As large parts of the Northern Hemisphere play the game of wondering if it will snow for Christmas, we thought it might be time to extend the question beyond the Earth, starting with locations closest to us and working out to planets orbiting other stars.
The Moon and Mercury
No.
Mercury and the Moon have no atmosphere, which is something of a requirement for snow. Both of them are also blazing hot during the day, although as we will discover later, that is not quite as much of an obstacle as it might seem. However, both do get very cold at night, thanks to that lasting so long. Consequently, if either had a thin atmosphere including water vapor, heat alone would not be an obstacle.
Venus
Venus of course has plenty of atmosphere, but is permanently far too hot for snow as we know it. However, Venus is so hot that pyrite minerals get vaporized at the surface, and can form a metallic frost on the tops of mountains. Whether this drifts down like snow is something future missions may reveal. Perhaps Venus, not Finland, is the true home of heavy metal snow.
Mars
Now things get interesting. The Martian polar icecaps are visible with medium-sized backyard telescopes, at least when the planet is particularly close to us. Moreover, they can be seen to wax and wane with the seasons.
Orbiters have revealed the icecaps are topped by a layer of frozen carbon dioxide (dry ice). That’s not surprising. The Martian atmosphere is 96 percent CO2, and carbon dioxide freezes at -78°C (-108°F). Unlike water, it doesn’t have a liquid phase, at least at Earthly or Martian atmospheric pressures, so there is no carbon dioxide rain. Instead, when it gets cold enough, some of the atmosphere solidifies.
Fortunately, that happens through a similar process to water vapor condensing in small flakes on Earth, rather than sudden blocks of solid CO2 forming.
Despite what an MMA fighter may tell you, Mars has a lot of Earth-like features, including its seasons, and when it’s winter in one hemisphere, carbon dioxide snow falls at the poles and on higher ground at temperate latitudes.
Water molecules have a lot of unusual features that CO2 doesn’t share, so carbon dioxide snowflakes are different – much smaller as they fall and inclined to pack together as cubes on the ground. The Martian atmosphere is so thin that future astronauts won’t need to worry about getting snowed in, but Dr Sylvain Piqueux of NASA’s Jet Propulsion Laboratory told the San Francisco Chronicle, “Enough falls that you could snowshoe across it.”
Mars even has something for the purists. Beneath the dry ice layer, the polar ice caps are mostly frozen water. Although it is probably a long time since this fell as snow, the Phoenix lander detected snow falling through the Martian upper atmosphere, and laser instruments revealed this was water snow, not carbon dioxide. The lander also found itself surrounded by frost in winter and water ice below ground.
The snow dissolved in the atmosphere long before reaching Phoenix. However, Martian mountains stretch considerably higher than the point where the snow was observed, which raises the question whether snow ever settles on the slopes of mountains like Olympus Mons.
We know there was a layer of ice there 20 million years ago, rather short by the standards of Martian history. Indeed, signs of water snow as recently as 400,000 years ago have been detected elsewhere on Mars, so maybe it is still happening.
The asteroid belt
The discovery of ice volcanoes was one of the big surprises of the Dawn mission to Ceres. We still don’t understand what powers these, but the material they spit out is thought to be a mix of ice and frozen methane. Some of their products flow like lava, but just as terrestrial volcanoes release ash there might be some ice particles thrown with more force that might slowly settle. Smaller asteroids seem like less likely candidates for this sort of thing, but then we didn’t really expect it for Ceres either.
The gas giants
Jupiter has clouds of ammonia and water ice, and sometimes these may condense, although at least one expert argues the products are probably more like hail than snow.
Neptune and Uranus are both thought to have a type of snow made of methane ice that has nucleated around a particles of photochemical haze. The snow never reaches these planets’ equivalent of a ground, instead turning back to gas at a currently-unknown depth, but looking down on these worlds from orbit, you’d still know it was snowing down there.
It is also thought that diamonds may fall from the sky on Uranus and Neptune. Somewhat curiously, this is almost always referred to as a “rain” of diamonds, but since the crystals of carbon are definitely solid, not liquid, it would be more accurate to say it snows diamonds.
