Venus is a very hot planet with a mean surface temperature of
740°K. It is so hot there that some metal compounds of chlorine,
fluorine, and sulfur are vaporized in some locales. Such metallic
"mists" could well coat out as "frost" on
the cooler, higher elevations. Actually, two outstanding Venusian
anomalies can be explained by such metallic "frosts."
Radar signals from earth are strongly reflected from the planet's
mountains and high plateaus. These regions may owe their unusually
high reflectance to metallic "frosts" consisting of
such radarbright minerals as pyrite, which is probably present
in vapor form at lower, hotter elevations.
During the 1978 Pioneer Venus mission, four instrumented
probes plunged into the Venusian atmosphere. All instruments
with external sensors on all four probes failed mysteriously
12.5 kilometers above the planet's surface. Thinking is that the
probes pierced a cloud deck of metallic vapor that condensed on
the cold sensor surfaces. (Anonymous; "Metal 'Frost' on Venus?"
Sky and Telescope, 90:13, August 1995.)