Cosmonaut Sergey Korsakov has captured a beautiful image of frost on the window of the International Space Station (ISS). Beautiful, but perhaps not unusual you may think, except, it is. This may be the first photo of ice crystals forming on a window on a spacecraft and no one knows how they formed.

The photo shows a crescent-shaped ice formation looming over the Earth, a bit like if we had an icy moon such as Europa or Enceladus. 

The image was shared by Korsakov on Twitter and Telegram in late May. On Telegram, the cosmonaut said that the ice crystals were long-lived, lasting for 24 hours, and that even after they melted, the condensation pattern remained. Its formation is not exactly clear though, and, surprisingly, there is no apparent scientific literature on this. 

When IFLScience reached out to ISS members NASA, the European Space Agency (ESA), and the Japanese Space Agency (JAXA) for any information, all three were hesitant to comment as the window may be in the Russian module of the ISS and clearly, no one is speaking to Roscosmos.

Without that knowledge, we can only speculate. The windows on the ISS are composed of multiple panes, with the internal one separated by air and the external ones by vacuum. The ice crystals could be within the panes or on the inside of the station. Some have suggested it may be the cosmonauts breathing on the window but the shape of the frost (an almost perfect circle) and the temperature of the ISS make it unlikely.

"The feature seems to have most ice at the borders of the circle –  this suggests the formation mechanism is acting equally from all sides, reaches a threshold to form ice at the circle edge, and then all water vapour is depleted before it gets to the middle," Dr James Lea, a glaciologist at the University of Liverpool, told IFLScience. 

"The bits of ice that extend into the middle are ice crystals being ice crystals and using themselves to help expand out –  they can't go outside the circle as it's too warm."

The circular shape, which is puzzling as well, could be the result of a temperature gradient in the glass. But it would depend on the windows properties, something that it is not currently clear.

"I suspect that the location of the ice reflects a temperature gradient across window. You might expect the contact point between the window and the hull of the space station at the window perimeter to be warmer than the centre of the pane." Dr Tom Whale, an expert on the role of ice nucleation in atmospheric science and cryobiology from the University of Warwick, told IFLScience. 

"If there is a little water vapour in a cavity between panes of the window, or perhaps a leak of relatively moist air from the space station into the cavity, it may be that ice tends to form at the point on the window where it gets cold enough for ice to form, and then grows inward from there. The circular shape of the ice may reflect a circular window geometry."

While this mystery might not get solved, the formation of ice crystals in microgravity is an active area of research. The JAXA has run experiments on the formation of ice crystals in orbit, discovering that ice crystals with “super” symmetry can form in space.

And as for Korsakov, this isn't the first time he's photographed something strange from the window of the ISS...