A significant martian dust storm is presently occupying much of the length of the great Valles Marineris canyon system, and is apparently still spreading. The event began as a small yellow cloud located just W. of Margaritifer Sinus on the S. border of Chryse on 2005 October 13. After a few days activity seemed to cease, but on October 18 there was a strong resurgence, with dust moving north across Chryse, whilst a second bright core appeared further west near Aurorae Sinus. Dust then rapidly spread along Valles Marineris. The region was observed by the Director before dawn on October 20, when the dust storm was seen to have expanded further, with partial obscuation of Mare Erythraeum. Another dust disturbance occurred on October 17, at the W. border of Mare Acidalium in SE Tempe. The lastest information, from October 21, shows that further bright dust cores have developed just south of Solis Lacus, and in northern Argyre. The region will soon be lost to view over the morning limb from the longitude of the UK, but in a few days’ time early evening observations will show it at the evening terminator if activity still persists.

      Beginning at Ls = 306 degrees (October 13), this dust storm – in terms of both martian date and location – is similar to the large regional storm that took place at Ls = 315 degrees (2003 December 13) near the end of the last opposition. It also resembles local or regional events from the 1990 and 1992 oppositions (in a very similar location) that occurred at Ls = 308 and 316 degrees respectively. For full details of the 2003 event, see the Final Interim Report on Mars at its 2003 opposition. For the historical storms see the writer’s monograph on Telescopic Martian Dust Storms. Other historical examples could be quoted, at similar Ls and in a similar location, right back to 1879.

      Historical records show that the present event is unlikely to develop into a planet-encircling event, but it could yet spread further. The seasonally latest encircling storm commenced in 1924 December at Ls = 311 degrees, but only storms that begun in Hellas, Noachis or near Solis Lacus ever achieved encircling status.

      Further observations are desirable and should be sent to me at once.

Richard McKim, Director

2005 October 21