Venus snow is a brightening of the radar reflection from the surface of Venus at high elevations (mountains). The nature of the "snow" on Venus was initially unknown. It could not be water ice, which cannot exist in the extremely hot and dry conditions of the Venusian surface. Posisble explanations included loose soil, different rates of weathering at high and low elevations; and chemical deposition at high elevation.
In radar images of the planet, smooth imsurfaces such as lava plains generally appear dark, while rough surfaces such as impcat debris appear bright. The composition of the rock also alters the radar return: conductive material, or material with a high dielectric constant, appears brighter. Therefor it was difficult to determine whether high-altitude areas were different in chemical composition or different in texture from the lowland.
Data from the radar mapper on the Pioneer Venus orbiter suggested an explanation in terms of chemical composition. It was hypothesized that the underlying rock contained iron pyrite or other metallic inclusions that would be very reflective. At the high temperatures found on the surface of Venus, these minerals would gradually evaporate. Faster weathering at high elevation might continually expose new material, causing the highlands to appear brighter than lowlands.
High resolution observations by the Magellan probe by 1995 began to favor the hypothesis that metallic compounds sublimate in lower, warmer altitudes and deposit in higher, cooler areas. Candidates included tellurium, pyrite, and other metal sulfides. More recent work has supported the mineral condensate scenario, and identified the "snow" as lead sulfide and bismuth sulfide precipitated from the atmosphere at altitudes above 2600 m.