Image Source
Here's whats been said:
We spotted what appears to be a particularly special VMC image we'd like you to look at. In the pictures from 2 July we were flying over the huge volcano chain on Mars, the Tharsis Montes. We spotted what appears to be an extremely long and thin cloud trailing from near the peak of the southernmost volcano, Arsia Mons, and we'd love to hear what you have to say about it!
The Observation:
VMC observed the pictures around 09:00 UTC on 2 July and they were transmitted to Earth and then posted onto the blog on 8 July. The images show a rotated view of Mars, with the North Pole to the top right of the image, and the South Pole to the bottom left. Load up the Celestia Script to see exactly where.
The three dark spots to the left of the image are the three Tharsis Montes, from left to right: Arsia Mons, Pavonis Mons and Ascraeus Mons. The cloud is the small white streak running diagonally from Arsia Mons on the left towards the top left of the image. At the time the image was taken Mars Express was at an altitude of around 6500 km, and VMC conducted a 30-minute observation, capturing 34 images of Mars.