A little while ago my kitchen sink had a serious clog. The emergency plumber worked on it for two hours. After failing to clear it with the manual plumbing snake and the much larger, electric plumbing snake, he finally tossed in the towel and poured a bottle of 80% sulphuric acid (chemical formula H2SO4) down the drain, which instantly fixed the problem. Because it’s so corrosive and has been used in acid attacks on people, he told me, the sulphuric acid is no longer available for sale to non-professionals. I told him the clouds on Venus are made of sulphuric acid. This information seemed to make his day and he asked if I know any other cool facts. Unfortunately, I answered “About planets, sure” instead of “The clouds on WASP 76 b are made of molten iron!!”
It is both true and very cool that other planets have clouds made of acid, iron, methane, titanium, and all sorts of other exotic chemistry. Clouds tickle our imaginations on many levels. When I was a child, I loved finding shapes in them; when I was a teenager, I loved psychoanalysing those shapes; and now that I’m a grown maths fan, I love contemplating their fractal nature. (Clouds are perfect fractals.) So it’s no surprise that acid and iron clouds are even more, well, metal. But how do they form? Why don’t they form on Earth? What makes a cloud a cloud, anyway?
When an atmospheric scientist says “cloud,” without specifying that it’s made of anything in particular, she generally means a water cloud like we have on Earth. Clouds form when water vapour–that is, H2O that has evaporated and is in gaseous form–condenses into its liquid phase while floating in the atmosphere. This doesn’t happen spontaneously just because it gets colder. The liquid water we encounter in our daily lives is usually attached to a solid surface: a water bottle, a glass, a window pane, the bottom of a puddle, and so on. In the atmosphere, though, water vapour can remain in gas form well below freezing temperatures. Unless temperatures are very low and the air is very saturated with vapour, water only condenses when it comes into contact with “cloud condensation nuclei” (CCN) or “cloud seeds.” These are solid particles of various types, small enough to float around in the air without settling to the ground but not tiny tiny, around a micrometre or a tenth of a micrometre, similar to the thickness of a human hair. Vapour can condense around CCNs.
There are many types of CCN in the Earth’s atmosphere: sea salt, mineral dust, soot, pollution, and more. Not just any particle can be a CCN. Clouds form around hygroscopic particles–particles that attract water. (The opposite is hydrophobic, a particle that repels water.) There are various reasons why a CCN could attract water, which are specific to the molecule’s electromagnetic and surface properties.
Let’s take sea salt as an example. Particles of sea salt get into the atmosphere from ocean spray. The chemical formula of sea salt is the same as for table salt, NaCl: one atom of sodium and one of chlorine. The chlorine steals an electron from the sodium, obtaining an overall negative charge, while the sodium becomes positive. The two are attracted to each other and form an ionic bond. The net charge is 0, but if you zoom in close, the area around the chloride ion is vaguely negative, while that around the sodium is positive. A molecule of H2O is similar, with the hydrogen atoms slightly positive and the oxygen slightly negative (although their bond is covalent, rather than ionic). The opposing charges attract the water vapour to the salt. When multiple H2O atoms are attracted to a salt particle, they also draw close to each other and form bonds with their neighbouring H2O–and there you have it, liquid water! A cloud is born. Or, in the case of your table salt, it just gets tacky.
The chemical process of attracting water vapour varies depending on the type of CCN. However, water clouds always need some kind of CCN to form. Is that true of clouds made by condensing other molecules? Not necessarily! In fact, we know very little about non-H2O cloud condensation processes on other planets.
Observations of Venus have shown that its clouds are made largely of liquid sulphuric acid, with some water. There may also be solid particles of sulphuric acid, iron(III) chloride, and other sulphur compounds present in the atmosphere. A mystery molecule known as “the unknown UV-blue absorber,” so called because it strongly absorbs UV light, is probably a solid particle. Any of these could be acting as cloud condensation nuclei. On the other hand, the sulphuric acid could be condensing on its own, without any need to cling to cloud seeds first. This would require a higher saturation of gaseous sulphuric acid, so that individual molecules are more likely to get close to each other and condense, but it’s not impossible. Clouds made of different molecules than water and in different environments can follow their own rules.
As if acid clouds and acid rain weren’t wild enough, last year a team of researchers detected what could be iron clouds and rain on an exoplanet. How can anyone guess at the weather on an environment as alien as WASP 76 b, a planet the size of Jupiter, closer to its star than Mercury is to the Sun, and with atmospheric temperatures around 2190 K (1916 Celsius or 3482 Fahrenheit!)?
WASP 76 b, like (probably) Proxima Centauri b and many other known exoplanets, is tidally locked. One side of the planet always faces its star and the other always faces away. In their study, the researchers looked at the dividing line where day becomes night, called the terminator. They found that the “evening” terminator, where the dayside gives way to the nightside, showed traces of vaporised iron in the atmosphere. On the other side of the planet, the “morning” terminator where night becomes day, there was no sign of the iron! Strong winds blow around the planet, carrying iron vapour from the dayside to the nightside, but apparently not carrying it back to the dayside. Something must be happening to the iron vapour on the dark side of the planet. Their conclusion? It could be condensing into liquid iron, forming clouds, and raining down on the nightside. (And perhaps solidifying again as it disappears into the depths of a gas giant…)
Like the sulphuric acid vapour on Venus, the iron vapour on WASP 76 b could be condensing into liquid with or without cloud condensation nuclei. If without, the atmosphere would need higher concentrations of iron vapour. Adding CCNs into the mix makes it easier to form iron clouds. But what particles could be present on WASP 76 b’s nightside to play this role is totally unknown. Fortunately for life on Earth, we have no direct experience of anything but our friendly, fluffy white clouds, and scientists have to content ourselves with peering through distant telescopes and running cloud simulations on our computers.