if you look at the picture again the "blades" on this "fan" are flat. They don't angle, which is what you would have in a conventional fan. the blades must angle to move the air either forward or backward. maybe who- or what-ever made this had some sort of advanced tchnology that made it possible to turn the blades to whatever angle they needed for that day.
the bar that cuts across the tunnel looks as if the "fan" would spin around it, again not the way of a conventional fan. if it spins that way how is it moving the air anywhere. spinning in that direction would simply create a vortex, if you will. the air would go niether forward or backward, but instead stay in the same place moving in a circle.
i agree with JASE. that it is some sort of ventilation shaft, but i don't think it's for bringing in fresh air. i think it is for expeling waste air and excess heat generated by whatever is underground. maybe there were "martians" there several thousand years ago and now they're dead or gone to some other place, and whatever is underground is leftovers of their civilization; machinery, or living quarters, or holding cells for prisoners or slaves.
or maybe they had some sort of underground air generating machine that they used these for. they would need some way to get the air to the surface. if that is the case then maybe the planet will be, someday, friendly to human life. all the probes need to do is figure out how to get to this machinery and turn it on.
just a little wild speculation fer ya. thanks for letting me ramble,
Anirtak
wild speculation mode=off