| The behavior of frozen carbon dioxide in the Martian polar caps is partly controlled by its optical properties. Modeling of polar deposits and clouds for the interpretation of remotely sensed measurements and for climatic energy balance calculations requires optical constants for CO{dollar}sb2{dollar} ice. Although thin film samples have been used to measure the real and imaginary indices of refraction in the strongly absorbing infrared wavelength bands and the real index in the visible wavelengths, very little accurate information has been available for the much weaker absorption between the strong bands. These weak absorption coefficients determine the emissivity of CO{dollar}sb2{dollar} frosts, such as in the Martian seasonal polar caps. A laboratory experiment was undertaken to improve on and extend the wavelength range of the previous data by measuring the transmission through thick samples of high quality, and determining the spectral absorption coefficient in wavelength regions of low absorption in the infrared spectral range 1.8-333 {dollar}mu{dollar}m (30-5555 cm{dollar}sp{lcub}-1{rcub}{dollar} in wavenumber). The optical path length between the two window surfaces in the sample chamber was adjustable from 1.6 to 107.5 mm in 5 geometrically spaced increments. The path length range allowed for accurate measurement of absorption coefficients from about 0.1 to 4000 m{dollar}sp{lcub}-1{rcub}{dollar}. Three window materials (CaF{dollar}sb2{dollar}, CsI, and crystal quartz) were used to span the measured wavelength range. A technique was developed to grow clear, thick CO{dollar}sb2{dollar} ice samples from the gas at a temperature of 150 K. The most important aspect of growing clear ice samples was strict control of the temperature. Techniques were developed to quantify the scattering of light by these samples; it typically contributed less than 20% of the total extinction. The spectral absorption was measured using a Fourier transform spectrometer. Four beamsplitters and three detectors were used to cover the wavelength range. The finest wavenumber resolution used was 0.14 cm{dollar}sp{lcub}-1{rcub}{dollar}. Wavenumber calibration was accomplished by examining water vapor absorption lines in the spectra. |
