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Ultraviolet Spectrometer (UVS)

NSSDC ID: 1971-051A-02
Mission Name: Mariner 9
Principal Investigator: Dr. Charles A. Barth

Description

The Mariner 9 ultraviolet spectrometer (UVS) experiment was designed to receive UV radiation (1100 to 3520 A) from the surface and atmosphere of Mars, scan selected bands of this radiation, and provide an intensity value as a function of wavelength on the basis of scan-cycle time. The scientific objectives of this experiment fell into two broad categories, UV cartography and UV aeronomy. The UV cartography involved measurements in the UV of the (1) local atmospheric pressure over the major portion of the planet, (2) local ozone concentration, (3) wave of darkening, (4) variability of surface features, (5) yellow clouds, blue haze, and blue clearing, and (6) local variations in the oxygen-ozone abundances for detecting signs of biological activity. The UV aeronomy involved measurements in the UV of the (1) composition and structure of the upper atmosphere as a function of latitude, longitude, and time, (2) variability of the rate of escape of atomic hydrogen from the exosphere, and (3) distribution and variability of the UV aurora and determination of the induced planetary magnetic field. In addition, when Mars was occulted from the instrument FOV, observations of strong stellar sources of UV were made. The optics and sensing portion of the UVS consisted of an Ebert grating spectrometer with two exit slits, a light baffle, an occulting slit telescope, and two photomultiplier tube (PMT) light sensors. The incident UV radiation passed through the baffling system, which eliminated any stray light, and entered into the telescope. The telescope primary mirror reflected the radiation to a secondary mirror through a preslit where it was focused onto the entrance slit of the Ebert spectrometer, which isolated monochromatic radiation from the incoming radiation. The radiation from the entrance slit filled half the Ebert mirror where it was collimated and reflected onto the grating (2160 lines/mm) so that the radiation filled the grating. The grating rotated over a small angle by means of a cam-follower drive and diffracted the radiation. Diffracted radiation of different wavelengths, depending on the grating angle, fell on the other half of the Ebert mirror, which focused it onto the two exit slits, thus providing the wavelength scan. The two photomultiplier tubes sensed radiation from their respective exit slit and were sensitive only to selected bands in the UV spectrum -- 1100 to 2000 A (channel 1) and 1450 to 3520 A (channel 2). Channel 1 was detected by the PMT with a cesium iodide photocathode and lithium fluoride window and included the data used in the UV aeronomy study. Channel 2 was detected by the PMT with a cesium telluride photocathode and sapphire window and included the data used in the UV cartography study. The UVS scanned the wavelength range with a 3-s period and a spectral resolution for first-order spectra of 15 A. The wavelength of any given photometric sample in the UV spectrum was known to plus or minus 5 A or better. The Mariner 9 Data Automation Subsystem (DAS) caused each channel to be sampled every 5 ms. Channel 2 was sampled 2.5 ms after channel 1. There were 200 samples/s/channel, a total of 400 UVS samples/s. Each sample was digitized to eight bits and one sign bit in the DAS. The instrument had a dynamic range of 200 Rayleighs per 20-A interval to 50 kiloRayleighs per 20-A interval for channel 1 and 200 Rayleighs per 20-A interval to 50 megaRayleighs per 20-A interval for channel 2. Channel 1 had a field of view sufficient to permit imaging a portion of the Martian surface subtending 0.19 by 1.9 deg of arc, while channel 2 was limited to a 0.19- by 0.55-deg field of view. Channel 1, at a slant range of 5,700 km, viewed a column of space 100 km above the Martian surface that was 24 by 240 km. Channel 2, on the other hand, at a vertical distance of 1,250 km, viewed a 2.25- by 6.5-km area at the subspacecraft point on the planet's surface, while at a vertical distance of 850 km the area viewed was 1.5 by 4.5 km. The UVS had four fundamental measuring geometries during an orbit: (1) bright limb, (2) illuminated disk, (3) terminator, and (4) dark limb. In addition to taking complete UV spectra, the instrument design also allowed for sampling at 1216 A (Lyman-alpha) to utilize a lower data rate mode. This allowed Lyman-alpha data to be taken for a large percentage of each orbit. The experiment began collecting excellent data soon after orbital insertion on November 13, 1971, and continued until April 2, 1972, when the experiment was shut off to conserve spacecraft power during solar occultation. The experiment was reactivated on June 8, 1972, after the spacecraft emerged from solar occultation. It continued to operate normally until 2200 UT on October 27, 1972, when the experiment was shut off along with the rest of the Mariner 9 spacecraft.

Alternate Names

  • UVS

Funding Agency

  • NASA-Office of Space Science Applications (United States)

Disciplines

  • Planetary Science: Atmospheres
  • Planetary Science: Biology
  • Planetary Science: Geology and Geophysics

Additional Information

Questions or comments about this experiment can be directed to: Dr. David R. Williams.

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Charles A. BarthPrincipal InvestigatorUniversity of Coloradocharles.barth@colorado.edu
Dr. Charles W. HordOther InvestigatorUniversity of Colorado 
Dr. J. B. PearceOther InvestigatorRadiophysics, Inc 
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