[forktheif]

Pretty big.

I've often wondered why they don't send a dragline digger to Mars, so they can dig a decent trench. When Spirit's wheel broke and they started dragging it along, digging a little trench, they made a nice discovery and that was with something not designed to dig at all.

If they could get a few feet down, who knows what they could find?!

[hahainternet]

The Exomars project will land a 2 metre vertical drill on the surface, sampling much deeper than anything done so far.

It's just a question of priorities. Drilling 4cm into a rock with a specific surface abrasion tool is a long way from plunging a 2 metre metal shaft into unknown terrain. Engineering this so it has no chance of jamming, breaking, overheating etc is not gonna be a trivial task. How do you lubricate a drill without contaminating the environment and with such a wide temperature variation?

[thaumasiotes]

What exactly is the downside of contaminating the environment?

[bad_alloc]

They sent Phoenix, which had a scoop and dug down a few centimeters: http://en.wikipedia.org/wiki/Phoenix_probe

[The_Double]

It may have been big, but curiosity has a scoop to scoop up dirt, a drill, and a laser that can vaporize rock to look inside of them. It's not like the rovers can only look at the surface.