MEASUREMENTS AND CALCULATIONS FOR ESTIMATING THE SPECTROMETRIC DETECTION LIMIT FOR CARBONATES IN MARTIAN SOIL

Theoretical considerations of Mars' climate evolution and conceptions about the weathering processes of martian crustal rocks suggest that c arbonate minerals might be present as likely secondary minerals in the soil and dust on Mars. However, only limited evidence for the existen ce of carbonates on the surface of Mars has been found. We investigate the spectral contrast (or band intensity) of diagnostic features as a function of varying carbonate content in Mars soil analogues. Specifi cally, we mixed siderite and dolomite/ankerite in samples of palagonit e and tholeiite. From measurements of spectral reflectance the carbona te band at similar to 4 mu m is chosen as the best feature for carbona te detection. At this wavelength, however, Mars' thermal emission must be taken into account. Thermal emission increases the total radiance in the considered wavelength region. Moreover, the shapes of spectral features are opposites in the reflectance and emittance spectra; i.e., a reflectance trough becomes an emittance peak. This results in parti al compensation of the diagnostic spectral features and should be cons idered in addition to the increased flux. Unfortunately, this reduces the possibility for spectroscopic carbonate detection. Furthermore, fo r increasing particle size the diagnostic spectral features are weaker . Using our results from reflectance and emissivity measurements for M ars soil analogues mixed with various percentages of carbonates, the c ontributions bf both reflected and emitted radiation can be examined. The spectra for the various mixtures show a drastic weakening of the 4 -mu m carbonate feature with increasing temperature. From these spectr a, obtained for several surface temperatures and grain size ranges, we estimate carbonate detection limits which suggest that possibly more than 7 wt% (at 275 K) or similar to 20 wt% (at 300 K) carbonate can be included in the surface but not observed spectroscopically. (C) 1996 Academic Press, Inc.