Mars: The cloud effect on the blue clearing in the Syrtis Major region

We conducted imaging observations of Mars in March 1997 and April 1999; the se apparitions were around its northern summer oppositions. Mars showed hig hly developed clouds that formed the aphelion cloud belt. Comparing the diu rnal variations of the apparent contrasts of Syrtis Major to the adjacent b right areas with those of the optical depths of the aphelion clouds over th ose regions, we conclude that the time when those regions are subject to th e morning clouds is around 0900 Martian Local Time (MLT) and that the diurn al variation of the degree of blue clearing strongly depends on the differe nce between the optical depth of H2O ice clouds over Syrtis Major and that over the adjacent bright area. Moreover, our simple linear regression analy ses of the degree of blue clearing to the optical depths of H2O ice clouds suggest that the key region in which the H2O ice cloud optical depth determ ines the degree of blue clearing is Syrtis Major.