Viking 2 and the frost of Mars

NASA’s Viking 2 lander confirms a surprising finding first detected in black-and-white images just days earlier: Mars has naturally occurring frost. Scientists try to determine, from images alone, if the frost forms from condensation due to overnight cold (as on Earth), or through some other atmospheric mechanism. But the finding does confirm enough moisture in the atmosphere to condense on the Martian surface, decades before surface water is confirmed on the red planet.