NASA camera spots unexpected flashes from Earth
Over 1.5 million kilometres from Earth, a NASA camera captured hundreds of flashes of light reflecting off our planet – and now scientists have deciphered the source.
OVER THE SPACE OF a year, a camera aboard a NASA Earth-observation and space weather satellite has caught hundreds of flashes coming from Earth – and now scientists have deciphered their source.
The flashes were captured by an Earth-facing camera aboard the NOAA’s Deep Space Climate Observatory, launched in February 2015. The so-called EPIC camera (Earth Polychromatic Imaging Camera) aboard the satellite has been taking almost-hourly images of our sunlit planet from its vantage point between the Earth and Sun.
Alexander Marshak, from NASA’s Goddard Space Flight Center, first noticed the light flashes occasionally appearing over oceans as he looked through that day’s EPIC images.
Investigating the flashes, Alexander and his colleagues found that similar reflections had once caught the attention of astronomer Carl Sagan back in 1993, when looking at images taken by the Galileo spacecraft when it turned its instruments away from Jupiter to collect data from Earth – with the aim of testing the question of whether they could detect signatures of life from afar.
“Large expanses of blue ocean and apparent coastlines are present, and close examination of the images shows a region of [mirror-like] reflection in ocean but not on land,” Sagan and his colleagues wrote of the glints at the time.
While these flashes of light reflected off oceans could have a simple explanation – sunlight hitting a smooth part of an ocean or lake and reflecting back to the sensor – when scientists took another look at the older Galileo images, they saw something Sagan had apparently missed: bright flashes over land as well.
“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,” Alexander said.
Instead the NASA scientists considered where else water might exist in the Earth system: ice particles high in the atmosphere. They conducted a series of experiments (detailed in a new paper published in Geophysical Research Letters) to confirm the cause of the distant flashes.
Having catalogued all 866 of the sunlight glints over land captured in the images, they reasoned that if the flashes were caused by reflected sunlight, they would be limited to certain spots on the globe – where the angle between the Sun and Earth is the same as the angle between the spacecraft and the Earth. They were right – which helped confirm the source wasn’t, for example, lightning flashes. “Lightning doesn’t care about the sun and EPIC’s location,” explained Alexander.
The scientists were also able to plot angles to determine that the light was reflecting off ice particles floating in the air almost horizontally, and that they were from a high altitude.
“The source of the flashes is definitely not on the ground. It’s definitely ice, and most likely solar reflection off horizontally oriented particles,” said Alexander, who is now investigating how common these particles are and whether they could have a measurable impact on how much sunlight passes through the atmosphere. If so, it’s a feature that could be incorporated into computer models of how much heat is reaching and leaving Earth.
The research could also be used to help other spacecraft studying exoplanets much further away.