Recent (Sept. 1996--Sept. 1997) observations of Mars submillimeter CO lines from the James Clerk Maxwell Telescope (JCMT) on Mauna Kea, Hawaii have revealed surprisingly cold temperatures as typical of the Mars atmosphere over the 50--80 km altitude region. Measurements of these J=2→3 and J=3→4 rotational transitions of Mars atmospheric 12CO provide unique seasonal coverage of global-averaged, dayside temperatures within this poorly observed mesosphere of Mars. At solar longitudes (Ls) of 8¿, 90¿, 150¿, and 187¿ in 1996--97, the mean atmospheric temperature for the 70--80 km (2.0--0.3 &mgr;bars) altitude region is observed to be at or below 120 K, which is only 10--15 K above the vapor saturation temperature for the CO2 Mars atmosphere. Consequently, local CO2 saturation conditions at these altitudes would be likely to exist and, in fact, were observed during the 3 AM descent entry of the Pathfinder spacecraft, on July 4, 1997. We argue that the blue wave clouds imaged from the Pathfinder lander 35--100 minutes prior to sunrise on Sol 39 are evidence of such CO2 ice formation within the 60--100 km altitude region; and that the 4.3 &mgr;m CO2 lines of solar scattered flux, in Mariner 6 and 7 infrared limb spectra of Mars are a direct spectral identification of CO2 ice cloud formation in the dayside Mars mesosphere. Simple considerations of these Pathfinder and Mariner 6 and 7 observations suggest 0.1--0.3 &mgr;m cloud particle radii, and particle number densities of the order 102 cm-3. On the basis of a variety of such day and night time measurements, we assert that the mesosphere of Mars does not exhibit extreme (25 K) diurnal temperature variations as maintained by the Pathfinder meteorology team; and that it is often substantially colder (>20 K) than determined from the Viking descent measurements in 1976 due to temporal variations in dust loading of this region. ¿ 1998 American Geophysical Union