Thanks everyone!
Morton - get the scope out in the next three nights - 9/9 seeing forecast for Sydney with a chance of clear skies
Bob, interesting question. Valles Marineris does show up, you just need to know what to look for. It's quite prominent in
Damian Peach's image from last week as the forked dark linear feature near the centre of his images. It should be visible in most images of that side of the planet. Valles Marineris is ~4,000km long and Mars' circumference is ~22,000km, so maybe it's not quite as large as you thought? Comparing the famous
Viking image on Wikipedia with telescope images, it would seem that one end doesn't have too much dark material, and the other stretches into Mare Erythraeum (various names with VM on
Damian's map from 12 years ago)