What are Asteroids Made Of?
Whenever light falls on an object like a lake, or a plant, or a rock, part of the light is absorbed, and part is reflected. How much light is reflected, and in what color (i.e., which part of the electromagnetic spectrum), determine the brightness and color the object will have to our eyes. For example, ice and snow reflect nearly 100% of the incident light all across the visible spectrum, so it appears white and very bright to the eye. Coal and soot reflects only a few percent of the incident light across the visible spectrum, so they appear dark and black. Young tree leaves reflect 10-20% of incident green light, but absorb nearly all the incident red and blue light, so they appear green and darkish. Typical rocks and soils reflect 20 to 40% of the incident light, most of it in the reddish part of the spectrum, so they commonly have relatively light brown colors.
If we measure the fraction of light reflected (R), by a particular object at each wavelength we can construct a graph of the fraction of light reflected as a function of wavelength: a reflectance spectrum. This has been done for many different materials. For example, the reflectance spectra of snow, coal, vegetation, and dry soil are shown in the Figure 1.
Figure 1. Reflectance spectra of several common terrestrial materials. The units of wavelength are microns: millionths of a meter. The visible part of the spectrum (that seen by the human eye) extends from only 0.35 to 0.7 microns, at the left end of the graph.
Notice that each reflectance spectrum is unique, like a fingerprint. The slopes and dips in each spectrum, called spectral features, are determined by the detailed types, number, and bonding of atoms in each substance. For example, light interacting with water molecules cause the deep dips in the ice spectrum at about 1.5 and 2.0 microns. Thus we can use the slopes and dips to determine what an object is made of — its composition.
Astronomers often use the term albedo for reflectance, and discuss albedo in decimal fractions. In the numerical language of albedo, the albedo of snow is about 0.99, coal is about 0.02, tree leaves are about 0.15, and common terrestrial rocks and sand are ~0.3.
Reflectance spectra have been measured for more than 500 asteroids at the time of this writing. Reflectance for a number of asteroids are shown in Figure 2. As you can see, asteroids have a wide range of overall albedos. Some, like Bamberga, are as dark as coal, while others, like Vesta and Juno, are about as reflective as common Earth rocks.
Figure 2. Reflectance spectra of several representative asteroids. The horizontal axis is wavelength in microns (10-6 meters). The visible range extends from 0.39 (violet) to 0.7 (red) microns. Wavelengths between 0.7 and 1.1 microns are called near infrared. The vertical axis is the reflectance (R), or the decimal fraction of incident light that is reflected back from the surface. These spectra were derived by combining relative reflectance at different wavelengths — the spectral features — with average albedos. The detailed spectral features were obtained by photometric measurements obtained at the Mount Wilson and Palomar Observatories and the Kitt Peak National Observatory. The average albedos were obtained from simultaneous visual and thermal measurements at Mauna Kea. Details are given in Chapman, C. R., D. Morrison, and B. Zellner (1975) Surface properties of asteroids: a synthesis of polarimetry, radiometry, and spectrophotometry. Icarus, vol. 25, pages 104-130.
Getting asteroid compositions from these spectra is not an easy task. Asteroid spectra do have distinct slopes and some show distinct dips that are characteristic of particular minerals. For example, the deep dip at 0.95 microns in the spectrum of Vesta indicates the presence of pyroxene, a mineral common in terrestrial volcanic flows. Apparently, however, the surfaces of asteroids are mixtures of minerals that tend to “wash out” distinct spectral features. Since meteorites originally come from asteroids, astronomers find it helpful to compare asteroid spectra with those characteristic of mineral combinations known to have come from space, namely, the meteorites. Sample meteorite spectra are shown in Figure 3. The meteorite spectra are drawn to the same scale as the asteroid spectra, and represent averages of the major meteorite classes.
Now the challenge begins! If you can find an asteroid spectrum that closely matches a meteorite spectrum, then you can begin to say something about the asteroid’s composition. Not only that, but with an understanding of the nature of meteorites and the geologic process that shaped them, you can begin to outline the asteroid’s geologic history as well.