describe its geologic history (see “Asteroids, Meteorites, and Geologic Processes”). So just by measuring the colors of an asteroid, you can learn a lot about it.

Table 1. Asteroid Classes

C "Carbonaceous" type, like C1 and C2 carbonaceous chondritic meteorites
S "Silicacious" type, like ordinary chondrites
M "Metallic" or "stony-iron" types, like irons or stony-iron meteorites
B "Modified Carbonaceous" type, like C3 and C4 carbonaceous chondritic meteorites
V "Vesta" type, like basaltic achondrite meteorites

Note: there are several other classes of asteroids, including F, G, E, etc., which represent variations on these basic types.