neufer wrote: https://en.wikipedia.org/wiki/Noctilucent_cloud wrote:
... (On August 28, 2006, scientists with the Mars Express mission announced that they found clouds of carbon dioxide crystals over Mars that extended up to 100 km above the surface of the planet; they are the highest clouds discovered over the surface of a planet.) ...
Interesting. In the referenced article (https://solarsystem.nasa.gov/news/2006/ ... nd-on-mars
), it seems that the definition of "visible" was pushed pretty far. It is also interesting that they say "over the surface of a planet". One can immediately consider the gas giants, and they are ruled out of this comparison if we do not know where the surface of the planet is, or if one exists. If we decide that Jupiter or Saturn have something we will agree to call a surface, then ... new record.
It was also instructive (for me) to look at the Wikipedia definition of a cloud:
In meteorology, a cloud is an aerosol comprising a visible mass of minute liquid droplets, frozen crystals, or particles suspended in the atmosphere above the surface of a planetary body. (Primary source, National Weather Service "Weather Terms".)
I'm surprised that this definition is already "solar-system-ready" (and beyond), not simply a definition that is appropriate for Earth. It is a sign of our progress in our experience with atmospheres that we want a definition that is so widely applicable. But this definition means the particles have to be suspended in the planet's atmosphere, so planetary rings and such are not going to qualify.