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Sizzling Snow

by Perry Pezzolanella

Snow, frost, and ice are common throughout the Solar System from Pluto’s methane ice capped peaks, Io’s sulfur dioxide frosted plains, and Mars’ carbon dioxide frost, to Earth’s water ice glaciers. Water ice even lurks in the permanently shadowed polar craters on the Moon and Mercury where temperatures never rise above -300ºF. There is even snow on Venus.

Venus? How can snow exist on a world where the average surface temperature is 870ºF? It is possible if the mind does not think of snow as something like a chilly winter wonderland. Discovery of metallic snow dates back to 1995 when data from the Magellan spacecraft that orbited Venus from 1990-94 was being analyzed. Magellan mapped the surface of Venus using synthetic aperture radar that revealed a mysterious brightening effect in the highlands on its imaging maps. Computers factoring the abundance of elements present on Venus with altitude, temperature, and pressure determined the brightening effect was due to a metal-coating snow, more like a frost, that coated the rugged high peaks, probably no more than a tenth of an inch thick. The chemical composition remained a mystery until now. The old theory was that gasses emitted from erupting volcanoes consisted of trace elements that could exist as a vapor in the hot atmosphere until they reached a high enough altitude where they would condense. The highest mountains on Venus such as Maxwell Montes at 37,000 feet are refreshingly cool at about 700ºF where some of the elements could react to form compounds that would condense out of the atmosphere and coat the cooler peaks. An early candidate was tellurium, but sulfur dioxide, the third most abundant gas in the atmosphere and a major contributor to the thick clouds, can readily react with tellurium creating tellurium sulfide, a stable gas unlikely to condense.

Scientists use a specific physical property called the dielectric constant to describe a material’s electrical conductivity. Typical volcanic rocks have a dielectric constant between 2 and 4, but Magellan found that the dielectric constant on the high peaks was much higher, nearly 100. To have a reading this high either a semiconductor or a conductor had to be present. There are two minerals that are a perfect match: lead sulfide (galena) and bismuth sulfide.

Volcanoes release gases containing lead and bismuth. Rocks at lower elevations also bake out these elements where the temperature exceeds 800ºF, which is well above the melting point of lead at 622ºF and bismuth at 521ºF. As the vapor rises a few miles into the atmosphere where the temperature falls, they react with the sulfur dioxide to form lead and bismuth sulfides, which then condense out onto the cooler highlands. The confirmation of lead is exciting because it will make it easier to date the age of the surface of Venus since lead isotope ratios are used for dating on Earth. The only problem is obtaining a sample of the soil to analyze.

Landing a spacecraft on Venus is a huge challenge due mainly to the intense heat averaging 870ºF. The extremely high pressure, 92 times Earth’s, has been overcome by using a titanium sphere that is also resistant to the corrosive sulfuric acid mist in the atmosphere. No spacecraft has lasted more than two hours, but new advances in high-temperature electronics is making it possible to survive longer on Venus. Plans for a long-lived lander are in the works within the next decade, but a sample return mission is decades away, probably after Mars, Europa, and Enceladus samples are collected and returned.

There is little, if any sun shining through the thick clouds, so images of snowcapped mountains on Venus are only a dream. There may be a change in color and texture of the soil at the snowline, but it will take a lander, rover, drone, balloon, or something with a camera to find out. Someday, centuries from now, we will go to Venus in person on a skiing vacation because who can pass up some of the hottest ski slopes in the Solar System where conditions are sizzling fast!