Rain!
Published on Aug 14, 2006 at 4:06 pm.
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Filed under planets.
It rained here Saturday night! Wow. For those of you who don’t know, we’re in a terrible drought here. This was the first rain that I’ve had at my house in almost 6 weeks, and the last time was just a sprinkle. So, this was the first significant rain in about 2.5 months. That got me to thinking about rain. We are all familiar with rain on Earth, but does it rain elsewhere in the Solar System?
Rain on Earth consists of water. The water evaporates from the ground, becomes water vapor and rises with warm air. At higher altitudes, the air has lower pressure and temperature, and so it can not hold as much water vapor as it does on the ground. The water condenses as drop
lets. Air currents keep the droplets aloft, making clouds. But, as droplets merge, they get bigger. If the droplets get big enough, then air currents no longer hold them aloft, and they fall as rain drops. Can this happen elsewhere in the Solar System?
The short answer is “no,†at least, not exactly the same. But that doesn’t mean that you don’t get rain elsewhere. Clearly, Mars is too cold for rain. Mercury and the Moon don’t have 
atmospheres to support a rain cycle. But Venus has rain or sorts. It is far too hot on Venus for rain to reach the surface. However, droplets can form in the clouds and fall down. But, this isn’t water! Venus’ clouds are sulfuric acid. So, Venus’ rain is like concentrated battery acid. Also, this acid rain doesn’t make it to the ground. Rather at lower altitudes, with higher pressure and temperature, the sulfuric acid breaks into sulfur dioxide and water vapor. These components then ride air currents to the tops of the clouds, where they recombine to form sulfuric acid again. This it totally unlike Earth’s rain cycle, where the rain stays as water the whole way through. On Venus, this cycle is one of the ways that heat makes it from below to above the cloud layers.
If Mars is too cold for rain, then planets farther out surely are, too, right? Well, not exactly. Jupiter and Saturn are clearly cold at their cloud tops. In fact, much of what you see when you look at these planets is frozen ammonia ice crystals in their upper cloud layers. However, the deeper that you go into these worlds, the warmer that it gets. In fact well below the cloud layers that we see from Earth are cloud layers that form at a level where the pressure and temperature permits liquid water droplets. Presumably, rain can happen there. But, the rain would never make it to the surface. As it f
ell deeper and deeper into the planet, the temperature and pressure would increase to the point that the water would no longer be liquid drops. It would evaporate. Then, air currents would carry it back up to repeat the process. We think. No one has ever seen this rain, though. But, the Galileo spacecraft detected towering clouds on Jupiter and lightning flashes like those that you find with thunderstorms. The Cassini spacecraft has found 
what appear to be lightning flashes on Saturn. So, perhaps you do have rain storms on these worlds. If so, they would be storms that would dwarf anything seen on Earth both in size and intensity.
Saturn has a further “rain†of sorts. Low in its atmosphere, helium can precipitate out and “rain†down to the inner parts of the planet. This has the effect of depleting helium in Saturn’s atmosphere (a puzzling result found decades ago by the Voyager probes) and warming the planet. This helium rain actually makes Saturn so warm that it radiates about double the energy that it gets from the Sun! It is possible that some similar mechanism may occur on Jupiter, but not to a great degree.
The Saturn system is also home to one other form of rain. 
On Saturn’s moon Titan, the temperature and pressure are such that very small changes in pressure or temperature can cause methane to be either solid, liquid, or gas. In fact, the moon is surrounded in methane clouds, and the Cassini spacecraft has seen what appear to be lakes and rivers of liquid methane on the Moon’s surface. Those findings are not confirmed yet, but they do look like lakes and rivers, and we do expect methane rain.
We know so little about Uranus and Neptune, that I can’t say much about them. I suppose that you could get methane rain on them the way that you get water rain on Jupiter and Saturn, but that would be pure speculation. We really don’t know. Nothing else in the Solar System is likely to be able to sustain rain.
So, there you have it. Earth isn’t the only place with rain. Now, if only I could get more on my lawn …
-Astroprof
(Images courtesy of NASA)
