Diurnal variation of Martian water-ice clouds in Tharsis region of the low latitude cloud belt: Observations in 1995-1999 apparitions

We investigated diurnal variations of water-ice clouds in the Tharsis region of Mars in the three apparitions from 1995 to 1999. The Tharsis morning cloud, centered near a point (120 degreesW, 10 degreesN) between Olympus Mons and Tharsis Montes, was the brightest among the morning clouds in Tharsis. Its optical thickness was about 0.6 near 9.5 h Martian local time and reduced to a minimum of about 0.2 around local noon in 1995. The minimum optical thickness was 0.4 in 1997, which also appeared around local noon. The optical thickness of the Tharsis morning cloud increased again in the early or mid afternoon in 1995 and 1997. In the late morning when the brightness of the morning cloud declined, bright afternoon clouds appeared over the major volcanoes in Tharsis. The optical thickness of the afternoon cloud over Olympus Mons was about 0.7 in the early afternoon in early summer, 1997. The Olympus afternoon cloud seems to be more active in early summer than in the first half of late spring. Those morning and afternoon clouds repeat the diurnal variation from late spring to at least early summer every Martian year.