Infrared radiometry experiment on Mariner 9.
A two-channel IR radiometer similar to that carried on Mariners 6 and 7 was included on Mariner 9 in order to extend and improve the surface coverage and spatial resolution obtained on the earlier flights. The minimum brightness temperature observed by the Mariner 9 radiometer over the shrinking south polar cap, slightly after Martian southern midsummer, is at least 25 K above the temperature that would correspond to frozen carbon dioxide.
Chase, S. C., Jr.↗