It’s winter here in Minnesota, and that means snowsports! Sure, it’s unusually warm this January, and our snow is in danger of melting, but we still love it. Fluffy flakes of frozen water, piled up and perfect for skiing, sledding, snowmobiling, and more!
But snow isn’t just a feature of Earth. Enceladus is believed to have deep powder, fallen as snow from its huge geysers. And other worlds have stranger snows….
Mars is famously thought of as a desert, but anybody with a large enough telescope can tell that it has seasonal icecaps at its poles. But it’s not just the kind of ice we know. The water ice at the poles is pretty much permanent, but the part that grows and recedes isn’t water. It’s carbon dioxide. For a long time, it’s been believed it grows as frost, but Mars Reconnaissance Orbiter discovered evidence that it doesn’t just precipitate out on the ground; it comes out in the atmosphere and falls as snow. Dry ice snow! It’s white, but in other respects very different from what we’re used to. Experiments on Earth suggest the “flakes” will actually be tiny cubes little larger than a grain of sand, so it might be more like skiing on a big mound of salt, except that dry ice melts at a low enough temperature that the friction of your skis could melt it — if it weren’t for the high pressure required to liquify CO2. It would go straight from solid to gas, and you could completely lose control. In fact, actual observations of the polar regions suggests that it regularly does sublimate, with explosive results, as it heats up in the spring. So maybe not the best place to ski.
Io is the innermost of Jupiter’s four largest moons, the Galilean satellites. The twin Voyager spacecraft quickly revealed that it was like no other moon, and is actually more volcanically active than the Earth is. Much more: we now know that like Europa, it has a vast subsurface ocean, but unlike Europa it’s not water. It’s molten rock, and mostly sulfur. This sulfur erupts out through Io’s many large volcanoes, rising hundreds of kilometers into space above the surface and then falling gently back down in an enormous spherical plume that took the Voyager mission scientists completely by surprise. As it falls, it freezes into a fine dust — effectively, snow. It creates vast white snowfalls on the moon’s surface, but they’re not our kind of snow. It’s sulfur dioxide. In August of 2001, the Galileo spacecraft fortuitously flew through one of these plumes, directly sampling the snow. I don’t think you’d want to ski there, though. Apart from the totally absent atmosphere and the lethal radiation from Jupiter, there’s also the fact that you’re underneath the plume of an active volcano. Um . . . I think I’d rather brave the lines at Big Sky or Aspen than try for that.
Okay, this one’s a bit more speculative, since the surface of Venus is nearly impossible to photograph shrouded as it is behind thick clouds of sulfuric acid and an unbelievably dense and hot atmosphere. The first landers to make the attempt were crushed before they reached the ground; the next ones, built like submarines, expired from the heat. So how could there be snow?
In the image above, there are some surprisingly bright areas next to some surprisingly dark areas, and the bright areas are all high latitude. That’s a radar image from the Magellan spacecraft, so don’t assume the light areas are actually brightly colored; all it really means is that they’re very reflective to radio wavelengths, while the black areas just soak it right up. Snow is very bright to radar, with its multitude of fine surface particles. And it’s not the only thing that can be bright, so bear in mind that maybe this one isn’t snow at all. But if it were, what material could possibly snow on Venus?
Scientists have speculated that it must be some kind of a metal compound forming a crystalline material, such as coloradoite (a blend of mercury and tellurium) or tellurobismuthite (bismuth and tellurium). Heavy metals, which would be liquid in the lower elevations, but like water on Earth, would freeze at higher altitudes. Definitely don’t contemplate the skiing here, though, even if it really is snow. Venus is off limits. If it weren’t for the active volcanism, crushing atmospheric pressure, heat sufficient to melt lead, violent winds at high altitude, and sulfuric acid rain during the descent, Venus might actually be a nice place. 😉 But it’s not, and whether this stuff is heavy metal snow or not, it’s certainly a reminder that although some worlds can be surprisingly Earthlike, others are anything but.