T he image above might suggest someone dropped a small planet in the middle of the Presumpscot River in Westbrook, Maine, but (shocker) that’s not the case. It’s actually a big disk made of ice that’s slowly rotating in place.
It would be reasonable to think the river’s current is propelling that rotation, but research suggests the object may be spinning all on its own. Giant disks like this form not infrequently during the cold winter months, and in 2016 some physicists decided to get to the bottom of the phenomenon. The group created a miniature ice disk and floated it in a tank, where they determined that the rotational force was generated by a vortex that formed beneath the disk. Water, you see, is at its most dense at 4°C (that’s 39.2°F). As the ice of the disk cools down the fluid surrounding it, that water reaches the four-degree mark and sinks. It flows down and horizontally, creating a swirling vortex. And the larger the temperature gradient—meaning the warmer the river water is compared to the frozen disk—the faster the cooling water will sink. That means the disk spins faster, too. it’ll spin because the cooling water will sink faster.
That also means spinning disks like this wouldn’t necessarily happen in any body of water. Some lakes may already be at 39.2°F or colder when water starts freezing, which means the cooled water next to the ice wouldn’t sink at all, and thus there wouldn’t be a vortex to rotate the disk.
The same phenomenon also happens you place an ice disk on a solid surface, such as a plate of aluminum. If the plate is warmer than the ice, it melts and creates a miniature vortex inside the pooled water beneath.
We can all agree that the science behind this photo is the coolest thing about it, but that doesn’t mean we can’t enjoy watching this drone footage of the disk set to almost-over-the-top dramatic music: