Telescopic observations of Mars, 1996-1997: Results of the Marswatch program

During the 1996-1997 apparition of Mars, Marswatch and the International Ma rs Patrol (IMP) of the Mars Section of the Association of Lunar and Planeta ry Observers (ALPO) received 2750 observations from 70 observers in 14 coun tries. Approximately 40% of these observations are CCD images, many of whic h were obtained through filters covering a spectral range from 330 to 820 n m. In addition, a large number of high quality visual observations were per formed through standard color filters, permitting more accurate assessments of surface and atmospheric phenomena. The apparition coverage extended thr ough 249 degrees (11 degrees to 260 degrees) of areocentric longitude (L-s) , permitting monitoring of the northern hemisphere spring/summer/autumn met erological activity, north polar cap (NPC) behavior, and albedo feature alt erations. There were few significant changes in albedo feature morphology during the 1996-1997 apparition. The north polar cap rift, the Rima Tenuis, which had been observed and imaged during each aphelic apparition since 1980, was not observed, perhaps due to a slowing in the NPC regression after 80 degrees L-s. The incidence of discrete clouds, orographic and localized, was higher than that predicted on the basis of the ALPO/IMP's martian meteorology stu dy that spanned 14 previous apparitions from 1964 to 1993. Discrete clouds were more numerous than in 1993 or 1995, peaking in late northern spring, 3 0 degrees L-s earlier than predicted. The Syrtis Blue Cloud was extensively observed and imaged between 66 degrees and 115 degrees L-s. The rate of oc currence of limb clouds was significantly lower than predicted for a given subseason. Their peak incidence occurred after aphelion, but their frequenc y decreased rapidly after 105 degrees L-s, nearly two martian seasons earli er than observed during previous apparitions. Limb hazes, bright in blue an d ultraviolet light, were more numerous and conspicuous throughout the nort hern summer, as predicted from the meteorology study. Arctic clouds increas ed in number around aphelion and were often large and bright during early n orthern summer. A variable north polar hood was observed after 160 degrees L-s. We find clear evidence for the existence of extensive equatorial cloud bands, which became numerous between 50 degrees and 120 degrees L-s, peaki ng in early northern summer (105 degrees L-s), and often merging into brigh t limb clouds. These bands were more frequent and prominent in 1996-1997 th an during the 1993 or 1995 apparitions and were extensively imaged in blue and ultraviolet light by both groundbased observers and by the HST. (C) 199 9 Academic Press.