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Infrared Radiometer (IRR)

NSSDCA ID: 1971-051A-01

Mission Name: Mariner 9
Principal Investigator:Dr. Gerry Neugebauer


The Mariner 9 infrared radiometer (IRR) experiment was designed to provide, over a wide coverage of the surface of Mars, brightness temperatures of the soil as a function of local time by measuring the energy radiated in the 8-to 12-micrometer and 18- to 25-micrometer wavelength bands. From these temperatures, the following information was derived: (1) the large-scale distribution of the thermal inertial of the surface materials, (2) the occurrence of irregularities in the cooling curve, (3) the existence of "hot spots" that may indicate sources of internal heat, and (4) the temperature of the polar cap and adjacent area. The instrument consisted of two telescope/thermopile-detector assemblies. Each assembly contained two lenses, a spectral filter, a field stop, and a thermopile-detector. The detectors in each assembly were identical. However, the lenses and filters through which the radiation must pass were of different materials so that one detector responded to the radiation in the 8-to 12-micrometer band (channel 1) whereas the other detector responded to radiation in the 18-to 25-micrometer band (channel 2). The channel 1 assembly, which had a field of view of 0.53 by 0.53 deg, had a germanium spectral filter and field lens and an infrared transmission (IRTRAN) -2 objective lens. The channel 2 assembly, which had a 0.7 by 0.7 deg field of view, had a silicon spectral filter, an IRTRAN-6 field lens, and an IRTRAN-6 objective lens. The detectors were 13-junction bismuth-antimony differential thermopiles, which generated a voltage in response to incident radiant heat flux. Channels 1 and 2 had sensitive areas of 0.25 by 0.25 mm and 0.4 by 0.4 mm, respectively. Radiation was measured from three sources (space, Mars, and a thermal reference source) by means of a three-position scan mirror rotated clockwise by a bidirectional digital stepping motor. The 42-s scan cycle was controlled by the Mariner 9 Data Automation Subsystem (DAS) and consisted of the following viewing modes each separated by a 0.25-s scan interval: planet (19.2 s), space (2.4 s), planet (18.0 s), and thermal reference (2.4 s). The radiation from the source being viewed at a given time entered the IRR, was reflected off the scan mirror, passed through the objective lens, spectral filter, and field lens, and was focused onto the detector. The detector then converted the incident radiant flux to a voltage. The IRR data samples were taken in pairs, each pair consisting of a channel-1 sample and a channel-2 sample. Data pairs appeared at 1.2-s intervals, while the interval between samples in a pair was 200 ms. The dynamic range of the instrument was optimal from 150 to 325 deg K. The sensitivity of the IRR was plus or minus 0.12 deg K at 300 deg K and plus or minus 0.6 deg K at 140 deg K. Mounted on the Mariner 9 planetary scan platform, the IRR had a 20-sq-cm aperture with an unobstructed view of Mars subtending a minimum of 15 deg half-angle. At 90 deg to this aperture and directly opposite the thermal reference plate (flat-black curved aluminum plate), the aperture had an unobstructed view of deep space subtending 20 by 20 deg. The IRR, which was boresighted with the narrow-angle TV camera to within plus or minus 0.3 deg, had a resolution at the subsatellite point on the surface of Mars of 20 by 20 km and 25 by 25 km for channels 1 and 2, respectively, from the periapsis altitude of 2000 km. The instrument was basically the same as that flown on the Mariner Mars 1969 missions, except that a focal-plane diaphragm (field stop) had been placed in front of the detectors to reduce the response to off-axis radiation. The experiment began collecting high-quality 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 turned back 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 turned off along with the rest of the Mariner 9 spacecraft.

Alternate Names

  • IRR
  • Infrared Radiometer
  • Mariner9/IRR


  • Planetary Science: Atmospheres
  • Astronomy: Infrared
  • Planetary Science: Geology and Geophysics

Additional Information

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



NameRoleOriginal AffiliationE-mail
Dr. Guido MunchOther InvestigatorMax-Planck-Institut fur Astronomie
Mr. Stillman C. Chase, Jr.Other InvestigatorSanta Barbara Research Center
Dr. Hugh H. KiefferOther InvestigatorUS Geological
Dr. Ellis D. MinerOther InvestigatorNASA Jet Propulsion
Dr. Gerry NeugebauerPrincipal InvestigatorCalifornia Institute of

Selected References

  • Chase, S. C., Jr., et al., Infrared radiometry experiment on Mariner 9, Science, 175, No. 4019, 308, Jan. 1972.
  • Kieffer, H. H., et al., Preliminary report on infrared radiometric measurements from the Mariner 9 spacecraft, J. Geophys. Res., 78, No. 20, 4291-4312, doi:10.1029/JB078i020p04291, July 1973.
  • Gatley, I., et al., Infrared observations of Phobos from Mariner 9, Astrophys. J., 190, No. 2, 497-503, doi:10.1086/152902, June 1974.
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