Mars Infrared Spectrometer : Lessons for Future Exploration

Introduction: Spectra returned by the 1969 Mariner Mars Infrared Spectrometer (IRS, 1.8-14.2 µm) have been used in a range of studies of the surface composition of Mars. IRS measured a wide spectral range with good spectral resolution and excellent signal to noise ratio (SNR). Table 1 lists spacecraft spectrometers that have returned spectra of Mars. IRS results, as well as a comparison of IRS, the 1971 IRIS, 1989 ISM, and 1997 TES spectra show that an unambiguous examination of the surface mineralogy of Mars requires: (1) high spectral resolution (~2-5 cm-1 in the thermal region, and ~10-20 cm-1 in NIR); (2) high SNR (>600); and (3) continuous spectra measured of the VIS-NIR, overtone, and fundamental regions. Radiometer measurements, such as those provided by Viking IRTM and the proposed 2001 THEMIS, can be used to map spectral type regions. However, fully utilizing a radiometer data set will require complementary high quality spectra for an unambiguous examination of the minerals present. Instrument: IRS used two circular variable interference filters with ~1% resolution to scan continuously two channels, from 1.8 to 6.0 µm and 3.7 to 14.2 µm [1]. Each spectrum contains approximately 1340 discrete measurements, scanned every ten seconds. Continuous spectrum: Huguenin [2] noted that the assignment of a single band is frequently ambiguous, while the assignment of a suite of bands is generally not. Continuous spectra such as those recorded by IRS allow an examination of more than one spectral region and feature, and so the least ambiguous study of surface mineralogy. Spectral range: The infrared spectrum can be divided into three approximate ranges that exhibit distinct behavior: VIS-NIR, (0.4-~3 µm); overtone, (~3-7 µm); and fundamental, (~7-50 µm). Each region contains complementary information. The information content of a spectrum increases significantly when it covers more than one spectral range. VIS-NIR region. This region measures mainly reflected light (0.4-3 µm). It is simpler to use for quantitative measurements of mineral abundance, and atmospheric corrections are also simpler than for thermal wavelengths. IRS shows that this range can be combined with thermal wavelengths to provide more information on the surface than either wavelength range alone [3]. Overtone region. This region (~3-7 µm) is particularly important for studies of Mars because finely-particulate minerals, such as those predicted to occur on Mars, commonly have strong, diagnostic overtone features, but very weak fundamental features (which