A canyon. This is from 2005 and was just released.
http://www.esa.int/SPECIALS/Mars_Express/SEM4CATHKHF_1.html
What do you think?
Why don't they send some criminal over there to dig some ground? No-body wants to go cuz it'd take like 10 years. And the Phoenix as a machine was programmed so it can haul the ground 8 times, and only with the last attempt it managed to get some ground for sample to bring to Earth for study.
Maybe they could find some fossiles or whatever from the canyon bottom.