Some of Earth’s most majestic features are its towering snowcapped mountains, reaching high enough that they can sculpt our world’s weather systems. But the mountains on Earth are by no means unique, and neither is the snow. Mountains on Venus are also capped with snow. Except that Venusian snow is mostly made from heavy metals.
As you might expect from a planet with such an alien atmosphere, the snow which caps the Venusian mountains is seemingly no less exotic. With the high temperatures on the planet’s surface, water ice is impossible (not that there is much water on Venus). It’s made from lead sulfide and bismuth sulfide, more commonly known as the minerals galena and bismuthinite.
We got our first glimpses under the corrosive clouds of Venus down into its dense, scorching interior late last century. Swathed in thick layers of cloud, roughly 50 kilometers (30 miles) deep, we have only two ways to see the surface of Venus — either descend to the harsh surface as the Soviet Venera landers did, or use radar to see through the clouds from orbit.
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.
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.
At higher altitudes, this mist condenses, forming shiny, metallic frost on the tops of the mountains. And Earth’s simmering sibling has plenty of high altitude terrain. Maxwell Montes, the tallest peak on Venus, stands at an altitude of 11 kilometers (6.8 miles) — 3 kilometers (1.8 miles) higher than Mount Everest.
Whether snow genuinely falls on Venus is still unknown, but it’s certainly possible. Sulfuric acid rains have been observed plentifully on Venus as virga — rain which evaporates before it hits the ground, just like over the rainforests on Earth.
The heavy metal snows can be observed anywhere on the surface of Venus over about 2.6 kilometers (1.6 miles). It may not be any coincidence that below this altitude Venus’s atmosphere is technically no longer a gas.
Over 96 percent of the Venusian atmosphere is carbon dioxide, and our sister planet has nearly 100 times as much atmospheric gas as Earth does. This is what causes the huge crushing pressure at Venus’s surface, but that pressure also has a rather strange effect on the gas.
At the pressures and temperatures found near the surface of Venus, carbon dioxide becomes a “supercritical fluid” — an unusual state of matter, partway between a liquid and a gas. Often used on Earth as an industrial solvent, this supercritical carbon dioxide is expected to be found on Venus, coincidentally, at altitudes below around 2 – 3 kilometers (1.2 – 1.8 miles).
While we’re a long way from taming Earth’s evil twin, meaning that it’s unlikely human eyes will see the surface of Venus directly anytime soon, one thing’s very likely. Both the lead sulfide and bismuth sulfide which make up Venusian snow are a light greyish color with a shiny, metallic lustre.
Which means the the mountaintops on Venus are probably beautiful, glinting in the sunlight that filters through the dense clouds. Who knows? Maybe in a few hundred years, the Venusian mountaintops might even be a tourist attraction.