The major Martian dust storms of 1971 and 1973

The two largest Martian dust storms on photographic record occurred during the past two apparitions. The general characteristics of these events are compared using the hourly photography from the International Planetary Patrol. Dust storms are believed to be seasonal events on Mars, and therefore both storms had been predicted, but both turned out to be larger than expected, since they were each more extensive than the famous 1956 storm.The 1971 storm was much larger than the 1973 storm in both intensity and duration, although the 1973 storm had a more rapid initial expansion. Both storms began in the southern hemisphere but the 1973 storm began later (during summer, nearly three months beyond perihelion). The 1971 storm lasted approximately twice as long as the 1973 storm.Maps of the first nine days of both storms are presented to facilitate a comparison of their initial outbreaks and growth. Graphs of changes in contrast of albedo features during the 1973 storm are based on microdensitometer tracings of Planetary Patrol photographs.The frequency and regularity of storms of this size are uncertain because of the lack of comprehensive photographic observation during past apparitions. Identification and evaluation of such events in the future requires the continuation of well-coordinated worldwide observing efforts.