CO2 ICE CLOUDS IN THE UPPER-ATMOSPHERE OF MARS

Recent (Sept. 1996-Sept. 1997) observations of Mars submillimeter CO l ines from the James Clerk Maxwell Telescope (JCMT) on Mauna Kea, Hawai i surprisingly cold temperatures as typical atmosphere over the 50-80 km altitude region. Measurements of these J=2->3 and J=3->4 rotational transitions of Mars atmospheric (CO)-C-12 provide unique seasonal cov erage of global averaged, dayside temperatures within this poorly obse rved mesosphere of Mars. At solar longitudes (L-s) of 8 degrees, 90 de grees, 150 degrees, and 187 degrees in 1996-97, the mean atmospheric t emperature for the 70-80 km (2.0-0.3 mu bars) altitude region is obser ved to be at or below 120 K, which is only 10-15 K above the vapor sat uration temperature for the CO2 Mars atmosphere. Consequently, local C O2 saturation conditions at these altitudes would be likely to exist a nd, in fact, were observed during the 3 AM descent entry of the Pathfi nder spacecraft, on July 4, 1997. We argue that the blue wave clouds i maged from the Pathfinder lander 35-100 minutes prior to sunrise on So l 39 are evidence of such CO2 ice formation within the 60-100 km altit ude region; and that the 4.3 mu m CO2 lines of solar scattered flux, i n Mariner 6 and 7 infrared limb spectra of Mars are a direct spectral identification of CO2 ice cloud formation in the dayside Mars mesosphe re. Simple considerations of these Pathfinder and Mariner 6 and 7 obse rvations suggest 0.1-0.3 mu m cloud particle radii, and particle numbe r densities of the order 10(2) cm(-3). On the basis of a variety of su ch day and night time measurements, we assert that the mesosphere of M ars does not exhibit extreme (25 K) diurnal temperature variations as maintained by the Pathfinder meteorology ream; and that it is often su bstantially colder (> 20 K) than determined from the Viking descent me asurements in 1976 due to temporal variations in dust loading of this region.