In 1990, when the Galileo satellite whipped around the Earth on its way to Jupiter, it caught sight of mysterious flashes of light coming from the surface of the Earth.

Twenty-five years later, the Deep Space Climate Observatory, or DSCOVR, which is stuck out in space between the Earth and sun almost four times as far away from the planet as the moon, was also capturing these mysterious flashes of light. In fact, DSCOVR captured more than 800 of these flashes between 2015 and 2016, and scientists have figured out what they are.

The puzzle was solved by Alexander Marshak, DSCOVR deputy project scientist at NASA’s Goddard Space Flight Center.

He had noticed the flashes sometimes showing up over oceans while he looked through images from the observatory’s on-board camera, Earth Polychromatic Imaging Camera, or EPIC.

But when Marshak went back and looked at the Galileo images as well as the images from EPIC.

“We found quite a few very bright flashes over land as well,” he said.

Since it seemed like bodies of water were not the source of the flash, Marshak joined forces with Tamas Varnai of the University of Maryland, Baltimore County, and Alexander Kostinski of Michigan Technological University, using data from EPIC to figure it out.

So what is it? Nearly horizontal ice particles in cirrus clouds that are three- to five-miles high up in the atmosphere.

That’s the conclusion from works published in Geophysical Research Letters.

So mystery solved, and now what to do with this information.

First, scientists can try and figure out how often these ice crystals form, and to see if they can be used in calculations of how much of the sun’s energy are reflected back into space.

Down the road, a similar observational approach could potentially help scientists determine if distant exoplanets have the same sort of reflection, meaning the planets could have an atmosphere, and worth further inspection as a potential home for future generations of humankind.