Spectral anomalies in the 11 and 12 μm region from the Mariner Mars 7 Infrared Spectrometer

Two hundred‐forty infrared spectra acquired by the 1969 Mariner Mars 7 Infrared Spectrometer (IRS), spanning the wavelength region 1.8–14.4 μm (5550–690 cm−1), have recently been recovered and calibrated in both wavelength and intensity. An examination of these IRS spectra has revealed absorptions at 11.25 and 12.5 μm that have not previously been reported for Mars. A search of the literature and spectral data bases shows that materials that exhibit a doublet at 11.25 and 12.5 μm are rare. In this paper we examine potential causes for these features and include a detailed discussion of carbonates, goethite, CO2 ice, and water ice. CO2 ice and water ice measured in transmission do not exhibit bands that match those recorded at 11.25 and 12.5 μm for Mars, which indicates that CO2 or water ice clouds are not the source of these features. Since these bands show no clear correlation with atmospheric path length, they are most likely caused by a surface material. In the IRS database they appear to be exceptionally intense in the western part of the Hellas basin. Goethite exhibits bands that are a good spectral match, but confirming whether goethite causes the features will require additional studies of the 20–50 μm region. These studies will require laboratory measurements of weathering coatings and an examination of spectra recorded of Mars by the 1971 Mariner Mars Infrared Interferometer Spectrometer (IRIS; 5–50 μm, 2000–200 cm−1) and the 1996 Thermal Emission Spectrometer (TES; 6–50 μm, 1667–200 cm−1).