A Mixed-Up Universe
Using the emissivity spectrum from a pure material, it is easy to identify its composition. The emissivity of quartz for example, is different from that of every other substance in the universe. But materials in the universe rarely are found in their pure state. What happens to emissivity when it is measured from a mixture of materials? Fortunately, the emissivity spectrum from the individual components of a mixture add together in a simple linear fashion. A rock for example, is a mixture of different minerals. If it is composed of quartz, feldspar, and hornblende, its emissivity spectrum will appear as a mixture of all of the absorption features of each mineral. A mixed spectrum at first may appear to be hopelessly convoluted, but the fact that the component spectra add together linearly allows it to be deconvolved.
Unmixing: Making Sense of It All
Any attempt to deconvolve (unmix) a mixed emissivity spectrum requires a library of pure spectra. The features of a mixed spectrum can be compared systematically to those of the endmember spectra from the library. Individual components of the mixture are identified by matching the features of its spectrum to those in the library. In addition to identifying what is there, we can also tell how much. The depth of the absorption features in the mixed spectrum is directly related to the abundance of the endmember components. For rocks, the ability to identify the minerals and how much of each is present, tells us what kind of rock it is.
