A Chilly Martian Night
Viking Lander 2 photographed frost on Mars in September 1977 during the Martian winter. A planet-wide dust storm had just subsided, and the theory evolved that both water and carbon-dioxide ice had frozen on dust particles in the atmosphere. Such particles were heavy enough to fall and give the scene around Viking a snow-like coating. However, frost was again photographed in 1979 (one Martian winter later) without the benefit of a dust storm. So, Mars theorists are in a quandary -- no dust, then no frost theory is reasonable.
(Anonymous; "Viking, Three Years Later," Eos, 60:635, 1979.)
Comment. Evidently, frost cannot form directly on the Martian surface as it does on earth due to the very low vapor pressure of water on the planet.