When orbiting space probes such as Pioneer Venus and Magellan used their radar instruments to look under the clouds though, they were met with something unusual. The highlands of Venus seemed unusually reflective, appearing much brighter than the lava plains of the Venusian lowlands.
This was quite a puzzle at first, which planetary scientists took some time to disentangle. There were a few possible explanations to choose from. A different surface texture on the mountaintops, perhaps looser soil, might cause them to appear differently, or a different kind of weathering at high altitude might cause the terrain to be different. Alternatively the surface at high altitude may be chemically different.
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After Magellan arrived in the orbit of Earth's twisted sister in the 1990s, it took some more detailed measurements. Everything pointed towards some form of chemical deposition occurring on the higher ground.
As we now understand it, the snow on Venus' surface is probably more similar to frost. On the lower Venusian plains, temperatures reach a searing 480°C (894°F). This is hot enough that reflective pyrite minerals on the planet's surface are vaporized, entering the atmosphere as a kind of metallic mist, leaving only the dark volcanic rocks like basalt in the Venusian lowlands.