Venus snow is a brightening of the radar reflection from the surface of

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

at high elevations. The "snow" appears to be a mineral condensate of

lead sulfide Lead sulfide refers to two compounds containing lead and sulfur: * Lead(II) sulfide, PbS, containing lead in the +2 oxidation state, naturally occurring as the mineral galena *Lead(IV) sulfide Lead(IV) sulfide is a chemical compound with the formu ...

and bismuth sulfide precipitated from the atmosphere at altitudes above . The nature of the "snow" was initially unknown. In radar images, smooth surfaces such as lava plains generally appear dark, while rough surfaces such as impact debris appear bright. The composition of the rock also alters the radar return: conductive material, or material with a high

dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insula ...

, appears brighter. It was therefore initially difficult to determine whether the high-altitude areas of Venus were different from the lowlands in chemical composition or in texture. Possible explanations included loose soil, different rates of weathering at high and low elevations, and chemical deposition at high elevation. It could not be water ice, which cannot exist in the extremely hot, dry conditions of the Venusian surface. Data from the radar mapper on the

Pioneer Venus The Pioneer Venus project was part of the Pioneer program consisting of two spacecraft, the Pioneer Venus Orbiter and the Pioneer Venus Multiprobe, launched to Venus in 1978. The program was managed by NASA's Ames Research Center. The Pio ...

orbiter suggested an explanation in terms of chemical composition. It was hypothesized that the underlying rock contained

iron pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue gi ...

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 Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...

at high elevation might continually expose new material, causing the highlands to appear brighter than lowlands. High-resolution radar observations by the

Magellan probe The ''Magellan'' spacecraft was a robotic space probe launched by NASA of the United States, on May 4, 1989, to map the surface of Venus by using synthetic-aperture radar and to measure the planetary gravitational field. The ''Magellan' ...

by 1995 began to favor the hypothesis that metallic compounds sublimate in lower, warmer altitudes and deposit in higher, cooler areas. Candidates included

tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionall ...

, pyrite, and other metal

sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds la ...

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References

{{Venus Geology of Venus Surface features of Venus