Fourier transform spectroscopy in remote sensing of solid planetary surfaces

Abstract

Fourier transform spectrometers have been used to obtain infrared emission spectra of solid planetary surfaces in the mid- and thermal-infrared region. This range is suitable for determining the mineralogical composition 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 surfaces is considerably complicated by the influence of morphological parameters on spectral behaviour. The surfaces of most of the airless planetary objects are covered with a regolith blanket. Particle size, grain shape, particle size distribution, the degree of covering and porosity of this fine-grained material strongly influence the thermal throughput and the spectral behaviour in the infrared. Laboratory emittance spectra of particulate minerals show that the spectral contrast of the fundamental vibration bands decreases with decreasing particle size. In the spectra of mineral powders with small grain sizes, weak overtone and combination tone bands instead of the vanishing fundamentals become increasingly important for the identification of minerals. Therefore, in the past infrared spectroscopy has been used rather sparingly for remote sensing. The chances for composition analysis of solid surfaces 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 interferometer (RRI) is especially well adapted for space missions. It uses two retroreflectors in full aperture, integrated into a rotational device. The retroreflectors move along a circular path for some degrees, tailored to give medium resolution (0.5–5 cm −1) typical for space missions. Moreover, the RRI is suitable for lightweight, rugged and compact construction, which has minimum sensitivity to vibrations and shocks.