Astronomy/Rain in other planets.
Dear Prof Courtney
Is there a possibility of rain occuring in other 8 Planets?
In general, the other planets cannot have rain in the sense that the Earth does, but there could be liquid droplets gradually falling relative to the surrounding gases in some part of the atmosphere of most of the other planets.
As discussed in "The 'Surfaces' of the Jovian Planets" (at http://cseligman.com/text/planets/outersurfaces.htm
), those planets have no surface such as the Terrestrial planets have, and in the lower part of their atmospheres the temperatures are extremely high (discussed in detail for Jupiter on the page just noted, and to a lesser extent true for all of them), so no liquids should exist in their "lower" atmospheres (there is a point where the temperature and pressure are so high that it is impossible to distinguish between a dense gas and a liquid -- referred to as the "triple point" in terms of the conditions, but that is not the same as the "place" where such conditions exist). Above that, liquid droplets could exist and almost certainly do exist for one kind of material or another, so there could be and probably is "rain" of some sort in the upper atmospheres of those planets.
Similarly, there should be liquid droplets (primarily of sulfuric acid) in the upper atmosphere of Venus, but further down it is too hot for such liquids to exist.
For the other three planets, the likelihood of "rain" is much lower. Mars could conceivably have some kind of liquid droplets in some parts of its atmosphere, but because the atmosphere is so thin and so cold, odds are that ice crystals (primarily of water and/or carbon dioxide) are the primary component of the occasional "clouds" seen in its atmosphere. Mercury's atmosphere is far too thin (essentially a vacuum) to have any kind of liquids, and although Pluto does have haze in parts of its very thin nitrogen atmosphere, they are probably microscopic bits of ice of some sort, and not liquid.
So for Venus and the Jovian planets I'd say yes, there probably are liquid droplets in some parts of the atmosphere, but nothing near the surface, no matter how that is defined. And for the other three planets I'd say no, there cannot be liquid droplets, but there might be icy bits for Mars and Pluto; and no, there cannot be anything of the sort for Mercury.