Telescopic Observations of Mars, 1996–1997: Results of the Marswatch Program

• Published in: Icarus (New York, N.Y. 1962) Vol. 138; no. 1; pp. 3 - 19
• Main Authors: Parker, D.C., Beish, J.D., Troiani, D.M., Joyce, D.P., Hernandez, C.E.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.03.1999
• Subjects: atmosphere; Mars; climate; surface
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1006/icar.1998.6056

During the 1996–1997 apparition of Mars, Marswatch and the International Mars Patrol (IMP) of the Mars Section of the Association of Lunar and Planetary Observers (ALPO) received 2750 observations from 70 observers in 14 countries. Approximately 40% of these observations are CCD images, many of which were obtained through filters covering a spectral range from 330 to 820 nm. In addition, a large number of high quality visual observations were performed through standard color filters, permitting more accurate assessments of surface and atmospheric phenomena. The apparition coverage extended through 249° (11° to 260°) of areocentric longitude (Ls), permitting monitoring of the northern hemisphere spring/summer/autumn meterological activity, north polar cap (NPC) behavior, and albedo feature alterations. 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°Ls. The incidence of discrete clouds, orographic and localized, was higher than that predicted on the basis of the ALPO/IMP's martian meteorology study that spanned 14 previous apparitions from 1964 to 1993. Discrete clouds were more numerous than in 1993 or 1995, peaking in late northern spring, 30°Lsearlier than predicted. The Syrtis Blue Cloud was extensively observed and imaged between 66° and 115°Ls. The rate of occurrence of limb clouds was significantly lower than predicted for a given subseason. Their peak incidence occurred after aphelion, but their frequency decreased rapidly after 105°Ls, nearly two martian seasons earlier than observed during previous apparitions. Limb hazes, bright in blue and ultraviolet light, were more numerous and conspicuous throughout the northern summer, as predicted from the meteorology study. Arctic clouds increased in number around aphelion and were often large and bright during early northern summer. A variable north polar hood was observed after 160°Ls. We find clear evidence for the existence of extensive equatorial cloud bands, which became numerous between 50° and 120°Ls, peaking in early northern summer (105°Ls), and often merging into bright limb clouds. These bands were more frequent and prominent in 1996–1997 than during the 1993 or 1995 apparitions and were extensively imaged in blue and ultraviolet light by both groundbased observers and by the HST.