Ultraviolet Spectrometer
Spectral measurements were made of the UV radiation emitted from the Martian atmosphere due to resonance scattering of solar radiation from the upper atmosphere, resonance reradiation, fluorescence, and photoelectron excitation of neutral and ionic constituents found in the lower part of the atmosphere. The following parameters were determined: the presence of certain atoms, ions and molecules in the upper and lower atmosphere, their respective scale heights, the degree of atmospheric Rayleigh scattering due to carbon dioxide, and surface reflectivity in the UV. The instrument was an Ebert-Fastie scanning monochromator with dual photomultiplier detectors, used in the focal plane of a reflecting planetary coronograph. Incoming light passed through a baffled light shade and struck the primary telescope mirror, which focused the light through a preslit onto a secondary mirror. From there, the light was focused onto the entrance slit of the spectrometer. Entering the spectrometer, the radiation was collimated by the first half of the Ebert mirror onto a diffraction grating. Diffracted light was then focused onto exit slits by the second half of the Ebert mirror. A separate exit slit was provided for each of the two detectors. The position of the spectral images with respect to the exit slits was controlled by cyclically scanning the grating, with a scan from low- to high-wavelength taking 2.82 s, and the grating return taking 0.18 s. The wavelength region from 1900 A to 4300 A was covered in first order as seen by one of the two slits, and the range from 1100 A to 2100 A measured in second order by the other. The photomultiplier detector used for the long-wavelength range operated in two gain modes, so that valid measurements could be made over the entire dynamic range from 100 to 10,000 Rayleighs. The spectral resolution of the instrument was 20 A at 2950 A in first order. A spectrum was produced every 3 s, and contained 600 values from each of the two detectors. Thirty-six values were used as fiducial period measurements and 564 for spectral measurements. Measurements of Lyman-alpha radiation at 1216 A were also taken after encounter. Less than 30 min of data were obtained from both channels during the Mariner 6 near-encounter equatorial scan on July 31, 1969. The quality of the data was comparable to the best obtained by rockets in the 130-km-and-above region of the earth's atmosphere.