Use of maximums in the infrared emission of particulate surfaces for compositional remote sensing is a recent development that has not been thoroughly investigated. We show that experimental conditions, such as particle size and packing, background temperature, atmospheric pressure, and uniformity of sample heating, all affect the wavelength of the emission maximum and the contrast of the spectrum. These effects emphasize the necessity for laboratory study of emission spectra under conditions which simulate the remote environment to provide appropriate information for correctly deciphering remote sensing data. When experimental conditions are held constant, simulating those on the moon, emission maximums are shown to be diagnostic of rock type.

The emission spectra of single particles of inorganic solids as a function of particle size have been recorded from 6 micro to 11.8 micro. For small optically thin particles, an emission maximum is produced in the reststrahlen region. The emission behavior is dominated by scattering and can be adequately described in terms of Mie absorption efficiency factors. As the particle size is increased, the emission band reverses its polarity, and the spectrum approaches that of a polished plate. The data provide source functions necessary for determining the emission behavior of particulate samples in which temperature gradients exist, such as on the lunar surface. The data are of particular interest for interpreting the spectral behavior of circumstellar silicate particles.

Infrared spectra have been acquired under simulated lunar conditions that demonstrate that, contrary to popular belief, features of high spectral contrast are available for small‐particle‐size samples. The spectral information occurs in the form of emission maxima that are associated with the principal Christiansen frequencies, and these maxima are diagnostic of gross composition. The features represent a 5 to 30% effect, depending on particle size and composition. The effect is explained in terms of the sharp thermal gradients produced close to the surface under lunar conditions.

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