[Previous] | [Session 3] | [Next]

[3.07] Martian daytime clouds observed by MOLA, TES, and MOC

P.G. Ford (MIT Ctr. for Space Res.), G.H. Pettengill (MIT Ctr. for Space Res., and Dept. of Earth, Atmos. and Planetary Sci.)

The MOLA lidar aboard Mars Global Surveyor (MGS) frequently observed dense clusters of cloud echoes. Those seen during the polar night were almost certainly composed of CO₂ ice crystals, but some 3% occurred in daylight, mostly at high latitudes. In the north, these daytime clouds were restricted to three seasons: late winter (260 \degr < L_s < 10 \degr), mid-spring (L_s ~60 \degr), and late fall (140 \degr < L_s < 190 \degr). This seasonal pattern was similar to that of ``non-reflective'' MOLA clouds, when the lidar received no echo from either surface or cloud. In the south, daytime clouds were seen in the fall (30 \degr < L_s < 70 \degr) and late winter (100 \degr < L_s < 190 \degr), but at significantly higher latitudes than non-reflective clouds. The TES instrument measured thermal emission profiles for 58% of the MOLA daylight clouds. In no instance did the emission temperature drop below 150K in the 15 \mu - 18 \mu wavelength range, indicating that these clouds were composed of H₂O ice or dust, but not CO₂ ice.

A distinctive class of daytime cloud was restricted to the late northern spring. Its MOLA echoes were closely grouped at low altitude (below 2 km, vs. 5 - 15 km for polar CO₂ clouds), and showed no evidence of trapped or propagating buoyancy waves. Because of the MGS sun-synchronous orbit, these clouds were observed in only two ranges of solar incidence angle: 50 - 55 \degr and > 75 \degr. In each instance, TES showed that these clouds lay at the boundary between an isothermal, stably stratified polar atmosphere and an adiabatic atmosphere overlying a warmer surface. Whenever they were imaged by the MOC wide-angle camera, the clouds were seen to lie at the extreme edge of the seasonal cap, and were most probably composed of trapped H₂O ice and/or dust that was being released as the winter CO₂ deposits evaporated.

[Previous] | [Session 3] | [Next]

Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.