FOURIER-TRANSFORM SPECTROSCOPY IN REMOTE-SENSING OF SOLID PLANETARY SURFACES

Fourier transform spectrometers have been used to obtain infrared emis sion spectra of solid planetary surfaces in the mid- and thermal-infra red region. This range is suitable for determining the mineralogical c omposition of the different planetary objects because even the highest -frequency fundamentals of silicates and other minerals appear in this region. However, the analysis of the emission spectra of solid surfac es is considerably complicated by the influence of morphological param eters on spectral behaviour. The surfaces of most of the airless plane tary objects are covered with a regolith blanket. Particle size, grain shape, particle size distribution, the degree of covering and porosit y of this fine-grained material strongly influence the thermal through put and the spectral behaviour in the infrared. Laboratory emittance s pectra of particulate minerals show that the spectral contrast of the fundamental vibration bands decreases with decreasing particle size. I n the spectra of mineral powders with small grain sizes, weak overtone and combination tone bands instead of the vanishing fundamentals beco me increasingly important for the identification of minerals. Therefor e, in the past infrared spectroscopy has been used rather sparingly fo r remote sensing. The chances for composition analysis of solid surfac es are discussed in relation to instrumental requirements for Fourier spectroscopy during planetary missions. An FT-IR spectrometer with its throughput and multiplex advantage is proposed for the spectroscopic sounding of the planetary surfaces. Moreover, the rotating reflector i nterferometer (RRI) is especially well adapted for space missions. It uses two retroreflectors in full aperture, integrated into a rotationa l device. The retroreflectors move along a circular path for some degr ees, tailored to give medium resolution (0.5-5 cm-1) typical for space missions. Moreover, the RRI is suitable for lightweight, rugged and c ompact construction, which has minimum sensitivity to vibrations and s hocks.