The Wildest Clouds in The Universe

Flying on a Delta MD90 jet on my way back from Munich, Germany to Tucson among gorgeous towering clouds glowing in exotic shades of yellow, orange, and purple. Amazing view – especially interesting is the all the different clouds we see are made of water.

How would clouds on exotic other planets look?

Earth is special (in the Solar System, but not in the Galaxy) that it hosts three phases of water simultaneously: liquid, gas, and solid (ice). This allows some fun physics to take place and lead to the hydrological cycle we all know. In the terrestrial atmosphere most water clouds form as the rising moist air cools down and water condensates to condensation nuclei and forms droplets; this process forms the low clouds (~1-2 km). Higher up in the atmosphere, where temperatures are very low, ice crystals will form clouds (>6km).

But we don’t need to go far to find exotic clouds: Earth’s hostile twin, Venus, has a thick carbon dioxide atmosphere (with a crushing surface pressure as high as 900 meters underwater on Earth!). As Venus has lost all of its water in the past, it does not have water clouds – but still has very thick clouds. And these clouds are made of hot droplets of sulphuric acid ()! The cloud layer is so thick that no sunlight reaches the surface directly. The thick yellowish cloud layer has blocked the views of surface from the early space probes.

Or think about the amazing Titan, Saturn’s large icy moon (larger than planet Mercury!). Titan is the only moon in the Solar System that has a significant atmosphere – in fact its atmospheric pressure is higher than that of Earth. But the surface temperature of Titan is so cold (only ~93 K) that it allows the three phases of methane on its surface. The Cassini orbiter, which keeps on returning amazing high-quality images from the Saturnian system, has photographed the formation and evolution of clouds in Titan’s atmosphere. The clouds of Titan are not well understood and remain an exciting field of research.

These planetary bodies are not exceptions: in fact, every Solar System planet that has an atmosphere has clouds, too. Depending on the temperature and the pressure (often called the “P-T profile”) and the composition of their atmospheres Solar System planets have different clouds. The very thin atmosphere of Mars allows the formation of tenuous water ice and carbon-dioxide clouds; Jupiter and Saturn sport clouds made of ammonia (), ammoniahydrosulfide (), and water; while the upper atmospheres of the even colder ice giants Uranus and Neptune also have methane () and clouds.

This bonanza of clouds raises the question: which are the craziest, most exotic clouds in the universe?
This is a difficult one, of course, as there are many candidates. My favorites are probably the clouds in hot super-Jupiters and brown dwarfs (see post on Substellar zoo). These gaseous planets and brown dwarfs have no solid surfaces but have extremely high pressures in their interiors. In their uppermost atmospheres, the layers visible to us, temperatures can exceed 1,800 K. In this class of objects iron can exist in two phases: in the hotter layer deeper down as gas and in the slightly cooler, lower pressure upper layers it will form droplets. Just as water droplets form massive clouds on Earth that can pour down heavy rain, hot super-Jupiters will have iron clouds that will drive heavy rain of hot molten iron droplets… Gives a whole new perspective on bad weather!

Back on the airplane, somewhere above New Mexico I miss another opportunity to get free pretzels, but still recall the beauty of our simple water clouds.