Hundreds of large flashes have been captured on camera reflecting off our planet, and they’ve helped NASA solve a mystery that stumped the likes of Carl Sagan more than two decades ago.
These flashes are so bright that they can be seen from space, and they were thought to be caused by sunlight reflecting off the ocean’s surface. But then NASA began spotting them on land, and no one could say why.
“We found quite a few very bright flashes over land as well,” says Alexander Marshak from NASA’s Goddard Space Flight Centre.
“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.”
The famous astronomer Carl Sagan first noticed strange flashes of light in images of Earth taken by the Galileo spacecraft in 1993.
The unmanned space probe had been launched four years earlier to study Jupiter and its moons, but Sagan and his team decided to use one of its flybys past Earth to look for signs of life on our own planet.
The idea was that if he could detect signs of life on Earth from space, any undiscovered extraterrestrial neighbors could do the same and know that our planet was inhabited from afar.
They discovered large glints of light reflecting like mirrors in Galileo’s images, but he could only find them in certain areas of the planet covered in water.
“Large expanses of blue ocean and apparent coastlines are present, and close examination of the images shows a region of specular [mirror-like] reflection in ocean, but not on land,” the team reported at the time.
It came as quite a surprise, then, when 24 years later, NASA detected 866 bursts of light between June 2015 and August 2016, all of which originated on Earth.
You can see them in the video below, which was captured by NASA’s Earth Polychromatic Imaging Camera (EPIC) onboard NOAA’s Deep Space Climate Observatory (DSCOVR):
When the NASA team compared the old Galileo photos, they discovered that Sagan and his team had overlooked an important detail: those strange reflections had also appeared on land during that survey.
Marshak and his team were tasked with explaining this strange phenomenon, so they cataloged and mapped all known flashes over land from Galileo and EPIC images.
They hypothesized that if the glints were caused by reflected sunlight, they would only appear in certain parts of the world – places where the angle between the Sun and Earth was the same as the angle between the spacecraft and Earth, allowing the spacecraft to pick up the reflected light.
That pattern did emerge, and it helped them rule out one possible cause of the glints: lightning.
“Lightning doesn’t care about the Sun and EPIC’s location,” says Marshak.
To determine what the sunlight was reflecting off, the team proposed that water is still the culprit, but this time up in the atmosphere rather than on the surface.
Using EPIC data, they were able to pinpoint the source of the glints, narrowing it down to 5 to 8 km (3 to 5 miles) above the surface, where cirrus clouds full of ice crystals hang.
The numbers in the EPIC and Galileo images matched perfectly when they modeled the direction of sunlight reflecting off hypothetical ice crystals that happened to be floating in the air on a horizontal angle.
“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,” says Marshak.
Because the study has not yet been peer-reviewed, certain aspects of the discovery may change once it has been independently verified.
However, the researchers are now investigating how common these horizontal ice crystals are, and if they’re having any significant impact on how much sunlight is being reflected through our atmosphere.
And at least we now know one thing for sure – these aren’t the signs of life Carl Sagan was looking for.