Catching dust storms
Fierce winds can whip Mars’s fine dry soil into large clouds
Although the Martian atmosphere is tenuous, it is quite capable of producing powerful dust storms. Winds of half the speed of sound have been recorded. Typically, the dust storm season starts at about Ls =240° and it continues to Ls=0°. This means that we can expect to see them from now until early February.
The Martian dust storm season has been studied in great detail, and there are three types of dust storm which occur:
Local: these dust storms are confined to very small regions like a corner of the Hellas Basin.
Regional: these dust storms may cover an entire region like Syrtis Major, or indeed a whole hemisphere.
Global: the largest, these dust storms cover the entire planet. During this time the entire globe can become featureless even to large telescopes.
Local and regional dust storms tend to be the most frequent. During the previous opposition in 2018, I was able to observe a regional dust storm at the Lowell Observatory, in Flagstaff, Arizona (see picture, above).
Truly global dust storms are quite rare – two prominent ones occurred in 1975 and 1977, around the time that the Viking missions were approaching Mars.
There are a number of sites on Mars that are well known for producing storms, including the Hellas Basin, Solis Lacus, Noachis and Chryse, and you should survey these regions whenever you can. Storms always start off as bright yellow or orange clouds. Local ones will require a 150mm telescope or larger to be seen, but regional ones can be visible in smaller instruments. You’ll find a red (W25), orange (W21) or yellow (W15) filter will also help enhance dust clouds and make them easier to see.
If a large storm does erupt, it is a good idea to record its progress; you can do this by sketching the region and plotting the size and location of the dust storm as it changes over time.