The surface composition of the northern ice cap in middle spring (after a winter's accumulation of seasonal dry ice) has been studied from orbit. The outer edges of the ice cap are contaminated with dust (0.15% by weight) and are mostly water ice. As one moves toward the pole, the surface water ice content decreases and is replaced by dry ice. The purity of the ice also increases. At the pole, the surface seasonal ice consists of essentially pure dry ice with little dust content and 30 parts per million of water ice.