Let It Snow (on K2-141b)

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.

Here's what hexagonal symmetry can do, working with water ice on Earth. Just imagine what might be possible on K2-141b! (photo by Lt. Cindy McFee - Edited version of Image:410px-Sun halo optical phenomenon.jpg., Public Domain, https://commons.wikimedia.org/w/index.php?curid=5267988 ).

What do scientists want to explore next in the solar system?

 

T'is the summer of the National Academies Planetary Decadal Survey a chance for the community to come together and motivate the next ten years of space missions. Because Planetary depends heavily on this kind of "Big Science" it makes sense to organize. The last time, Mars Sample Return was the winner and since MSR looks to feature prominently in the coming decade it will be interesting to see how everything plays out. For now, the Science White Papers are in! But the Mission White Papers and the Community White Papers have yet to come - those will be in August and September, respectively.

By Hemani Kalucha

July 15 marked the submission of Science White Paper proposals to the Planetary Science Decadal Survey. Every 10 years, the scientific community makes a plan of what they want to explore next in the universe; this is called the decadal survey. It is written by roughly a 100 scientists, all with different expertise across a range of fields. Based on the science they want to accomplish, they develop mission proposals and budgets that strongly influence space agency activity for that decade. The next decadal survey will be published in 2023, and as a part of this process, the survey committee asks for the input of scientists around the world, which is published in the form of whitepapers. The whitepapers are open to everyone and can be read here: https://www.nationalacademies.org/our-work/planetary-science-and-astrobiology-decadal-survey-2023-2032. I, myself, got the chance to be a co-author on my first whitepaper this time (pictured above), which made me feel a little less like a new grad student and more like a real scientist!

Out of the 341 papers submitted, roughly 48 were about Mercury/The moon, 37 were about Venus, 74 were about Mars, 41 were about giant planets (Jupiter, Saturn, Uranus, Neptune), and 50 were about ocean worlds (Titan, Enceladus, Europa). The whitepapers explored themes like prebiotic organics, habitability, solar system formation, geological evolution, atmospheric evolution, and more. As I was going through the list, a few titles really stuck out to me, and so I’ve shared them below with a little description of the scientific gap they are trying to address. There are, of course, whitepapers on expected topics such as Mars Sample Return, Mars subsurface research, the return to the Moon, and the radiation belts of Jupiter. The reasons for these missions have been justified several times, so I won’t go into them here.  

Dive, Dive, Dive: Accessing the Subsurface of Ocean Worlds: This paper and a host of other papers recommend a development program of ocean-world exploration surface technology this decade, in order to achieve a subsurface mission to first Enceladus and then Europa in the 2033-2042 decade. The technology required includes drills that can reach 15-20 km into Europa’s ice sheet, sample handling systems that can manage liquid samples, reliable through-ice communication systems (tethered and wireless), guidance and navigation systems capable of identifying in-ice hazards, and compact power systems for deep ice probes. Although this sounds like a daunting set of requirements, a lot of technology that is used in deep ocean exploration on Earth can give heritage to these types of missions, which is an exciting crossover! 

Venus, an astrobiology target: This paper suggests that there could be life living in the acidic clouds of Venus! Venus is thought to have sustained oceans in its past for a long period of time – 600 million years, which allows for a source of water activity in the clouds. In addition, clouds on Venus have earthlike temperatures and pressures, suggesting the possibility of airborne microbial species. Even though the clouds are highly acidic, the paper claims that bacteria like the acidophiles found on Earth may be able to survive. One of many obstacles is the fact that though airborne bacteria have been detected on earth, airborne reproduction is yet to be found, which means if bacteria do exist in the clouds of Venus, they might still need to land on the surface occasionally. However, the surface is extremely hostile to bacteria…needless to say, I appreciated the creativity of this paper, even though it may be hard to find life on Venus.  

Science and technology requirements to explore caves in our solar system: the idea of accessing the martian subsurface using caves has been discussed for some time now. It turns out that caves exist on a host of other bodies in the solar system: Titan, Ceres, Venus, Europa, Enceladus, Ganymede, Io, and even Vesta the asteroid! Caves are extremely useful features: they provide access to the geological record without the need for drilling, and preserve volatiles, ice, and organic matter, all while sheltering the material from radiation! This paper therefore suggests that a large effort of exploration go towards cave exploration. This would begin with identifying caves all over the solar system using flybys and orbiters, and then exploring caves of high interest with drones, crawlers, and microbots. One day, it may even be possible to set up sensor networks inside the caves for continuous monitoring.