One of the major questions posed by the Mariner 9 data was the composition of the residual polar cap left when the winter polar cap, made of frozen carbon dioxide, retreated in midsummer. A major controversy existed over whether this summer cap seas also frozen carbon dioxide or was frozen water.
According to Viking data, the temperatures of the residual cap are near -68° to -63°C, making a case for water frost. Also. the brightness of the frost "indicates it has a lot of dirt mixed in with it. The dirty nature of the ice had also been seen now by the orbital imaging system." Apparently there is no permanent reservoir of carbon dioxide in the polar regions of Mars, a finding that tends to rule out the theory of a rapid climate change induced by the instability of the carbon dioxide on the planet. "This means we still don't have an adequate explanation of how the atmosphere could have been of sufficient density to sustain the liquid water that appears to have flowed at one time in streams and rivers on the surface of Mars,'' said Kieffer.