If you were to watch Earth from space, you might notice sudden flashes of light. This has been known for decades, but now scientists have found out what’s causing them: ice crystals.
The flashes were first discovered in the early 1990s by Carl Sagan and colleagues. Now, NASA’s DSCOVR (Deep Space Climate Observatory) has provided enough data to understand them – the glints are caused by horizontal, high-altitude ice crystals. So far, DSCOVR’s Earth Polychromatic Imaging Camera (EPIC) has observed 866 of them.
Sagan’s team used the Galileo probe, which launched in 1989, as it passed near Earth in 1990. The team wanted to find out if they could tell something about life on Earth by looking at pictures of our planet. They thought the flashes were reflections on the oceans, but afterwards scientists realized that the glints appeared over land too.
“We found quite a few very bright flashes over land as well," Alexander Marshak, DSCOVR deputy project scientist at NASA’s Goddard Space Flight Center, said in a statement. "When I first saw it I thought maybe there was some water there, or a lake the sun reflects off of. But the glint is pretty big, so it wasn’t that."
Another initial explanation was lighting. Electrical discharges in the atmosphere could be bright enough to be seen by DSCOVR, which is located 1,500,000 kilometers (930,000 miles) from Earth. However, when the team considered the position of the Sun and the craft when the images were taken, the flashes only appeared when the angle between the EPIC, Earth, and our Sun matched.
“Lightning doesn’t care about the sun and EPIC’s location,” Marshak added. "The source of the flashes is definitely not on the ground. It’s definitely ice, and most likely solar reflection off of horizontally oriented particles."
The EPIC instruments can measure how high clouds are, and in this case they were able to estimate that the flashes originate in high cirrus clouds 5 to 8 kilometers (3 to 5 miles) over the surface of the Earth.
This ice crystal discovery has consequences both near and far. Planetary scientists will certainly be curious to understand how much sunlight is reflected into space by these atmospheric formations. It’s unclear at the moment if there’s enough of them to make a sizable contribution to the energy budget of the atmosphere.
The other consequence is instead related to exoplanets. Similar to what Sagan had envisioned, these flashes might be visible using sophisticated instruments (like the upcoming James Webb Space Telescope) and might tell us a lot about a distant planet's atmosphere.
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