When we Canadians think of snow, it's of the water-ice variety. And though every snowflake is unique, they're all made of the same stuff and share a similar geometry. But depending on the temperature-pressure regime and the composition, many different other types of snow are possible on other planets. On Mars, it can be both water ice or carbon dioxide ice snows. For Saturn, ammonia ice snow is common. But on exoplanets, still stranger snows can fall to a planet's surface. Yet, as Giang describes below, there might be some surprising parallels.
By Giang Nguyen
K2-141b is a recently discovered lava planet where almost half the planet is a giant magma ocean that we’ve estimated to be 140 km deep. Since the planet orbits so close to its star, temperatures are hot enough to vapourize magma which forms a thin mineral vapour atmosphere. My work on the planet involves simulating atmospheric and oceanic flow given likely compositions of K2-141b. As this work on the initial characterization of K2-141b’s atmosphere is coming to an end, I’ll be expanding on my work with more complexities for my next project.
One of the complexities that I’ve been very interested in is the condensation process on K2-141b as well as incorporating more constituents into our previously idealized pure atmosphere. This would help me understand and envision what the weather on K2-141b would be like. On Earth, when I think of weather, I think of things like wind, clouds, and precipitation. Clouds are particularly noteworthy here as they precede rain and snow. On Earth, clouds and precipitation usually involve water but on Mars, carbon dioxide plays a large role as well. On K2-141b, the hydrological cycle likely depends on silicates.
Silica (silicon dioxide) is the dominant composition of a rocky planet’s crust where up to half of the crust is expected to be silica. The crust is also likely made up with magnesium oxide and titanium oxide. However, since the atmosphere is likely to be made up of very volatile compounds, we expect sodium, silicon monoxide and oxygen along with trace amounts of iron and magnesium. As these molecules are evaporated from the hot dayside where the pressure is high, they are pushed by winds to the cold nightside where the pressure is low. When the atmosphere cools, the mineral vapour condenses into liquid droplets, likely forming clouds, and rains out onto the magma ocean. However, when the atmosphere is cool enough where silica condenses as a solid, interesting things happen.
On Earth, snow forms when water vapour directly turns into ice; liquid droplets that freezes do not become snow but instead become sleet. During the snow making process, water will freeze onto existing ice crystals in a pattern specific to the water ice’s hexagonal crystal structure. Although we tend not to think of solid material as ice, it is useful to think so on lava planets where rocks are simply frozen magma. Therefore, when the mineral vapour condenses, the result would be clouds of mineral. As silicon monoxide and oxygen freezes out and combine, you essentially get quartz.
Quartz is a common mineral on Earth and is useful in keeping time (although modern timekeeping standards now rely on quantum time constants and radioactive decay). One can easily imagine seeing quartz on beaches and in the wild, but it is much harder to imagine the quartz falling out of the sky as snow on K2-141b. How different would snowflakes be on this lava planet than on Earth?
Well, as mentioned before, the typical photogenic Earth snowflake has 6 arms due to the hexagonal structure of water ice. Quartz also has a hexagonal crystal structure, so a quartz snowflake should also follow suit with 6 arms where each arm should have fractal hexagonal pattern. Significant impurities in K2-141b’s atmosphere such as iron or magnesium that binds with the quartz would give the snow a much more diverse colour palette.
The most abundant impure molecule expected to bind with the quartz should be iron (as FeO or just Fe). Note that sodium is actually the most volatile molecule, but it is fairly scarce on the crust which invites skepticism on how much sodium can be evaporated into the atmosphere. Impurities in crystal composition gives colour to an otherwise clear gemstone and iron can transform the colourless quartz into an array of beautiful gems. On Earth, quartz have different varieties when iron impurity is present: rose quartz, purple amethyst, yellow citrine, and green prasiolite.
Of the quartz variety mentioned above, their prevalence on Earth can be directly influenced by humans as most of the citrine and prasiolite are created from heat treatments of the natural occurring amethyst. However, it can be pretty hot on a planet that’s half molten magma so these minerals may form naturally on K2-141b. Traces of magnesium can also find themselves bound to silicates and iron and form other gems such as peridot or chrysolite (gem-quality olivine).
Ultimately, weather systems are largely unknown for exoplanets and our proposed atmospheric and surface compositions are educated guesses at best. However, it is exciting to envision a planet with pink, purple, green, and yellow Earth-like snowflakes made of quartz flying through the sky. Imagine what twilight would look like when even the crystal clouds have a slight purple or pink hue. In conclusion, if this isn’t one of the most cinematic sci-fi setting ever, I don’t know what is.
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