Laboratory experiments for absolute temperature sounding of cold cometary surfaces using an IR Fourier transform spectrometer

Abstract

A new method has been applied to extract absolute temperature information from the emission spectra of cold (T?-227 K) cometary analogue materials. To test the method, the emission spectra of three samples in the spectral range 710-2000 cm-1 with a spectral resolution of 15.4 cm-1 have been taken. Two of the samples have been prepared during the KOSI-7 comet simulation experiment. They consist od mixtures of water, ice and dust (kerogen,dunite and carbon). The third sample is pure montmorrillonite powder.The new method implies fitting a model curve based on Planck's law tothe emission spectra measured and solving not only for the temperature of the object in M different spectral channels. In order to solve this highly non-linear inversion problem, radiometrically calibrated emission spectra in at least M+2 spectral channels have to be taken. Ideally 3 (M+2) spectral channels should be measured in order minimize noise. The uncertainty of the calculated temperatures is of the order of +- 5 K, taking into account the errorof calibration temperatures (+- 5 K) and the drift of the spectrometer during measurement (+- 1.5% over 15 min). Further, it isshown that radiometric calibration of the Fourier transform spectrometer has to be performed using magnitude and phase spectra by applying complex mathematics. This seems to be a typical problem in a temperature range where the black-body temperature of the instrumental radiance is of the order or even higher than the temperature of the emitting object. Laboratory experiments are considered to be a good method for the measurement of spectral emittances of cometary analogue materials.