Frost forms when water in the air condenses directly from water vapor to ice and forms carpets of minute crystals that whiten our lawns in early fall. Hoarfrost forms the same way but the grains grow larger to make delicate feathers of ice. Today across parts of northern Minnesota, rime coated trees, poles and even fire hydrants in long pointy needles of ice. Rime ice forms when supercooled water droplets in the air — often suspended in a thin fog — freeze onto surfaces that are 32° F (0° C) or colder. “Supercooled water” is water that’s below the freezing point but still remains a liquid or as droplets.
I noticed haziness in the air when I stepped outside this morning, but not until I left my neighborhood and drove to a higher elevation (with a lower air temp.) did I see the transformation. Honestly, it was porcupine quills everywhere. Every edge or pointed surface sprouted long needles of the stuff. Despite their spiky appearance the crystals were very delicate and easy to crumble. While my wife shopped for fabric supplies, I wandered a mall parking lot with my phone like a kid at Disneyworld. Best parking lot ever.
For where I live, frost and even hoarfrost are common but rime is unusual. Other planets and even our moon have ice but frost? I’m sure you could find some on Jupiter’s moon Europa and Saturn’s Enceladus, but closer to home, Mars is your best bet. NASA’s Viking 2 lander photographed a thin covering of water-ice frost at its landing sight in Utopia Planitia on May 18, 1979. The Phoenix lander recorded frost more recently in 2008.
Scientists think that Martian dust picks up water in the atmosphere. Then when it gets cold enough for carbon dioxide to condense into ice, it sticks to the dust and weighs it down, making it fall and coat the ground. Whether Mars has rime is unknown — maybe in some deep underground cavern, where moist air rises from some subterranean spring and dresses the cold rocks in frosty festoons.
On Earth, hoarfrost and rime rarely last beyond the morning hours, but conditions were just right to sustain the show the entire day. Winter may sting, but these needles only pricked the imagination.