Photo: Wikimedia Commons
On Earth, we get snow, rain, fog, hail, and sleet, and all of them are basically the same thing: water.For a true change of weather, you need to go to other worlds.
Here’s a tour of what to expect on a trip through our solar system.
Scientists have known for years that the polar caps of Mars are made of a combination of water ice and dry ice (or frozen carbon dioxide -- the same stuff that makes fog when you dump it into a pot of water).
But how does it get there?
The ice caps grow and recede with the seasons (in the Hubble images above, the carbon dioxide is receding with the onset of spring), so either the carbon dioxide is freezing directly out of the atmosphere, or it's snowing.
Scientists working with data from Mars Reconnaissance Orbiter recently solved the puzzle: MRO detected clouds of carbon dioxide crystals, and clear evidence of snow falling out of them. The snow would not fall as flakes, but as tiny cuboctohedrons (which have eight triangular faces and six square faces).
On the surface, Mars snow probably looks like granulated sugar.
Once thought to be our sister planet, Venus is, in actuality, a hellhole.
The surface is over 462 degrees C (864 degrees F) -- easily hot enough to melt lead -- and the atmospheric pressure is about 92 times the pressure on Earth at sea level. It's also bone dry (water is baked out of the soil).
But high up above the slowly rotating surface, where the winds whip violently, Venus is enshrouded by clouds of sulfuric acid (shown here in ultraviolet light from the Hubble Telescope). When it rains, the acid falls down to about 25 km before evaporating -- at these temperatures, even sulfuric acid can't stay liquid.
The vapor rises back up to recondense as clouds, giving Venus a liquid cycle confined entirely to the upper atmosphere.
Venus isn't the only hellhole in the solar system.
Jupiter's moon Io would fit the bill pretty well, too. It's riddled with active volcanoes, covered in brimstone, and hiding a subsurface ocean of lava. And it snows the sort of snow you might get when Hell freezes over, because it too is made of brimstone: sulfur, and, more specifically, sulfur dioxide, which were detected when the Galileo orbiter flew through the volcanic plumes on its kamikaze mission in September 2003.
Molten sulfur, heated to the boiling point below the surface of Io by torturous tidal flexing, sprays out of the volcanoes like a geyser would spray water on Earth. In the cold, airless void of space, the sulfur dioxide quickly crystalizes into tiny flakes; most of it falls back to the surface as a fluffy yellow snow.
Galileo's sensors indicated that the particles were very small, perhaps 15-20 molecules apiece, so the snow would look extremely fine on the surface. In the photo above, the broad white semi circle of material is sulfur dioxide snow from a plume called Amirani.
Titan is Saturn's largest moon, and the pictures revealed by Cassini and the Huygens lander show a world that looks surprisingly Earthlike, with riverbeds, lakes, and clouds. (The radar image above shows the shores of Kraken Mare, the largest known lake on Titan, with rivers flowing into it.)
But this is deceptive. Titan is much colder: What looks like rock is water ice, and what looks like water is natural gas.
A methane cycle (much like the water cycle on Earth) exists on Titan, driving seasonal rains that follow patterns (much like the ones tropical monsoons follow on Earth). When the season is right, the rain falls, filling vast but shallow basins bigger than our Great Lakes. As the seasons change, the lakes slowly evaporate.
The vapor makes its way up into the atmosphere and condenses into clouds; the clouds drift to the other hemisphere as the weather shifts, and when the rain falls, it starts the next loop of the cycle.
The environments on gas giant planets are extreme in many ways; one is that there is a depth within them at which the atmospheric pressure is so great that exotic forms of matter appear, such as metallic helium and hydrogen.
If the models are correct, above Jupiter's rocky core lies a deep ocean of liquid metallic hydrogen. Helium is a little harder to compress into a metallic form, so it doesn't mix with this ocean.
It is heavier than hydrogen, though; scientists believe it falls through the metallic hydrogen ocean like droplets falling through the atmosphere, until it gets deep enough to become metallic.
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