While Mars has abundant evidence of past and present water in frozen form and features suggesting ancient water flow, liquid water has never been directly observed on the Martian surface. The thin atmosphere and low pressure mean water would quickly evaporate or freeze. Ancient riverbeds (option A) have been imaged, evidence of recent water flows (option B) like gullies have been documented, and frozen water (option D) has been confirmed at the poles and underground. Liquid water remains unconfirmed on the surface.