Icy moons
Most of the Moons of Jupiter, Saturn, and beyond don’t need to dream of a white Christmas, because they’re permanently covered in ice, barring the odd asteroid strike – but does any fall from the sky? Permanent atmospheres are rare, but some of them may have something close. The famous geysers of Enceladus spit out ice particles. Some of these escape the weak gravity to form Saturn’s E-ring, but some almost certainly drift back down, slowly creating snow-like conditions across much of the little moon. It’s not seasonal, but it’s still likely to be exceptionally beautiful.
Even where snow may not fall, there is a possibility some of these moons’ surfaces are quite soft, more like newly fallen snow than ice. You couldn’t watch it fall, but you could still enjoy playing in it.
Europa is also a candidate for something a little different: underwater snow. Also known as frazil ice, this nearly pure ice falls like snow through the salty waters beneath ice shelves around Antarctica, and may occur on Europa and other icy moons with internal oceans.
The special moons: Io and Titan
Instead of ice volcanoes, Io has the real thing, and more than anywhere else in the Solar System. Parts of Io look like they’re covered in snow, but it’s actually sulfur dioxide, and possibly some other sulfur-rich compounds. Io’s volcanoes are so active, it has a sort of thin sulfur-based atmosphere, at least most of the time. This collapses every time Io passes through Jupiter’s shadow and the temperature drops by about 20°C (36°F), falling as something akin to snow.
Since Io orbits Jupiter every 1.8 days, and spends about 2 hours of that in the giant planet’s shadow, that means you don’t have to wait too long for a snow day on Io. Of course the snow smells foul, and is probably toxic, but you’d need to be in a spacesuit anyway, so that shouldn’t stop you making snowmen, although arguably they’d be pretty demonic.
These white areas on Io are sulfur compounds that have fallen as snow when the moon passes into Jupiter’s shadow.
Image credit: NASA/JPL/USGS
Like Earth’s, Titan’s atmosphere is mostly nitrogen. The next most common component is methane, which has both melting and boiling points quite close to Titan’s average temperature (−179.5°C, -291°F). That means Titan sometimes rains methane, and when it does it’s thought more complex hydrocarbons may be washed out as well, possibly in solid form, creating a black snow. Under the right circumstances, even the methane may snow out on its own.
Pluto
Ok, it’s not a planet, but that doesn’t mean Pluto can’t be pretty cool, as well as very, very cold. It’s covered in nitrogen ice, with a little frozen methane and carbon monoxide: so do these ever fall as snow? It seems so. Pluto’s atmosphere is very thin, but also mostly nitrogen. It seems some of its ice turns to gas in whichever hemisphere is experiencing summer. This builds up at first, but eventually the winter hemisphere cools enough for snow to start. Pluto’s long year means it takes a very long time between snowfalls, but it is expected that between 2035-2050 it will start snowing in Pluto’s southern hemisphere. That’s probably too soon for us to send a mission, so we might need to wait another 120 years until it’s the northern hemisphere’s turn.
We love you Pluto, and if we can keep Earth habitable, one day we might enyoy the nitrogen snow falling in your long winters.
Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
We don’t know enough about other Kuiper Belt Objects to really know, but the best guess is that the larger ones are quite similar to Pluto, just with more dramatic seasons from having such elongated orbits.
Beyond the Solar System
Almost certainly Earth-like planets exist where H2O snow is a common feature, but we have yet to confirm any. Our sample of exoplanets is skewed towards gas giants and rocky worlds close enough to their star to be very hot. We’ve checked some of the second category for atmospheres, but even if planets like TRAPPIST-1d turn out to have water vapor in the air, we will probably need to observe cooler ones for water snow, and they’re harder to study.
It’s easier to observe the atmospheres of gas giants, and we’ve discovered some with quite extraordinary substances falling from their atmospheres as winds carry them to the night side. These include titanium dioxide (used in sunscreen), falling as snow. In other cases the products have a wider liquid phase, such as iron and jewels like rubies, so they fall as rain instead.
All of these planets are far too hot for us to ever visit, so even if humanity voyages to the stars, we’ll never get to have snow-fights in any of these substances – your call if that’s a good or bad thing.
Under colder circumstances, Neptune-style diamond snow might be quite a common feature of carbon-rich gas giants.
