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.
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