Hi George & Brett, & Jeff - without further investigation, I suppose it is possible that the "spiral cloud" phenomenon you referred/linked to is possibly just one instance of clouds creating long shadows around Arsia Mons - but for my money the 3 Global Surveyor images were all taken very shortly after the Autumnal Equinox in the Northern Hemisphere in 2001, 2003 & 2005 respectively.
Your (slightly later) image on August 29th 2003 was a few weeks before the Winter Solstice. (Northern Hemisphere)
I'll set these dates down here for anyone's consideration...& I haven't got back into re-reading Dr. Harrison's comments again except to note that in the 2nd link I posted in Post #14 above, the most current data, we can read the following:
<"Mars just experienced its northern hemisphere winter solstice on 16 October. In the months leading up to the solstice, most cloud activity disappears over big volcanoes like Arsia Mons; its summit is covered with clouds throughout the rest of the martian year.
However, a seasonally recurrent water ice cloud, like the one shown in this image, is known to form along the southwest flank of this volcano – it was previously observed by Mars Express and other missions in 2009, 2012 and 2015.">
George's link is to the September 2005 Global Surveyor images & the comments in that older article: <"One of the repeated weather phenomena occurs each year near the start of southern winter over the martian volcano, Arsia Mons. The volcano is located near 9°S, 121°W. Each year, just before southern winter begins, sunlight warms the air on the slopes of the volcano"> would suggest that the recent (September 2018) cloud & shadow are out of kilter with the season(s) in which these spiral formations develop!!!
However, if we suggest that the statements <"One of the repeated weather phenomena occurs each year near the start of southern winter over the martian volcano, Arsia Mons. The volcano is located near 9°S, 121°W. Each year, just before southern winter begins, sunlight warms the air on the slopes of the volcano"> should actually read "southern summer" or "northern winter " it would accord more with the latest events..!
To me this is the only possibility that makes any sense, ie, the 2005 Global Surveyor article got their North & South mixed up! 
- that & the fact that the article George refers to is incorrect as far as the seasons are concerned if you look at the ready-reckoner here: http://www.planetary...s-calendar.html
Don't shoot me if I'm wrong, but Pat agrees so we may both be quite crackers...but I'll shoulder the blame for convincing her of my view! 
Brett, I noticed that comment...you deserve full credit for picking this shadow up 
...I only became aware of it in our own images after you posted your own here: fortunately in some ways I must have been suffering from processing overload at the time so I only processed one of the images displaying it in our own data. (red filter, UT 11:47 & rgb UT 11:49)
I'll look again at the blue filter & have already processed 2 red filter captures ( 11:53 & 11:59) that also display the shadow.....hoping more still to come plus rgb's although as Mars rotates further in the later times the shadow shortens etc...but there might be the remote possibility of an enhanced animation focusing on this feature... 
Edited by Kokatha man, 26 October 2018 - 10:05 PM.
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