[dr_lew]

...so we come up with 0.1 mm for a 4 day period with this figure ... so I don't know.

Oops! 1.0 mm in 4 days. A lot closer but still a stretch, especially with the shaded location.

[dr_lew]

Here's another skeptic from the New Mars blog:

"At that temperature and pressure the sublimation rate of water ice should be imperceptible."

As for me, the more I think about it, the more implausible it seems that centimeter size chunks of water ice could vanish by sublimation in 4 days in that trench. It doesn't take much reinforcement ( as per this citation ) to bolster my confidence.