I've just seen the movie and may have missed the explanation, but why did Mark remove the roof from the Rover towards the end, before he tries going to Ares 4 landing site ?
This is a vestigial element from the book. In the books, there are 2 working rovers, and he hooks them up together. In the second rover, which is acting as a trailer, he needs to store a bunch of large equipment from the hab that will sustain life support (specifically heat and air). To fit the equipment in the rover, he removes a portion of a wall, and integrates the equipment into the rover and seals it up using some of the material from a pop tent (a piece of equipment in the books).
In the film, this sequence is not present. But they include the section of him removing a chunk of the rover's wall and roof and patching it up.
From Chapter 20 of the book:
LOG ENTRY: SOL 376
I’m finally done with the rover modifications!
The tricky part was figuring out how to maintain life support. Everything else was just work. A lot of work.
I haven’t been good at keeping the log up to date, so here’s a recap:
First I had to finish drilling holes with the Pathfinder-murderin’ drill. Then I chiseled out a billion little chunks between the holes. Okay, it was 759 but it felt like a billion.
Then I had one big hole in the trailer. I filed down the edges to keep them from being too sharp.
Remember the pop-tents? I cut the bottom out of one and the remaining canvas was the right size and shape. I used seal-strips to attach it to the inside of the trailer. After pressurizing and sealing up leaks as I found them, I had a nice big balloon bulging out of the trailer. The pressurized area is easily big enough to fit the oxygenator and atmospheric regulator.