At 10:53:13 UT the Mars Webcam takes this new image of the phenomenon taking place on the volcano Arsia Mons.
The shadow of the plume is definitely “black” and remains so even in photos much more overexposed, as in the image taken um minute and a half later that is almost completely overexposed!!!
(see https://www.flickr.com/photos/esa_marswebcam/44698359825/)
This means that the plume is so opaque that to an observer placed under it the Sun would be totally invisible.
If that plume was really made of water ice crystals, you would wonder where all that water comes from!
In fact, in that area and in this period of the Martian year, if we condense to the ground all the water contained in the atmosphere we would get a layer of water only a hundredth of a millimeter thick (see http://www-mars.lmd.jussieu.fr/mcd_python/cgi-bin/mcdcgi.py?datekeyhtml=1&ls=278.1&localtime=0.&year=2018&month=10&day=29&hours=10&minutes=26&seconds=8&julian=2458420.9348148145&martianyear=34&martianmonth=10&sol=528&latitude=all&longitude=all&altitude=0&zkey=3&isfixedlt=off&dust=1&hrkey=1&zonmean=off&var1=mtot&var2=none&var3=none&var4=none&dpi=80&islog=off&colorm=jet&minval=&maxval=&proj=cyl&plat=&plon=&trans=&iswind=off&latpoint=&lonpoint= ).
Do you really think that such a small amount of water can feed a cloud so dense as to obscure the Sun?
Image titled: https://image.ibb.co/jU8OtA/44887592234-cc2fb4bcb2-o.jpg
Original image: https://www.flickr.com/photos/esa_marswebcam/44887592234/
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