That bothered me too. Other issues that bothered me:

• The gravity depicted on the surface of Mars was unrealistic.
• It is currently considered unfeasable by NASA for us to take off from the surface of Mars and escape the planet's gravity.
• The severe radiation on the surface wasn't addressed. The HAB should have been underground.

Beyond these relatively small issues though, and the fact that Matt Damon got like 80% of the screen time, the movie was surprisingly good.